Serial Communication Interface

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The Unist Quantum™ communicates to machine tools using the RS-232 serial communication protocol. This video will explain the basics of RS-232 serial communic. The communication links, across which computers (or parts of computers) talk to one another, may be either serial or parallel. A parallel link transmits several streams of data simultaneously along multiple channels (e.g., wires, printed circuit tracks, or optical fibers); whereas, a serial link transmits only a single stream of data.

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Introduction

Serial Peripheral Interface (SPI) is an interface bus commonly used to send data between microcontrollers and small peripherals such as shift registers, sensors, and SD cards. It uses separate clock and data lines, along with a select line to choose the device you wish to talk to.

Suggested Reading

Stuff that would be helpful to know before reading this tutorial:

Serial Communication

Asynchronous serial communication concepts: packets, signal levels, baud rates, UARTs and more!

Binary

Serial communication interface pdf
Binary is the numeral system of electronics and programming..so it must be important to learn. But, what is binary? How does it translate to other numeral systems like decimal?

Shift Registers

An introduction to shift registers and potential uses.

Logic Levels

Learn the difference between 3.3V and 5V devices and logic levels.

What's Wrong with Serial Ports?

A common serial port, the kind with TX and RX lines, is called 'asynchronous' (not synchronous) because there is no control over when data is sent or any guarantee that both sides are running at precisely the same rate. Since computers normally rely on everything being synchronized to a single “clock” (the main crystal attached to a computer that drives everything), this can be a problem when two systems with slightly different clocks try to communicate with each other.

To work around this problem, asynchronous serial connections add extra start and stop bits to each byte help the receiver sync up to data as it arrives. Both sides must also agree on the transmission speed (such as 9600 bits per second) in advance. Slight differences in the transmission rate aren't a problem because the receiver re-syncs at the start of each byte.

(By the way, if you noticed that '11001010' does not equal 0x53 in the above diagram, kudos to your attention to detail. Serial protocols will often send the least significant bits first, so the smallest bit is on the far left. The lower nybble is actually 0011 = 0x3, and the upper nybble is 0101 = 0x5.)

Asynchronous serial works just fine, but has a lot of overhead in both the extra start and stop bits sent with every byte, and the complex hardware required to send and receive data. And as you've probably noticed in your own projects, if both sides aren't set to the same speed, the received data will be garbage. This is because the receiver is sampling the bits at very specific times (the arrows in the above diagram). If the receiver is looking at the wrong times, it will see the wrong bits.

A Synchronous Solution

SPI works in a slightly different manner. It's a 'synchronous' data bus, which means that it uses separate lines for data and a 'clock' that keeps both sides in perfect sync. The clock is an oscillating signal that tells the receiver exactly when to sample the bits on the data line. This could be the rising (low to high) or falling (high to low) edge of the clock signal; the datasheet will specify which one to use. When the receiver detects that edge, it will immediately look at the data line to read the next bit (see the arrows in the below diagram). Because the clock is sent along with the data, specifying the speed isn't important, although devices will have a top speed at which they can operate (We'll discuss choosing the proper clock edge and speed in a bit).

One reason that SPI is so popular is that the receiving hardware can be a simple shift register. This is a much simpler (and cheaper!) piece of hardware than the full-up UART (Universal Asynchronous Receiver / Transmitter) that asynchronous serial requires.

Receiving Data

Note: You may not recognize the COPI/CIPO labels for SPI pins. SparkFun has joined with other members of OSHWA in a resolution to move away from using 'Master' and 'Slave' to describe signals between the controller and the peripheral. Check out this page for more on our reasoning behind this change. You can also see OSHWA's resolution here.

You might be thinking to yourself, self, that sounds great for one-way communications, but how do you send data back in the opposite direction? Here's where things get slightly more complicated.

In SPI, only one side generates the clock signal (usually called CLK or SCK for Serial ClocK). The side that generates the clock is called the 'controller', and the other side is called the 'peripheral'. There is always only one controller (which is almost always your microcontroller), but there can be multiple peripherals (more on this in a bit).

When data is sent from the controller to a peripheral, it's sent on a data line called COPI, for 'Controller Out / Peripheral In'. If the peripheral needs to send a response back to the controller, the controller will continue to generate a prearranged number of clock cycles, and the peripheral will put the data onto a third data line called CIPO, for 'Controller In / Peripheral Out'.

Notice we said 'prearranged' in the above description. Because the controller always generates the clock signal, it must know in advance when a peripheral needs to return data and how much data will be returned. This is very different than asynchronous serial, where random amounts of data can be sent in either direction at any time. In practice this isn't a problem, as SPI is generally used to talk to sensors that have a very specific command structure. For example, if you send the command for 'read data' to a device, you know that the device will always send you, for example, two bytes in return. (In cases where you might want to return a variable amount of data, you could always return one or two bytes specifying the length of the data and then have the controller retrieve the full amount.)

Note that SPI is 'full duplex' (has separate send and receive lines), and, thus, in certain situations, you can transmit and receive data at the same time (for example, requesting a new sensor reading while retrieving the data from the previous one). Your device's datasheet will tell you if this is possible.

Chip Select (CS)

There's one last line you should be aware of, called CS for Chip Select. This tells the peripheral that it should wake up and receive / send data and is also used when multiple peripherals are present to select the one you'd like to talk to.

The CS line is normally held high, which disconnects the peripheral from the SPI bus. (This type of logic is known as “active low,” and you’ll often see used it for enable and reset lines.) Just before data is sent to the peripheral, the line is brought low, which activates the peripheral. When you're done using the peripheral, the line is made high again. In a shift register, this corresponds to the 'latch' input, which transfers the received data to the output lines.

Multiple peripherals

There are two ways of connecting multiple peripherals to an SPI bus:

  1. In general, each peripheral will need a separate CS line. To talk to a particular peripheral, you'll make that peripheral's CS line low and keep the rest of them high (you don't want two peripherals activated at the same time, or they may both try to talk on the same CIPO line resulting in garbled data). Lots of peripherals will require lots of CS lines; if you're running low on outputs, there are binary decoder chips that can multiply your CS outputs.
  1. On the other hand, some parts prefer to be daisy-chained together, with the CIPO (output) of one going to the COPI (input) of the next. In this case, a single CS line goes to all the peripherals. Once all the data is sent, the CS line is raised, which causes all the chips to be activated simultaneously. This is often used for daisy-chained shift registers and addressable LED drivers.

Note that, for this layout, data overflows from one peripheral to the next, so to send data to any one peripheral, you'll need to transmit enough data to reach all of them. Also, keep in mind that the first piece of data you transmit will end up in the last peripheral.

This type of layout is typically used in output-only situations, such as driving LEDs where you don't need to receive any data back. In these cases you can leave the controller's CIPO line disconnected. However, if data does need to be returned to the controller, you can do this by closing the daisy-chain loop (blue wire in the above diagram). Note that if you do this, the return data from peripheral 1 will need to pass through all the peripherals before getting back to the controller, so be sure to send enough receive commands to get the data you need.

Programming for SPI

Many microcontrollers have built-in SPI peripherals that handle all the details of sending and receiving data, and can do so at very high speeds. The SPI protocol is also simple enough that you (yes, you!) can write your own routines to manipulate the I/O lines in the proper sequence to transfer data. (A good example is on the Wikipedia SPI page.)

If you're using an Arduino, there are two ways you can communicate with SPI devices:

  1. You can use the shiftIn() and shiftOut() commands. These are software-based commands that will work on any group of pins, but will be somewhat slow.

  2. Or you can use the SPI Library, which takes advantage of the SPI hardware built into the microcontroller. This is vastly faster than the above commands, but it will only work on certain pins.

You will need to select some options when setting up your interface. These options must match those of the device you're talking to; check the device's datasheet to see what it requires.

  • The interface can send data with the most-significant bit (MSB) first, or least-significant bit (LSB) first. In the Arduino SPI library, this is controlled by the setBitOrder() function.

  • The peripheral will read the data on either the rising edge or the falling edge of the clock pulse. Additionally, the clock can be considered 'idle' when it is high or low. In the Arduino SPI library, both of these options are controlled by the setDataMode() function.

  • SPI can operate at extremely high speeds (millions of bytes per second), which may be too fast for some devices. To accommodate such devices, you can adjust the data rate. In the Arduino SPI library, the speed is set by the setClockDivider() function, which divides the controller clock (16MHz on most Arduinos) down to a frequency between 8MHz (/2) and 125kHz (/128).

  • If you're using the SPI Library, you must use the provided SCK, COPI and CIPO pins, as the hardware is hardwired to those pins. There is also a dedicated CS pin that you can use (which must, at least, be set to an output in order for the SPI hardware to function), but note that you can use any other available output pin(s) for CS to your peripheral device(s) as well.

  • On older Arduinos, you'll need to control the CS pin(s) yourself, making one of them low before your data transfer and high afterward. Newer Arduinos such as the Due can control each CS pin automatically as part of the data transfer; see the Due SPI documentation page for more information.

Interested in learning more foundational topics?

See our Engineering Essentials page for a full list of cornerstone topics surrounding electrical engineering.

Resources and Going Further

Tips and Tricks

  • Because of the high speed signals, SPI should only be used to send data over short distances (up to a few feet). If you need to send data further than that, lower the clock speed, and consider using specialized driver chips.

  • If things aren't working the way you think they should, a logic analyzer is a very helpful tool. Smart analyzers like the Saleae USB Logic Analyzer can even decode the data bytes for a display or logging.

Advantages of SPI:

  • It's faster than asynchronous serial

  • The receive hardware can be a simple shift register

  • It supports multiple peripherals

Disadvantages of SPI:

  • It requires more signal lines (wires) than other communications methods

  • The communications must be well-defined in advance (you can't send random amounts of data whenever you want)

  • The controller must control all communications (peripherals can't talk directly to each other)

  • It usually requires separate CS lines to each peripheral, which can be problematic if numerous peripherals are needed.

Further Reading

Check out the Wikipedia page on SPI, which includes lots of good information on SPI and other synchronous interfaces.

This page presents a more correct way to set up an SPI network amongst your embedded devices, particularly for use with an Arduino microcontroller.

A number of SparkFun products have SPI interfaces. For example, the Bar Graph Breakout kit has an easy-to-use SPI interface that you can use to turn any of 30 LEDs on or off.

Other communication options:

Serial Communication

Asynchronous serial communication concepts: packets, signal levels, baud rates, UARTs and more!

Analog to Digital Conversion

The world is analog. Use analog to digital conversion to help digital devices interpret the world.

I2C

An introduction to I2C, one of the main embedded communications protocols in use today.

AST-CAN485 Hookup Guide

The AST CAN485 is a miniature Arduino in the compact form factor of the ProMini. In addition to all the usual features it has on-board CAN and RS485 ports enabling quick and easy interfacing to a multitude of industrial devices.

Now that you’re a pro on SPI, here are some other tutorials to practice your new skills:

MP3 Player Shield Music Box

Music Box Project based on the Dr. Who TARDIS.

Using the Serial 7-Segment Display

How to quickly and easily set up the Serial 7-Segment Display and the Serial 7-Segment Display Shield.

SparkFun BME280 Breakout Hookup Guide

A guide for connecting the BEM280 sensor to a microcontroller, and for using the Sparkfun Arduino library.

Raspberry Pi SPI and I2C Tutorial

Learn how to use serial I2C and SPI buses on your Raspberry Pi using the wiringPi I/O library for C/C++ and spidev/smbus for Python.
(Redirected from Serial Communication Interface)
Block diagram for a UART

A universal asynchronous receiver-transmitter (UART/ˈjuːɑːrt/) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel. It was one of the earliest computer communication devices, used to attach teletypewriters for an operator console. It was also an early hardware system for the Internet. The electric signaling levels are handled by a driver circuit external to the UART. Two common signal levels are RS-232, a 12-volt system, and RS-485, a 5-volt system. Specialised UARTs are used for automobiles, smart cards and SIMs. Early teletypewriters used current loops. A UART is usually an individual (or part of an) integrated circuit (IC) used for serial communications over a computer or peripheral device serial port. One or more UART peripherals are commonly integrated in microcontroller chips. A related device, the universal synchronous and asynchronous receiver-transmitter (USART) also supports synchronous operation.

Transmitting and receiving serial data[edit]

The universal asynchronous receiver-transmitter (UART) takes bytes of data and transmits the individual bits in a sequential fashion.[1] At the destination, a second UART re-assembles the bits into complete bytes. Each UART contains a shift register, which is the fundamental method of conversion between serial and parallel forms. Serial transmission of digital information (bits) through a single wire or other medium is less costly than parallel transmission through multiple wires.

The UART usually does not directly generate or receive the external signals used between different items of equipment. Separate interface devices are used to convert the logic level signals of the UART to and from the external signalling levels, which may be standardized voltage levels, current levels, or other signals.

Communication may be simplex (in one direction only, with no provision for the receiving device to send information back to the transmitting device), full duplex (both devices send and receive at the same time) or half duplex (devices take turns transmitting and receiving).

Data framing[edit]

The idle, no data state is high-voltage, or powered. Halo 4 pc free download full version no survey. This is a historic legacy from telegraphy, in which the line is held high to show that the line and transmitter are not damaged. Each character is framed as a logic low start bit, data bits, possibly a parity bit and one or more stop bits. In most applications the least significant data bit (the one on the left in this diagram) is transmitted first, but there are exceptions (such as the IBM 2741 printing terminal).

The start bit signals the receiver that a new character is coming. The next five to nine bits, depending on the code set employed, represent the character. If a parity bit is used, it would be placed after all of the data bits. The next one or two bits are always in the mark (logic high, i.e., '1') condition and called the stop bit(s). They signal to the receiver that the character is complete. Since the start bit is logic low (0) and the stop bit is logic high (1) there are always at least two guaranteed signal changes between characters.

If the line is held in the logic low condition for longer than a character time, this is a break condition that can be detected by the UART.

Serial

Receiver[edit]

All operations of the UART hardware are controlled by an internal clock signal which runs at a multiple of the data rate, typically 8 or 16 times the bit rate. The receiver tests the state of the incoming signal on each clock pulse, looking for the beginning of the start bit. If the apparent start bit lasts at least one-half of the bit time, it is valid and signals the start of a new character. If not, it is considered a spurious pulse and is ignored. After waiting a further bit time, the state of the line is again sampled and the resulting level clocked into a shift register. After the required number of bit periods for the character length (5 to 8 bits, typically) have elapsed, the contents of the shift register are made available (in parallel fashion) to the receiving system. The UART will set a flag indicating new data is available, and may also generate a processor interrupt to request that the host processor transfers the received data.

Communicating UARTs have no shared timing system apart from the communication signal. Typically, UARTs resynchronize their internal clocks on each change of the data line that is not considered a spurious pulse. Obtaining timing information in this manner, they reliably receive when the transmitter is sending at a slightly different speed than it should. Simplistic UARTs do not do this, instead they resynchronize on the falling edge of the start bit only, and then read the center of each expected data bit, and this system works if the broadcast data rate is accurate enough to allow the stop bits to be sampled reliably.

Serial Communication Interface Lectures

It is a standard feature for a UART to store the most recent character while receiving the next. This 'double buffering' gives a receiving computer an entire character transmission time to fetch a received character. Many UARTs have a small first-in, first-out (FIFO) buffer memory between the receiver shift register and the host system interface. This allows the host processor even more time to handle an interrupt from the UART and prevents loss of received data at high rates.

Transmitter[edit]

Transmission operation is simpler as the timing does not have to be determined from the line state, nor is it bound to any fixed timing intervals. As soon as the sending system deposits a character in the shift register (after completion of the previous character), the UART generates a start bit, shifts the required number of data bits out to the line, generates and sends the parity bit (if used), and sends the stop bits. Since full-duplex operation requires characters to be sent and received at the same time, UARTs use two different shift registers for transmitted and received characters. High performance UARTs could contain a transmit FIFO (first in first out) buffer to allow a CPU or DMA controller to deposit multiple characters in a burst into the FIFO rather than have to deposit one character at a time into the shift register. Since transmission of a single or multiple characters may take a long time relative to CPU speeds, a UART maintains a flag showing busy status so that the host system knows if there is at least one character in the transmit buffer or shift register; 'ready for next character(s)' may also be signaled with an interrupt.

Application[edit]

Transmitting and receiving UARTs must be set for the same bit speed, character length, parity, and stop bits for proper operation. The receiving UART may detect some mismatched settings and set a 'framing error' flag bit for the host system; in exceptional cases, the receiving UART will produce an erratic stream of mutilated characters and transfer them to the host system.

Typical serial ports used with personal computers connected to modems use eight data bits, no parity, and one stop bit; for this configuration, the number of ASCII characters per second equals the bit rate divided by 10.

Some very low-cost home computers or embedded systems dispense with a UART and use the CPU to sample the state of an input port or directly manipulate an output port for data transmission. While very CPU-intensive (since the CPU timing is critical), the UART chip can thus be omitted, saving money and space. The technique is known as bit-banging.

History[edit]

Serial Communication Interface

Some early telegraph schemes used variable-length pulses (as in Morse code) and rotating clockwork mechanisms to transmit alphabetic characters. The first serial communication devices (with fixed-length pulses) were rotating mechanical switches (commutators). Various character codes using 5, 6, 7, or 8 data bits became common in teleprinters and later as computer peripherals. The teletypewriter made an excellent general-purpose I/O device for a small computer.

Gordon Bell of DEC designed the first UART, occupying an entire circuit board called a line unit, for the PDP series of computers beginning with the PDP-1.[2][3]According to Bell, the main innovation of the UART was its use of sampling to convert the signal into the digital domain, allowing more reliable timing than previous circuits that used analog timing devices with manually adjusted potentiometers.[4] To reduce the cost of wiring, backplane and other components, these computers also pioneered flow control using XON and XOFF characters rather than hardware wires.

DEC condensed the line unit design into an early single-chip UART for their own use.[2]Western Digital developed this into the first widely available single-chip UART, the WD1402A, around 1971. This was an early example of a medium-scale integrated circuit. Another popular chip was the SCN2651 from the Signetics 2650 family.

An example of an early 1980s UART was the National Semiconductor8250 used in the original IBM PC's Asynchronous Communications Adapter card.[5] In the 1990s, newer UARTs were developed with on-chip buffers. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the popular National Semiconductor 16550 has a 16-byte FIFO, and spawned many variants, including the 16C550, 16C650, 16C750, and 16C850.

Depending on the manufacturer, different terms are used to identify devices that perform the UART functions. Intel called their 8251 device a 'Programmable Communication Interface'. MOS Technology 6551 was known under the name 'Asynchronous Communications Interface Adapter' (ACIA). The term 'Serial Communications Interface' (SCI) was first used at Motorola around 1975 to refer to their start-stop asynchronous serial interface device, which others were calling a UART. Zilog manufactured a number of Serial Communication Controllers or SCCs.

Starting in the 2000s, most IBM PC compatible computers removed their external RS-232COM ports and used USB ports that can send data faster. For users who still need RS-232 serial ports, external USB-to-UART bridges are now commonly used. They combine the hardware cables and a chip to do the USB and UART conversion. FTDI is a significant commercial supplier of these chips.[6] Although RS-232 ports are no longer available to users on the outside of most computers, many internal processors and microprocessors have UARTs built into their chips to give hardware designers the ability to interface with other chips or devices that use RS-232 or RS-485 for communication.

Structure[edit]

A UART usually contains the following components:

  • a clock generator, usually a multiple of the bit rate to allow sampling in the middle of a bit period
  • input and output shift registers
  • transmit/receive control
  • read/write control logic
  • Autobaud measurement (optional)
  • transmit/receive buffers (optional)
  • system data bus buffer (optional)
  • First-in, first-out (FIFO) buffer memory (optional)
  • Signals needed by a third party DMA controller (optional)
  • Integrated bus mastering DMA controller (optional)

Special transceiver conditions[edit]

Overrun error[edit]

An 'overrun error' occurs when the receiver cannot process the character that just came in before the next one arrives. Various devices have different amounts of buffer space to hold received characters. The CPU or DMA controller must service the UART in order to remove characters from the input buffer. If the CPU or DMA controller does not service the UART quickly enough and the buffer becomes full, an Overrun Error will occur, and incoming characters will be lost.

Underrun error[edit]

An 'underrun error' occurs when the UART transmitter has completed sending a character and the transmit buffer is empty. In asynchronous modes this is treated as an indication that no data remains to be transmitted, rather than an error, since additional stop bits can be appended. This error indication is commonly found in USARTs, since an underrun is more serious in synchronous systems.

Framing error[edit]

A UART will detect a framing error when it does not see a 'stop' bit at the expected 'stop' bit time. As the 'start' bit is used to identify the beginning of an incoming character, its timing is a reference for the remaining bits. If the data line is not in the expected state (high) when the 'stop' bit is expected (according to the number of data and parity bits for which the UART is set), the UART will signal a framing error. A 'break' condition on the line is also signaled as a framing error.

Parity error[edit]

A parity error occurs when the parity of the number of one-bits disagrees with that specified by the parity bit. Use of a parity bit is optional, so this error will only occur if parity-checking has been enabled.

Break condition[edit]

A break condition occurs when the receiver input is at the 'space' (logic low, i.e., '0') level for longer than some duration of time, typically, for more than a character time. This is not necessarily an error, but appears to the receiver as a character of all zero-bits with a framing error. The term 'break' derives from current loop signaling, which was the traditional signaling used for teletypewriters. The 'spacing' condition of a current loop line is indicated by no current flowing, and a very long period of no current flowing is often caused by a break or other fault in the line.

Some equipment will deliberately transmit the 'space' level for longer than a character as an attention signal. When signaling rates are mismatched, no meaningful characters can be sent, but a long 'break' signal can be a useful way to get the attention of a mismatched receiver to do something (such as resetting itself). Computer systems can use the long 'break' level as a request to change the signaling rate, to support dial-in access at multiple signaling rates. The DMX512protocol uses the break condition to signal the start of a new packet.

UART models[edit]

A dual UART, or DUART, combines two UARTs into a single chip. Similarly, a quadruple UART or QUART, combines four UARTs into one package, such as the NXP 28L194. An octal UART or OCTART combines eight UARTs into one package, such as the Exar XR16L788 or the NXP SCC2698.

ModelDescription
WD1402AThe first single-chip UART on general sale. Introduced about 1971. Compatible chips included the Fairchild TR1402A and the General Instruments AY-5-1013.[7]
Exar XR21V1410
Intersil 6402
CDP 1854 (RCA, now Intersil)
Zilog Z8440Universal synchronous and asynchronous receiver-transmitter. 2000 kbit/s. Async, Bisync, SDLC, HDLC, X.25. CRC. 4-byte RX buffer. 2-byte TX buffer. Provides signals needed by a third party DMA controller to perform DMA transfers.[8]
Z8530/Z85C30This universal synchronous and asynchronous receiver-transmitter has a 3-byte receive buffer and a 1-byte transmit buffer. It has hardware to accelerate the processing of HDLC and SDLC. The CMOS version (Z85C30) provides signals to allow a third party DMA controller to perform DMA transfers. It can do asynchronous, byte level synchronous, and bit level synchronous communications.[9]
8250Obsolete with 1-byte buffers. These UARTs' maximum standard serial port speed is 9600 bits per second if the operating system has a 1 millisecond interrupt latency. 8250 UARTs were used in the IBM PC 5150 and IBM PC/XT, while the 16450 UART were used in IBM PC/AT-series computers.
8251
Motorola 6850
6551
Rockwell 65C52
16450
82510This UART allows asynchronous operation up to 288 kbit/s, with two independent four-byte FIFOs. It was produced by Intel at least from 1993 to 1996, and Innovastic Semiconductor has a 2011 Data Sheet for IA82510.
16550This UART's FIFO is broken, so it cannot safely run any faster than the 16450 UART. The 16550A and later versions fix this bug.
16550AThis UART has 16-byte FIFO buffers. Its receive interrupt trigger levels can be set to 1, 4, 8, or 14 characters. Its maximum standard serial port speed if the operating system has a 1 millisecond interrupt latency is 128 kbit/s. Systems with lower interrupt latencies or with DMA controllers could handle higher baud rates. This chip can provide signals that are needed to allow a DMA controller to perform DMA transfers to and from the UART if the DMA mode this UART introduces is enabled.[10] It was introduced by National Semiconductor, which has been sold to Texas Instruments. National Semiconductor claimed that this UART could run at up to 1.5 Mbit/s.
16C552
16650This UART was introduced by Startech Semiconductor which is now owned by Exar Corporation and is not related to Startech.com. Early versions have a broken FIFO buffer and therefore cannot safely run any faster than the 16450 UART.[11] Versions of this UART that were not broken have 32-character FIFO buffers and could function at standard serial port speeds up to 230.4 kbit/s if the operating system has a 1 millisecond interrupt latency. Current versions of this UART by Exar claim to be able to handle up to 1.5 Mbit/s. This UART introduces the Auto-RTS and Auto-CTS features in which the RTS# signal is controlled by the UART to signal the external device to stop transmitting when the UART's buffer is full to or beyond a user-set trigger point and to stop transmitting to the device when the device drives the CTS# signal high (logic 0).
1675064-byte buffers. This UART can handle a maximum standard serial port speed of 460.8 kbit/s if the maximum interrupt latency is 1 millisecond. This UART was introduced by Texas Instruments. TI claims that early models can run up to 1 Mbit/s, and later models in this series can run up to 3 Mbit/s.
16850128-byte buffers. This UART can handle a maximum standard serial port speed of 921.6 kbit/s if the maximum interrupt latency is 1 millisecond. This UART was introduced by Exar Corporation. Exar claims that early versions can run up to 2 Mbit/s, and later versions can run up to 2.25 Mbit/s depending on the date of manufacture.
16C850
16950128-byte buffers. This UART can handle a maximum standard serial port speed of 921.6 kbit/s if the maximum interrupt latency is 1 millisecond. This UART supports 9-bit characters in addition to the 5- to 8-bit characters that other UARTs support. This was introduced by Oxford Semiconductor, which is now owned by PLX Technology. Oxford/PLX claims that this UART can run up to 15 Mbit/s. PCI Express variants by Oxford/PLX are integrated with a first party bus mastering PCIe DMA controller. This DMA controller uses the UART's DMA mode signals that were defined for the 16550. The DMA controller requires the CPU to set up each transaction and poll a status register after the transaction is started to determine if the transaction is done. Each DMA transaction can transfer between 1 and 128 bytes between a memory buffer and the UART. PCI Express variants can also allow the CPU to transfer data between itself and the UART with 8-, 16-, or 32-bit transfers when using programmed I/O.
16C950
16954Quad-port version of the 16950/16C950. 128-byte buffers. This UART can handle a maximum standard serial port speed of 921.6 kbit/s if the maximum interrupt latency is 1 millisecond. This UART supports 9-bit characters in addition to the 5–8 bit characters that other UARTs support. This was introduced by Oxford Semiconductor, which is now owned by PLX Technology. Oxford/PLX claims that this UART can run up to 15 Mbit/s. PCI Express variants by Oxford/PLX are integrated with a first party bus mastering PCIe DMA controller. This DMA controller is controlled by the UART's DMA mode signals that were defined for the 16550. The DMA controller requires the CPU to set up each transaction and poll a status register after the transaction is started to determine if the transaction is done. Each DMA transaction can transfer between 1 and 128 bytes between a memory buffer and the UART. PCI Express variants can also allow the CPU to transfer data between itself and the UART with 8-, 16-, or 32-bit transfers when using programmed I/O.
16C954
16C1550/16C1551UART with 16-byte FIFO buffers. Up to 1.5 Mbit/s. The ST16C155X is not compatible with the industry standard 16550 and will not work with the standard serial port driver in Microsoft Windows.
16C2450Dual UART with 1-byte FIFO buffers.
16C2550Dual UART with 16-byte FIFO buffers. Pin-to-pin and functional compatible to 16C2450. Software compatible with INS8250 and NS16C550.
SCC2691Currently produced by NXP, the 2691 is a single channel UART that also includes a programmable counter/timer. The 2691 has a single-byte transmitter holding register and a 4-byte receive FIFO. Maximum standard speed of the 2692 is 115.2 kbit/s.

The 28L91 is an upwardly compatible version of the 2691, featuring selectable 8- or 16-byte transmitter and receiver FIFOs, improved support for extended data rates, and faster bus timing characteristics, making the device more suitable for use with high performance microprocessors.

Both the 2691 and 28L91 may also be operated in TIA-422 and TIA-485 modes, and may also be programmed to support non-standard data rates. The devices are produced in PDIP-40, PLCC-44 and 44 pin QFP packages, and are readily adaptable to both Motorola and Intel buses. They have also been successfully adapted to the 65C02 and 65C816 buses. The 28L91 will operate on 3.3 or 5 volts.

SCC28L91
SCC2692Currently produced by NXP, these devices are dual UARTs (DUART), consisting of two communications channels, associated control registers and one counter/timer. Each communication channel is independently programmable and supports independent transmit and receive data rates.

The 2692 has a single-byte transmitter holding register and a 4-byte receiver FIFO for each channel. Maximum standard speed of both of the 2692's channels is 115.2 kbit/s.

The 26C92 is an upwardly compatible version of the 2692, with 8-byte transmitter and receiver FIFOs for improved performance during continuous bi-directional asynchronous transmission (CBAT) on both channels at the maximum standard speed of 230.4 kbit/s. The letter C in the 26C92 part number has nothing to do with the fabrication process; all NXP UARTs are CMOS devices.

The 28L92 is an upwardly compatible version of the 26C92, featuring selectable 8- or 16-byte transmitter and receiver FIFOs, improved support for extended data rates, and faster bus timing characteristics, making the device more suitable for use with high performance microprocessors.

The 2692, 26C92 and 28L92 may be operated in TIA-422 and TIA-485 modes, and may also be programmed to support non-standard data rates. The devices are produced in PDIP-40, PLCC-44 and 44 pin QFP packages, and are readily adaptable to both Motorola and Intel buses. They have also been successfully adapted to the 65C02 and 65C816 buses. The 28L92 will operate on 3.3 or 5 volts.

SC26C92
SC28L92
SCC28C94Currently produced by NXP, the 28C94 quadruple UART (QUART) is functionally similar to a pair of SCC26C92 DUARTs mounted in a common package, with the addition of an arbitrated interrupt system for efficient processing during periods of intense channel activity. Some additional signals are present to support the interrupt management features and the auxiliary input/output pins are arranged differently than those of the 26C92. Otherwise, the programming model for the 28C94 is similar to that of the 26C92, requiring only minor code changes to fully utilize all features. The 28C94 supports a maximum standard speed of 230.4 kbit/s, is available in a PLCC-52 package, and is readily adaptable to both Motorola and Intel buses. It has also been successfully adapted to the 65C816 bus.
SCC2698BCurrently produced by NXP, the 2698 octal UART (OCTART) is essentially four SCC2692 DUARTs in a single package. Specifications are the same as the SCC2692 (not the SCC26C92). Due to the lack of transmitter FIFOs and the small size of the receiver FIFOs, the 2698 can cause an interrupt 'storm' if all channels are simultaneously engaged in continuous bi-directional communication. The device is produced in PDIP-64 and PLCC-84 packages, and is readily adaptable to both Motorola and Intel buses. The 2698 has also been successfully adapted to the 65C02 and 65C816 buses.
SCC28L198Currently produced by NXP, the 28L198 OCTART is essentially an upscaled enhancement of the SCC28C94 QUART described above, with eight independent communications channels, as well as an arbitrated interrupt system for efficient processing during periods of intense channel activity. The 28L198 supports a maximum standard speed of 460.8 kbit/s, is available in PLCC-84 and LQFP-100 packages, and is readily adaptable to both Motorola and Intel buses. The 28L198 will operate on 3.3 or 5 volts.
Z85230Synchronous/Asynchronous modes, 2 ports. Provides signals needed by a third party DMA controller needed to perform DMA transfers. 4-byte buffer to send, 8-byte buffer to receive per channel. SDLC/HDLC modes. 5 Mbit/s in synchronous mode.
Hayes ESP1 KB buffers, 921.6 kbit/s, 8-ports.[12]
Exar XR17V352, XR17V354 and XR17V358Dual, Quad and Octal PCI Express UARTs with 16550 compatible register Set, 256-byte TX and RX FIFOs, Programmable TX and RX Trigger Levels, TX/RX FIFO Level Counters, Fractional baud rate generator, Automatic RTS/CTS or DTR/DSR hardware flow control with programmable hysteresis, Automatic Xon/Xoff software flow control, RS-485 half duplex direction control output with programmable turn-around delay, Multi-drop with Auto Address Detection, Infrared (IrDA 1.1) data encoder/decoder. They are specified up to 25 Mbit/s. DataSheets are dated from 2012.
Exar XR17D152, XR17D154 and XR17D158Dual, Quad and Octal PCI bus UARTs with 16C550 Compatible 5G Register Set, 64-byte Transmit and Receive FIFOs, Transmit and Receive FIFO Level Counters, Programmable TX and RX FIFO Trigger Level, Automatic RTS/CTS or DTR/DSR Flow Control, Automatic Xon/Xoff Software Flow Control, RS485 HDX Control Output with Selectable Turn-around Delay, Infrared (IrDA 1.0) Data Encoder/Decoder, Programmable Data Rate with Prescaler, Up to 6.25 Mbit/s Serial Data Rate. DataSheets are dated from 2004 and 2005.
Exar XR17C152, XR17C154 and XR17C158Dual, Quad and Octal 5 V PCI bus UARTs with 16C550 Compatible Registers, 64-byte Transmit and Receive FIFOs, Transmit and Receive FIFO Level Counters, Automatic RTS/CTS or DTR/DSR Flow Control, Automatic Xon/Xoff Software Flow Control, RS485 Half-duplex Control with Selectable Delay, Infrared (IrDA 1.0) Data Encoder/Decoder, Programmable Data Rate with Prescaler, Up to 6.25 Mbit/s Serial Data Rate. DataSheets are dated from 2004 and 2005.
Exar XR17V252, XR17V254 and XR17V258Dual, Quad and Octal 66 MHz PCI bus UARTs with Power Management Support, 16C550 compatible register set, 64-byte TX and RX FIFOs with level counters and programmable trigger levels, Fractional baud rate generator, Automatic RTS/CTS or DTR/DSR hardware flow control with programmable hysteresis, Automatic Xon/Xoff software flow control, RS-485 half duplex direction control output with selectable turn-around delay, Infrared (IrDA 1.0) data encoder/decoder, Programmable data rate with prescaler. DataSheets are dated from 2008 and 2010.

UART in modems[edit]

Serial Communication Interface Mcq

Modems for personal computers that plug into a motherboard slot must also include the UART function on the card. The original 8250 UART chip shipped with the IBM personal computer had a one character buffer for the receiver and the transmitter each, which meant that communications software performed poorly at speeds above 9600 bit/s, especially if operating under a multitasking system or if handling interrupts from disk controllers. High-speed modems used UARTs that were compatible with the original chip but which included additional FIFO buffers, giving software additional time to respond to incoming data.

A look at the performance requirements at high bit rates shows why the 16-, 32-, 64- or 128-byte FIFO is a necessity. The Microsoft specification for a DOS system requires that interrupts not be disabled for more than 1 millisecond at a time. Some hard disk drives and video controllers violate this specification. 9600 bit/s will deliver a character approximately every millisecond, so a 1-byte FIFO should be sufficient at this rate on a DOS system which meets the maximum interrupt disable timing. Rates above this may receive a new character before the old one has been fetched, and thus the old character will be lost. This is referred to as an overrun error and results in one or more lost characters.

A 16-byte FIFO allows up to 16 characters to be received before the computer has to service the interrupt. This increases the maximum bit rate the computer can process reliably from 9600 to 153,000 bit/s if it has a 1 millisecond interrupt dead time. A 32-byte FIFO increases the maximum rate to over 300,000 bit/s. A second benefit to having a FIFO is that the computer only has to service about 8 to 12% as many interrupts, allowing more CPU time for updating the screen, or doing other chores. Thus the computer's responses will improve as well.

See also[edit]

Serial Communication Interface In Embedded System

References[edit]

Serial Communication Interface Ppt

  1. ^Adam Osborne, An Introduction to Microcomputers Volume 1: Basic Concepts, Osborne-McGraw Hill Berkeley California USA, 1980 ISBN0-931988-34-9 pp. 116–126
  2. ^ abC. Gordon Bell, J. Craig Mudge, John E. McNamara, Computer Engineering: A DEC View of Hardware Systems Design, Digital Press, 12 May 2014, ISBN1483221105, p. 73
  3. ^Allison, David. 'Curator, Division of Information Technology and Society, National Museum of American History, Smithsonian Institution'. Smithsonian Institution Oral and Video Histories. Retrieved 14 June 2015.
  4. ^Oral History of Gordon Bell, 2005, accessed 2015-08-19
  5. ^Technical Reference 6025008(PDF). Personal Computer Hardware Reference Library. IBM. August 1981. pp. 2–123.
  6. ^'FTDI Products'. www.ftdichip.com. Retrieved 22 March 2018.
  7. ^Interfacing with a PDP-11/05: the UART, blinkenbone.com, accessed 2015-08-19
  8. ^'Zilog Product specification Z8440/1/2/4, Z84C40/1/2/3/4. Serial input/output controller'(PDF). 090529 zilog.com
  9. ^'Zilog Document Download'(PDF). www.zilog.com. Retrieved 22 March 2018.
  10. ^'FAQ: The 16550A UART & TurboCom drivers 1994'. Retrieved January 16, 2016.
  11. ^T'so, Theodore Y. (January 23, 1999). 'Re: Serial communication with the 16650'. The Mail Archive. Retrieved June 2, 2013.
  12. ^bill.herrin.us - Hayes ESP 8-port Enhanced Serial Port Manual, 2004-03-02

Further reading[edit]

Serial Communication Interface In Microprocessor

  • Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; ISBN978-1-931-44806-2.
  • Serial Port Complete: Programming and Circuits for RS-232 and RS-485 Links and Networks; 1st Edition; Jan Axelson; Lakeview Research; 306 pages; 1998; ISBN978-0-965-08192-4.
  • Serial port and Microcontrollers: Principles, Circuits, and Source Codes; 1st Edition; Grzegorz Niemirowski; CreateSpace; 414 pages; 2013; ISBN978-1-481-90897-9.
  • Serial Programming (Wikibook).

External links[edit]

  • FreeBSD Serial and UART Tutorial, includes standard signal definitions, history of UART ICs, and pinout for commonly used DB25 connector.
  • UART Tutorial for Robotics, contains many practical examples.

Serial Communication Interface In Microprocessor

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