Asynchronous serial transmission

The diagram below shows this concept: Here earlier state of line was 1 when a character has to be send a 0 is send and character bit are transferred. Difference between serial and parallel transfer — Serial Parallel Require single line to send data Require multiple line Less error and simple model Error prone and complex working Economical Expensive Slower data transfer Faster data transfer Used for long distance used for short distance Example:Computer to Computer Example:Computer to Printer.

Next program to perform AND operation in nibbles of 8 bit number. Recommended Articles. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment?

Please use ide. Load Comments. What's New. Synchronous transmission modes are used when large amounts of data must be transferred very quickly from one location to the other. The speed of the synchronous connection is attained by transferring data in large blocks instead of individual characters. Synchronous transmission synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component.

A continual stream of data is then sent between the two nodes. The data blocks are grouped and spaced in regular intervals and are preceded by special characters called syn or synchronous idle characters. See the following illustration. After the syn characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin.

An analogy of synchronous transmission would be the transmission of a large text document. Before the document is transferred across the synchronous line, it is first broken into blocks of sentences or paragraphs. The blocks are then sent over the communication link to the remote site. The timing needed for synchronous connections is obtained from the devices located on the communication link. All devices on the synchronous link must be set to the same clocking.

Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the bytes is correctly interpreted and received. In contrast, asynchronous transmission works in spurts and must insert a start bit before each data character and a stop bit at its termination to inform the receiver where it begins and ends. The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character.

When you use the Arduino UNO, then the usual way way to communicate asynchronously is to use the Serial object, which is an instance of the class HardwareSerial. The UART does most of the work and only when a byte has been received or a byte can be sent, an interrupt is raised. The standard solution is the SoftwareSerial library.

There are three main problems, though. First, it is necessary that the falling edge of the start bit is detected as accurately as possible. This is done using the pin-change interrupt on the receiving line. So other interrupts are counter-productive. They might lead to misinterpreting the received bit stream by sampling the bits to late. For example, if data is received with a bit rate of , then the bit time is If the millis interrupt is raised just before the falling edge of a start bit, then the detection of the start bit is delayed for 6.

With a little bit of other variations, one easily misinterprets the data stream. Second, sending and receiving data needs accurate timing and for this reason interrupts are disabled during that time and no other things can go on. Since the receiving routine waits into the stop bit, only one half of a bit time is available for processing a received byte.

If too many bytes are received in short order, the receive buffer of 64 bytes might overflow. Third, even slow bit rates can lead to problems. This implies that the millis interrupts, which is raised every millisecond, cannot be served on time. There are at least two alternatives to SoftwareSerial. It uses only a minimal amount of code, but is extremely accurate in timing.

Similar to SoftwareSerial, almost the entire frame time is blocked for interrupts. Receiving data can either be done by polling, i. In the later case, there is only a one byte receive buffer, which might easily lead to missing a data byte. Our final candidate is AltSoftSerial , which uses a quite different methodology than the bit-banging technique of the last two libraries.

It uses the input capture feature of Timer 1 on the ATmegaP for capturing the time when a signal edge occurs on the input line. And this is done in an interrupt driven way, which means that the interrupt latency imposed by this method is significantly shorter than 9. This is still much better than what the other methods impose, but may be prohibitive for higher bit rates.

In addition, it is claimed that the library can tolerate interrupt latency of almost one bit time. Generation of bytes to be transmitted is done using the output compare feature of the same timer in an interrupt driven way as well. So, compared wit the two bit-banging methods, this library does require significantly less MCU cycles. There is a price to pay for that, of course: the input and output pin are fixed and one cannot use the PWM functionality of the pins associated with Timer 1.

So which one is the best alternative? SoftwareSerial is the most flexible one. You can use any pins as input and output. And one can even setup more than one SoftwareSerial instance, but only one can be active at any time. It seems like a good fit for the smaller ATtinys. Finally, AltSoftSerial relying on timers instead of delay loops is very accurate and consumes the least amount of compute cycles.

In the next section, we have a look at what communication speeds the libraries can reliably deal with and how much deviation in the bit rate they tolerate. How accurate is the timing when sending and how tolerant are the libraries when receiving data?

In order to measure the accuracy of the bit rate when sending, I employed my Saleae logic analyzer to measure the generated bit rates. There are a few interesting observations to make. First, even for the hardware UART, it is not always possible to generate a bit rate that is close to the nominal one. For and bps, the real bit rate is 2. Even worse, for bps, it is 3. The reason for these deviations has been already mentioned: The AVR baud rate generator cannot generate all rates.

The next interesting observation is that SoftwareSerial should probably not be used with bit rates above bps.