{"id":683,"date":"2012-09-04T21:38:00","date_gmt":"2012-09-04T19:38:00","guid":{"rendered":"https:\/\/www.fussylogic.co.uk\/blog\/?p=683"},"modified":"2012-10-08T17:52:52","modified_gmt":"2012-10-08T16:52:52","slug":"avr-uart-transmit-and-stdio","status":"publish","type":"post","link":"https:\/\/www.fussylogic.co.uk\/blog\/?p=683","title":{"rendered":"AVR UART Transmit and stdio"},"content":{"rendered":"

The setup for a UART on AVR is such common code that I wonder if possibly it\u00e2\u20ac\u2122s just assumed to be understood and explained. This is a quick article to fill in the gaps. First let\u00e2\u20ac\u2122s talk initialisation. This is often the most important part of using any hardware peripheral on an embedded processor. In the case of the AVR UARTs, avr-libc<\/code> includes the very convenient setbaud.h<\/code> to help us (I\u00e2\u20ac\u2122m using \u00e2\u20ac\u02dcatmega8\u00e2\u20ac\u2122 register names here, but the principle is the same whatever member of the family you use)1<\/a><\/sup>:<\/p>\n

static<\/span> void<\/span> initUART()\n{\n    \/\/ UCSRA    RXC   TXC   UDRE  FE   DOR  PE    U2X   MPCM<\/span>\n    \/\/ UCSRB    RXCIE TXCIE UDRIE RXEN TXEN UCSZ2 RXB8  TXB8<\/span>\n    \/\/ UCSRC    URSEL UMSEL UPM1  UPM0 USBS UCSZ1 UCSZ0 UCPOL<\/span>\n    \/\/ UDR      ------------ UART Data Register -------------<\/span>\n    \/\/ UBRRH    URSEL -     -     -    ----- UBRR[11:8] -----<\/span>\n    \/\/ UBRRL    ---------------- UBRR[7:0] ------------------<\/span>\n    \/\/<\/span>\n\n    \/\/ Assume F_CPU has been set to CPU frequency<\/span>\n\n    \/\/ Use setbaud.h to calculate the registers for us<\/span>\n#   define BAUD 9600<\/span>\n#   include <util\/setbaud.h><\/span>\n    \/\/ Set baud rate before enabling the UART<\/span>\n    UBRRH = UBRRH_VALUE;\n    UBRRL = UBRRL_VALUE;\n#   if USE_2X<\/span>\n    UCSRA |= _BV(U2X);\n#   else<\/span>\n    UCSRA &= ~_BV(U2X);\n#   endif<\/span>\n\n    \/\/ Enable receiver and transmitter<\/span>\n    UCSRB |= _BV(TXEN) | _BV(RXEN);\n    \/\/ Set frame format: 8 data; 0 stop bits<\/span>\n    \/\/ Note: UCSRC and UBRRH share the same I\/O address.  The URSEL bit<\/span>\n    \/\/ is used to choose which of the registers the lower 7 bits are<\/span>\n    \/\/ written to.  For URSRC, include URSEL in the set, otherwise don't<\/span>\n    UCSRC |= _BV(URSEL) | (0<\/span> << USBS) | (3<\/span> << UCSZ0);\n}<\/code><\/pre>\n
Outputting a byte is then very simple. We need only trigger the UART peripheral.<\/p>\n
UDR = '\\n'<\/span>;<\/code><\/pre>\n
We should be a little careful though. The UDR<\/code> register might be in use sending an earlier byte. We should check for that before overwriting it.<\/p>\n
static<\/span> void<\/span> uart_putchar( char<\/span> c )\n{\n    \/\/ Wait for data register empty flag<\/span>\n    while<\/span>( !( UCSRA & _BV(UDRE) ) )\n        ;\n    \/\/ Begin next transmit<\/span>\n    UDR = c;\n}<\/code><\/pre>\n
Now you want to be able to use printf()<\/code>. What do you do?<\/p>\n
Conveniently, avr-libc<\/code> is pretty excellent. It includes a stdio<\/code> implementation that is retargettable. Here\u00e2\u20ac\u2122s the magic line\u00e2\u20ac\u00a6<\/p>\n
static<\/span> FILE UARTstdout = FDEV_SETUP_STREAM(\n    uart_putchar,\n    NULL,\n    _FDEV_SETUP_WRITE );<\/code><\/pre>\n
Then to use this structure in printf()<\/code> and all the other stdio<\/code> functions, we simply add the following to our initialisation:<\/p>\n
stdout = &UARTstdout;<\/code><\/pre>\n
The _FDEV_SETUP_WRITE<\/code> flag makes UARTstdout<\/code> a write-only handle, which is what we\u00e2\u20ac\u2122d expect for stdout<\/code>. We need to slightly modify our uart_putchar()<\/code> to match what FDEV_SETUP_STREAM()<\/code> expects:<\/p>\n
static<\/span> int<\/span> uart_putchar( char<\/span> c, FILE *stream )\n{\n    \/\/ Wait for data register empty flag<\/span>\n    while<\/span>( !( UCSRA & _BV(UDRE) ) )\n        ;\n    \/\/ Begin next transmit<\/span>\n    UDR = c;\n}<\/code><\/pre>\n
Now we\u00e2\u20ac\u2122ve got a FILE<\/code> variable, we need to tell stdio<\/code> to use it.<\/p>\n
int<\/span> main(void<\/span>)\n{\n    initUART();\n    stdout = &UARTstdout;\n\n    \/\/ ... other initialisation<\/span>\n\n    printf( "Hello, World!<\/span>\\r\\n<\/span>"<\/span> );\n\n    while<\/span>(1<\/span>)\n        ;\n    \n    return<\/span> 0<\/span>;\n}<\/code><\/pre>\n
It\u00e2\u20ac\u2122s pretty wasteful to force the inclusion of printf()<\/code> just to write an unformatted string of course; but I presume you\u00e2\u20ac\u2122ll have better uses for your printf()<\/code>.<\/p>\n
We\u00e2\u20ac\u2122ve not discussed interrupts at all here; and for good reason: interrupts often complicate matters considerably, but once under your control they make your program much more reactive and responsive. They\u00e2\u20ac\u2122re also vital for power control, since you can put your processor to sleep and only wake up when an event occurs. I\u00e2\u20ac\u2122ll cover interrupt handling in embedded systems in a future article.<\/p>\n
\n
\n
    \n
  1. \n
    The _BV()<\/code> macro is another handy one provided by avr-libc<\/code>, it\u00e2\u20ac\u2122s simply<\/p>\n
    #define _BV(bit) (1 << (bit))<\/span><\/code><\/pre>\n
    \u00e2\u2020\u00a9<\/a><\/li>\n<\/ol>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
    The setup for a UART on AVR is such common code that I wonder if possibly it\u00e2\u20ac\u2122s just assumed to be understood and explained. This is a quick article to fill in the gaps. First let\u00e2\u20ac\u2122s talk initialisation. This is often the most important part of using any hardware peripheral on an embedded processor. In\u2026 Read More »<\/a><\/span><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[39,37,38,6],"_links":{"self":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/683"}],"collection":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=683"}],"version-history":[{"count":8,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/683\/revisions"}],"predecessor-version":[{"id":809,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=\/wp\/v2\/posts\/683\/revisions\/809"}],"wp:attachment":[{"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=683"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=683"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.fussylogic.co.uk\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=683"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}