Using the USART of AVR Microcontrollers.
Welcome to the third part of my RS232 serial communication tutorial. Till now we saw the basics of RS232 communication and made our level converter. Now its time to understand the USART of AVR microcontroller and write the code to initialize the USART and use it to send and receive data.
Like many microcontrollers AVR also has a dedicated hardware for serial communication this part is called the USART - Universal Synchronous Asynchronous Receiver Transmitter. This special hardware make your life as programmer easier. You just have to supply the data you need to transmit and it will do the rest. As you saw serial communication occurs at standard speeds of 9600,19200 bps etc and this speeds are slow compared to the AVR CPUs speed. The advantage of hardware USART is that you just need to write the data to one of the registers of USART and your done, you are free to do other things while USART is transmitting the byte.
Also the USART automatically senses the start of transmission of RX line and then inputs the whole byte and when it has the byte it informs you(CPU) to read that data from one of its registers.
The USART of AVR is very versatile and can be setup for various different mode as required by your application. In this tutorial I will show you how to configure the USART in a most common configuration and simply send and receive data. Later on I will give you my library of USART that can further ease you work. It will be little complicated (but more useful) as it will have a FIFO buffer and will use interrupt to buffer incoming data so that you are free to anything in your main() code and read the data only when you need. All data is stored into a nice FIFO(first in first out queue) in the RAM by the ISR.
USART of AVR Microcontrollers.
The USART of the AVR is connected to the CPU by the following six registers.
- UDR - USART Data Register : Actually this is not one but two register but when you read it you will get the data stored in receive buffer and when you write data to it goes into the transmitters buffer. This important to remember it.
- UCSRA - USART Control and status Register A : As the name suggests it is used to configure the USART and it also stores some status about the USART. There are two more of this kind the UCSRB and UCSRC.
- UBRRH and UBRRH : This is the USART Baud rate register, it is 16BIT wide so UBRRH is the High Byte and UBRRL is Low byte. But as we are using C language it is directly available as UBRR and compiler manages the 16BIT access.
So the connection of AVR and its internal USART can be visualized as follows.
 |
Fig- AVR USART registers. |
Registers Explained
In order to write programs that uses the USART you need to understand what each register's importance. The scheme behind using the AVR USART is same as with any other internal peripheral (say ADC). So if you are new to this topic please see this tutorial, it shows you the basic idea of using peripherals.
I am not going to repeat what is already there in the datasheets, I will just tell about what is required for a quick startup. The datasheets of AVR provides you with all the details of every bit of every register so please refer to it for detailed info. Note bit names with RED background are of our interest here.
UDR: Explained above.
UCSRA: USART Control and Status Register A
*****************************************
Bit No |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
Name |
RXC |
TXC |
UDRE |
FE |
DOR |
PE |
U2X |
MPCM |
Initial Val |
0 |
0 |
1 |
0 |
0 |
0 |
0 |
0 |
RXC this bit is set when the USART has completed receiving a byte from the host (may be your PC) and the program should read it from UDR
TXC This bit is set (1) when the USART has completed transmitting a byte to the host and your program can write new data to USART via UDR
UCSRB: USART Control and Status Register B
*****************************************
Bit No |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
Name |
RXCIE |
TXCIE |
UDRIE |
RXEN |
TXEN |
UCSZ2 |
RXB8 |
TXB8 |
Initial Val |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
RXCIE: Receive Complete Interrupt Enable - When this bit is written one the the associated interrupt is enabled.
TXCIE: Transmit Complete Interrupt Enable - When this bit is written one the the associated interrupt is enabled.
RXEN: Receiver Enable - When you write this bit to 1 the USART receiver is enabled. The normal port functionality of RX pin will be overridden. So you see that the associated I/O pin now switch to its secondary function,i.e. RX for USART.
TXEN: Transmitter Enable - As the name says!
UCSZ2: USART Character Size - Discussed later.
For our first example we won't be using interrupts so we set UCSRB as follows
UCSRB=(1<<RXEN)|(1<<TXEN);
UCSRC: USART Control And Status Register C
*****************************************
Bit No |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
Name |
URSEL |
UMSEL |
UPM1 |
UPM0 |
USBS |
UCSZ1 |
UCSZ0 |
UCPOL |
Initial Val |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
IMPORTANT : The UCSRC and the UBRRH (discussed below) register shares same address so to determine which register user want to write is decided with the 7th(last) bit of data if its 1 then the data is written to UCSRC else it goes to UBRRH. This seventh bit is called the
URSEL: USART register select.
UMSEL: USART Mode Select - This bit selects between asynchronous and synchronous mode. As asynchronous mode is more popular with USART we will be using that.
UMSEL |
Mode |
0 |
Asynchronous |
1 |
Synchronous |
USBS: USART Stop Bit Select - This bit selects the number of stop bits in the data transfer.
USBS |
Stop Bit(s) |
0 |
1 BIT |
1 |
2 BIT |
UCSZ: USART Character size - These three bits (one in the UCSRB) selects the number of bits of data that is transmited in each frame. Normally the unit of data in MCU is 8BIT (C type "char") and this is most widely used so we will go for this. Otherwise you can select 5,6,7,8 or 9 bit frames!
UCSZ2 |
UCSZ1 |
UCSZ0 |
Character Size |
0 |
0 |
0 |
5Bit |
0 |
0 |
1 |
6Bit |
0 |
1 |
0 |
7Bit |
0 |
1 |
1 |
8Bit |
1 |
0 |
0 |
Reserved |
1 |
0 |
1 |
Reserved |
1 |
1 |
0 |
Reserved |
1 |
1 |
1 |
9Bit |
So we set UCSRC as follows
UCSRC=(1<<URSEL)|(3<<UCSZ0);
UBRR: USART Baud Rate Register:
*********************************
This is the USART Baud rate register, it is 16BIT wide so UBRRH is the High Byte and UBRRL is Low byte. But as we are using C language it is directly available as UBRR and compiler manages the 16BIT access. This register is used by the USART to generate the data transmission at specified speed (say 9600Bps). To know about baud rate see the previous tutorial. UBRR value is calculated according to following formula.
Where fosc is your CPU frequency say 16MHz
Baud Rate is the required communication speed say 19200 bps (see previous tutorial for more info).
Example:
For above configuration our UBRR value comes to be 51.083 so we have to set
UBRR=51;
in our initialization section. Note UBRR can hold only integer value. So it
is better to use the baud rates that give UBRR value that are purely integer
or very close to it. So if your UBRR value comes to be 7.68 and you decided
to use UBRR=8 then it has high error percentage, and communication is
unreliable!
Initialization of USART
Before using the USART it must be initialized properly according to need. Having the knowledge of RS232 communication and Internal USART of AVR you can do that easily. We will create a function that will initialize the USART for us.
#include <avr/io.h>
#include <inttypes.h>
void USARTInit(uint16_t ubrr_value)
{
//Set Baud rate
UBRR= ubrr_value;
/*Set Frame Format
>> Asynchronous mode
>> No Parity
>> 1 StopBit
>> char size 8
*/
UCSRC=(1<<URSEL)|(3<<UCSZ0);
//Enable The receiver and transmitter
UCSRB=(1<<RXEN)|(1<<TXEN);
}
Now we have a function that initializes the USART with a given UBRR value.
That's it for now friends. In next tutorial we will learn how to send and receive data over RS232 channel. Don't forget to post your opinion or doubts or any suggestion I would be very happy to see them. So don't wait post a comment now !
Finally, I’m starting to ‘get it’
December 14th, 2008 at 10:29 amnext will be I2C?? This will be greate…
December 15th, 2008 at 3:57 pmHi AM,
Ya I will surly come up with a I2C tutorials but before that I will complete USART tutorial by providing fuctions for reading and writing data.Then I will present my library for USART which is slightly complex (internally) but is more relibale and usefull. After that i will go for I2C
December 15th, 2008 at 5:04 pmits very very interesting…
December 25th, 2008 at 1:11 am[...] USART of AVR micro. After understanding the USART of AVR we have also written a easy to use function to initialize the USART. That was the first step to use RS232. Now we will go see how we can [...]
December 29th, 2008 at 11:52 amhi
I had been doing a project on polygraph with the help of program given by cornell university.but i dont knw which compiler they have used. v tried 2 use code vision but when v r xecuting prgm in proteus professional even if MATLAB is sending charector ’s’,its not working.i would like 2 know which compiler they would have used.The prog is given below
CODE REMOVED TO SHORTEN THE COMMENT.
January 3rd, 2009 at 5:51 amDear friend, so nice tutorial!!!
regards:
Enrique.
January 8th, 2009 at 2:04 amHi, I have ATmega8 and standart system clock is 1MHz. What I must change in code? and what baud rate need i set? from datasheet: Fosc=1MHz, Baud rate= 9600, UBRR= 6. Its correct? pls help.
March 4th, 2009 at 10:59 pmFormula for calculation of UBRR is clearly given and explained above.
Also not EVERY baud rate possible. So Baud rate of 9600 on 1MHz CPU gives UBRR=5.51 and since we can use only integer for UBRR, using 6 as UBRR has huge error.
So go for other baud rates.
I recommend you to go with BAUD = 19200 which gives UBRR=2.25=2
March 5th, 2009 at 8:37 amOK and when I use external crystal? 7,3728 MHz UBRR=47 no errors, but i dont know what i must change in code and in avr studio??
March 5th, 2009 at 9:15 pmThanks for the handy info. It really helped clear some things up about programming for RS-232.
March 6th, 2009 at 3:46 amHello Newman,
The two baud rate you calculated are correct. As you can see the above function we created takes UBRR value as argument, so simply pass the calculated values. Thats it !
Are you comfortable with c programming ? I mean do u understand what I said above.
See next tutorial’s code.
http://extremeelectronics.co.in/avr-tutorials/using-the-usart-of-avr-microcontrollers-reading-and-writing-data/
In the end I have given a complete demo program
Have a look at the main() function where we init USART by calling
USARTInit(51) //UBRR=51
Here 51 is UBRR value.
Simply change only this value to one you calculated and complile.
If you have any more doubts pls post in the forum (see side bar navigation), I will try by best to explain.
Have a nice day !
March 6th, 2009 at 8:54 amHi Avinash,
Your website is a recent discovery for me. I am very impressed with your tutorials. They are very clear and easily understood. Thank you!
I am working with an ATmega32 processor and I am unsure exactly how the UART, baud rate, internal, and external clocks all work together. Can you advise me on this?
Some colleagues of mine are struggling with the Input Capture function of the same mcu as above. Do you have any information they can read to make this function clearer?
Cheers,
Paul
March 15th, 2009 at 1:10 pmHello Paul,
Thanks you liked my works.:)
I can’t get
“I am unsure exactly how the UART, baud rate, internal, and external clocks all work together. Can you advise me on this?”
please tell in some details so that I can help you.
Have A Nice Day
March 15th, 2009 at 7:14 pmPaul when you use a internal rc oscilator 8MHz uart is no so reliable, when is good use external crystal with 7,3728MHz than is all correct. baud rate is speed sending data, i use 9600 baud rate…
March 15th, 2009 at 10:02 pmWhy is the internal RC oscillator not so reliable? For the external crystal what size of capacitors should I be using? I could calculate it but if you guys know already then maybe you could save me some time.
March 16th, 2009 at 1:35 pm2x 22pF, look at datasheet atmega32
March 16th, 2009 at 2:49 pmHello,
I thank newman for helping Paul.
Good work.
As newman has said use 22pF capacitors.
For complete circuit see
http://extremeelectronics.co.in/hardwares/home-made-avr-devboard/
Internal RC is only for not time critical application. Because the frequency change with temperature. And USART is a time critical application. Crystal Oscillators are used in such application because they are very stable over temperature changes.
March 16th, 2009 at 4:59 pmrs 232 and USART, are they a same method(protocol)?
sory if my question to sally..
April 10th, 2009 at 11:44 pmHello Apiz
RS232 is the “PROTOCOL” that means the standard “rule” for communication.
While USART is Universal Synchronous Asynchronous Receiver Transmmiter
It is a piece of hardware (commonly inside the MCU) that is used for RS232 Communication.
So now you know RS232 is a “PROTOCOL” while USART is a HARDWARE.
April 11th, 2009 at 7:47 amwow. now i understand how it actually works.
May 28th, 2009 at 11:47 amthanks
Hey very cool tutorial…is will this code also work with the UART on an ATMega168 MCU?
June 29th, 2009 at 8:58 pmthank you!it was intresting!
June 30th, 2009 at 12:59 amHello Dave,
It will work with ATmega168 but you have to replace register names. See datasheet for equivalent name. Like UDR becomes UDR0 and UCSRA becomes UCSR0A.
Thats all
June 30th, 2009 at 8:22 am[...] http://extremeelectronics.co.in/avr-tutorials/using-the-usart-of-avr-microcontrollers/ [...]
July 20th, 2009 at 6:55 amHello Avinash!
Great tutorial series. Its a noble work to share knowledge.
I really enjoy the simplicity of your tutorials.
Keep Going!!
August 11th, 2009 at 5:41 pmBest tutorials found online for newbies
thank you so much
now I dont have to guess the meaning of the code anymore
great job!
thanks again
August 29th, 2009 at 11:55 amHey Avinash,
Just have a question in regard to the RS232 protocol. Can it be used to transfer entire blocks of data into the Atmega chips memory?
for example, if we want to store the data line “12345″ in a special matrix in the memory, how should we go about it?
Thanks
September 3rd, 2009 at 7:15 am[...] grips with the black art of AVR USART comms. The guys at extremeelectonics have published a very comprehensive tutorial on the various registers related to the AVR USART peripherial, how to set them, [...]
September 17th, 2009 at 6:57 pmfor communication with microcontroller through hyper terminal, do v need to give any self & remote remote address
September 21st, 2009 at 9:55 ammitul tailor Says:
for communication with microcontroller through hyper terminal, do v need to give any self & remote remote address in the above given program
September 21st, 2009 at 9:56 ami dont know who are u Mr. Avinash,bt tell u have been amazing…so lucid explanation…just unbeleivable…I shal be gladly waitin to receive ur mail if u can kindly let me know about urself…
September 30th, 2009 at 12:51 amHello abhinash,i want to communicate 2 ATMEGA16 through RF.if i connect 433mhz reciever and transmitter in RXD and TXD respectively in each ATMEGA16.Then if i’ll transmitt data frm one microcontroller then then,My question is -data will be receive by another ATMEGA16 or the same ATMEGA16?because both ATMEGA16 has 433mhz receiver..plz help me.
October 4th, 2009 at 11:14 pmHello Avinash!
These are Great and easily understood tutorial.Thank you very very much!!!!
November 25th, 2009 at 4:04 pmIn future tutorial can you Plese add some webcam or similer Cam interface to AVR family,I am waiting eagerly,Thanks again
Hello Avinash
Thanks a lot for your excellent articles. This is best place to start with for any AVR or infact any microcontroller beginner.
The way you explain the concepts is uncanny.
December 2nd, 2009 at 9:16 amDear Sir
December 21st, 2009 at 9:45 pmI am vishal working on a project in which i have interface RF Module with ATMEGA 32L AVR Microcontroller for wireless communication.
I have already established the communication through wire using USART and adler32 protocol, But when i had connected rf module to the MC, I cannot communicate.
I had connected rf TX module to the TX PIN of AVR, and RF RX module to the RX pin of MC, Also i had made a common ground with powersupply, Still i cannot transmit the desired signal.
Should i connect some IC in between, or is there is another problem
Waiting for the solution
Regards
vishal
(vishalmeshram2007@rediffmailcom)
sir amazing tutorials i read from a lot of sites but nothing as clear as dis one……
December 30th, 2009 at 8:41 pmsir my objective is to create a usart communication between two microcontroller.shld the information transmitted be first converted to binary form before transmitting through tx line or is the conversion internal. similarly if i want to see the data received should i convert the received data from binary to normal form
hello avinash
January 12th, 2010 at 4:05 pmnice job avinash, this is the best avr beginers forum i found on internet.
i have a problem with with an usart implementation.
i tried implementing an usart code using internal RC oscillator at 4MHz using code vision.The hyperterminal displayed junk values(like “..[ “) instead of the chars “1? “2? or “3?,irrespective the value in UDR.
i even implemented the code given above, but the heyperterminal wasn’t accepting any input or displaying any output.
what could be the fault in this case?
is there a way i can rectify it?
there is also external crystal ocsilator on board but how to use it? as i am using codevision compiler it asks me oscilator frequncy
thanks
[...] Using the USART of AVR Microcontrollers. [...]
January 17th, 2010 at 11:48 amThanks for the information! I personally really like your article. This is a great website. I will make sure that I stop back again!.
February 17th, 2010 at 4:33 am