The USART of the AVR max. Embedded. The USART of the AVRHey folks Lets continue with our tutorials on Serial Communication. Until now, we have seen a general introduction to serial communication, followed by a tutorial on the concepts of RS 2. So here we are with the AVR communication protocols series, starting with the most basic ones, UART and USARTWe will move on to SPI and TWI I2. C later. Some images used in this tutorial are taken directly from and are a courtesy of the AVR datasheets for ATMega. ATMega. 163. 2 microcontrollers. Contents. UART and USARTThe UART and USART have already been discussed here. Anyways, lets have a quick recap. UART stands for Universal Asynchronous ReceiverTransmitter. From the name itself, it is clear that it is asynchronous i. USART stands for Universal Synchronous Asynchronous ReceiverTransmitter. Atmega8 Uart Program' title='Atmega8 Uart Program' />This is of the synchronous type, i. If you refer to the USART section in the datasheet of any AVR microcontroller, you will find several features listed there. Some of the main features of the AVR USART are Full Duplex Operation Independent Serial Receive and Transmit RegistersAsynchronous or Synchronous Operation. Master or Slave Clocked Synchronous Operation. High Resolution Baud Rate Generator. Supports Serial Frames with 5, 6, 7, 8, or 9 Data bits and 1 or 2 Stop Bits. Atmega8 Uart Program' title='Atmega8 Uart Program' />USART Layout How to set it up Before we continue, please note that the AVR USART is fully compatible with the AVR UART in terms of register bit locations, baud rate generation, transmitterreceiver operations and buffer functionality. Export Reportviewer To Pdf Landscape. So let us now have a quick look at how to set up USART in general. We will discuss in detail later. The first step is to set the baud rate in both, the master and the slave. The baud rate has to be the same for both master and slave. Set the number of data bits, which needs to be sent. Get the buffer ready In case of transmission from AVR to some other device, load it up with the data to be sent, whereas in case of reception, save the previous data so that the new received data can be overwritten onto it. Then enable the transmitterreceiver according to the desired usage. One thing to be noted is that in UART, there is no master or slave since master is defined by the Micro. Controller, which is responsible for clock pulse generation. Hence Master and Slave terms occur only in the case of USART. Master C is the one which is responsible for Clock pulse generation on the Bus. USART Pin Configuration. Now lets have a look at the hardware pins related to USART. The USART of the AVR occupies three hardware pins pins Rx. D USART Receiver Pin ATMega. Pin 2 ATMega. 163. Pin 1. 4Tx. D USART Transmit Pin ATMega. Pin 3 ATMega. 163. Pin 1. 5XCK USART Clock Pin ATMega. Pin 6 ATMega. 163. Pin 1Modes of Operation. Atmega8 Uart Program' title='Atmega8 Uart Program' />Firstly i try my M16 usart communication to my pc. M16 on the hyper terminal. Arduino. Official Atmel AVR development tools and evaluation kits contain a number of starter kits and debugging tools with support for most AVR devices. Free AVR ATmega Xmega Bootloader with Intel hex Support and Automatic Baudrate Adjustment Autobaud chip45boot2. The USART of the AVR can be operated in three modes, namely Asynchronous Normal Mode. Asynchronous Double Speed Mode. Synchronous Mode. Asynchronous Normal Mode. In this mode of communication, the data is transmittedreceived asynchronously, i. XCK pin. The data is transferred at the BAUD rate we set in the UBBR register. This is similar to the UART operation. Asynchronous Double Speed Mode. This is higher speed mode for asynchronous communication. In this mode also we set the baud rates and other initializations similar to Normal Mode. The difference is that data is transferred at double the baud we set in the UBBR Register. Setting the U2. X bit in UCSRA register can double the transfer rate. Setting this bit has effect only for the asynchronous operation. Set this bit to zero when using synchronous operation. Setting this bit will reduce the divisor of the baud rate divider from 1. Note however that the Receiver will in this case only use half the number of samples reduced from 1. For the Transmitter, there are no downsides. Synchronous Mode. This is the USART operation of AVR. Norton System Works Activation'>Norton System Works Activation. When Synchronous Mode is used UMSEL 1 in UCSRC register, the XCK pin will be used as either clock input Slave or clock output Master. Baud Rate Generation. The baud rate of UARTUSART is set using the 1. Globe Capital Market Ltd Odin here. UBRR register. The register is as follows UBRR Register Click to EnlargeSince AVR is an 8 bit microcontroller, every register should have a size of 8 bits. Hence, in this case, the 1. UBRR register is comprised of two 8 bit registers UBRRH high and UBRRL low. This is similar to the 1. ADC register ADCH and ADCL, remember. Since there can be only specific baud rate values, there can be specific values for UBRR, which when converted to binary will not exceed 1. Hence there are only 1. UBRR1. 1 0. We will learn how to calculate the value of UBRR in a short while in this post. The USART Baud Rate Register UBRR and the down counter connected to it functions as a programmable prescaler or baud rate generator. The down counter, running at system clock FOSC, is loaded with the UBRR value each time the counter has counted down to zero or when the UBRRL Register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate generator clock output FOSCUBRR1. The transmitter divides the baud rate generator clock output by 2, 8, or 1. The baud rate generator output is used directly by the receivers clock and data recovery units. Below are the equations for calculating baud rate and UBRR value Baud Rate Calculation Click to EnlargeBAUD Baud Rate in BitsSecond bps Always remember, Bps BytesSecond, whereas bps BitsSecondFOSC  System Clock Frequency 1. MHz or as per use in case of external oscillatorUBRR Contents of UBRRL and UBRRH registers. Frame Formats. A frame refers to the entire data packet which is being sentreceived during a communication. Depending upon the communication protocol, the formats of the frame might vary. For example, TCPIP has a particular frame format, whereas UDP has another frame format. Similarly in our case, RS2. If you have gone through the loopback test discussed in the previous tutorial, you will notice that we have chosen options such as 8 bits data, 1 stop bit, no parity, etc. This is nothing but the selection of a frame format A typical frame for USARTRS2. However a vast number of configurations are available 3. Frame Format Click to EnlargeOrder of Bits. Start bit Always lowData bits LSB to MSB 5 9 bitsParity bit optional Can be odd or evenStop bit 1 or 2 Always highA frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after the data bits, before the stop bits. When a complete frame is transmitted, a new frame can directly follow it, or the communication line can be set to an idle high state. Here is the frame format as mentioned in the AVR datasheet Frame Format Click to EnlargeNote The previous image not the above one, the one before that of Frame Format has a flaw in it If you can find it, feel free to comment below Let me see how many of you can spot itAnd Im not kidding, there is a mistake Setting the Number of DATA Bits. The data size used by the USART is set by the UCSZ2 0, bits in UCSRC Register. The Receiver and Transmitter use the same setting. Note Changing the settings of any of these bits on the fly will corrupt all ongoing communication for both the Receiver and Transmitter. Make sure that you configure the same settings for both transmitter and receiver.