mirror of
https://github.com/rfivet/stm32bringup.git
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161 lines
4.9 KiB
HTML
161 lines
4.9 KiB
HTML
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>2.3 Hello There!</title>
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<link type="text/css" rel="stylesheet" href="style.css">
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</head>
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<body>
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<h1>2.3 Hello There!</h1>
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Looking for a <i>“you had me at hello”</i> moment? Let’s see how serial
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transmission works for you.
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<h2>Implementation</h2>
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I make a copy of <b>board.c</b> into <b>usart1tx.c</b> to add support for the
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USART1 peripheral.
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<p>
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In order to make a first transmission, the peripherals have to be
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initialized. As the TX/RX of USART1 are mapped on pin PA9 and PA10, I
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need to configure GPIOA first.
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<ul>
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<li> GPIOA needs to be enabled via RCC AHB Enable Register as GPIOs are on
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AHB bus.
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<li> PA9 and PA10 set to alternate mode.
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<li> Alternate function USART1 selected for PA9 and PA10.
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</ul>
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Then USART1 can be configured:
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<ul>
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<li> USART1 enabled via RCC APB2 Enable Register as USARTs are on the APB
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bus.
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<li> Baud rate set to 9600 bauds.
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<li> USART itself and transmission needs to be enabled via the Control
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Register (CR1).
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</ul>
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By default the transmission format is 8N1: 8 bit data, no parity and 1
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stop bit.
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<pre>
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/* USART1 9600 8N1 */
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RCC_AHBENR |= RCC_AHBENR_IOP( A) ; /* Enable GPIOA periph */
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GPIOA[ MODER] |= 0x0A << (9 * 2) ; /* PA9-10 ALT 10, over default 00 */
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GPIOA[ AFRH] |= 0x110 ; /* PA9-10 AF1 0001, over default 0000 */
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RCC_APB2ENR |= RCC_APB2ENR_USART1EN ;
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USART1[ BRR] = 8000000 / 9600 ; /* PCLK [8MHz] */
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USART1[ CR1] |= USART_CR1_UE | USART_CR1_TE ; /* Enable USART & Tx */
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</pre>
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Sending data is done by writing in the Transmission Data Register (<b>TDR</b>).
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To check if it is ready for transmission you must check the state of the
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TX Empty (<b>TXE</b>) bit in the Interrupt & Status Register (<b>ISR</b>).
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<p>
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I write a basic <code>kputc()</code> function that does busy waiting if the
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<b>TDR</b> is not empty and insures that LF are mapped to CR LF. The ‘k’ in
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kputc refer to ‘kernel’, as kputc is a low level function that will be used
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mostly for debugging. With the busy wait and the recursive code this
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implementation is definitively not optimal, but it’s functional and
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that’s what matter most at this stage.
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<pre>
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void kputc( unsigned char c) {
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static unsigned char lastc ;
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if( c == '\n' && lastc != '\r')
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kputc( '\r') ;
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/* Active wait while transmit register is full */
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while( (USART1[ ISR] & USART_ISR_TXE) == 0) ;
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USART1[ TDR] = c ;
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lastc = c ;
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}
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</pre>
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The high level C function I need for this simple test is <code>puts()</code>.
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I make my own implementation but I keep the same declaration as the standard
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header that come with the C compiler.
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<pre>
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int puts( const char *s) {
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while( *s)
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kputc( *s++) ;
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kputc( '\n') ;
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return 0 ;
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}
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</pre>
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Finally I use a standard C implementation for <b>hello.c</b>.
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<pre>
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/* hello.c -- hello there */
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#include <stdio.h>
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#include <stdlib.h>
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int main( void) {
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puts( "hello, world") ;
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return EXIT_SUCCESS ;
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}
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</pre>
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<h2>Build</h2>
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To build I update the software composition in <b>Makefile</b> by adding a new
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<code>SRCS</code> line.
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<pre>SRCS = startup.c usart1tx.c hello.c</pre>
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Calling make, I can see that there is now some variable in <b>BSS</b> section
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of the RAM. It is <code>lastchar</code> local to <code>kputc()</code>. Because
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of word alignment <code>BSS</code> occupies 4 bytes.
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<pre>
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$ make
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f030f4.elf
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text data bss dec hex filename
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413 0 4 417 1a1 f030f4.elf
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f030f4.hex
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f030f4.bin
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</pre>
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<h2>Testing</h2>
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After flashing the board with the new executable, I place back the
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<b>BOOT0</b> jumper and press the reset button, the board user LED blinks
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as usual but I can see the RX LED on the USB to UART adapter flash
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briefly when I release the reset button.
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<p>
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On Windows PC, if I use PuTTY or Arduino IDE to open <b>COM4</b> at 9600
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baud, every time I press and release the reset button I can see ‘hello,
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world’ displayed on a new line in the terminal window.
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<p>
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On Linux, when I plug in the USB to UART adapter, it enumerates as
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<b>/dev/ttyUSB0</b>, so it is compatible with the USB driver for serial
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ports. If I try to open it with Arduino IDE, I get an error message as I
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need to belong to <b>dialout</b> group to open that TTY for reading and
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writing.
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<pre>sudo usermod -a -G dialout $USER</pre>
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Once added to <b>dialout</b>, I can open <b>/dev/ttyUSB0</b> at 9600 baud in
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Arduino IDE, each time I press and release the board RESET button, I can see
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‘hello, world’ displayed on a new line in the Serial Monitor window.
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<h2>Checkpoint</h2>
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I have now a functional serial transmission channel through <b>USART1</b>. I
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have only a first implementation for <code>puts()</code>, but I will add
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support for other stdio functions when needed.
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<p>
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<a href="24_stm32flash.html">Next</a>, I will switch to an open source tool
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for flashing over serial connection that works on both Windows and Linux.
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<hr>© 2020-2024 Renaud Fivet
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</body>
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</html>
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