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USART with Standard I/O
This example takes advantage of the big Flash memory space and uses the standard library functions printf, and fgets to implement a simple interactive application.
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‎examples/stm32/f4/nucleo-f411re/usart-stdio/Makefile

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##
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## This file is part of the libopencm3 project.
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##
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## Copyright (C) 2015 Chuck McManis <[email protected]>
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##
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## This library is free software: you can redistribute it and/or modify
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## it under the terms of the GNU Lesser General Public License as published by
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## the Free Software Foundation, either version 3 of the License, or
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## (at your option) any later version.
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##
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## This library is distributed in the hope that it will be useful,
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## but WITHOUT ANY WARRANTY; without even the implied warranty of
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## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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## GNU Lesser General Public License for more details.
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##
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## You should have received a copy of the GNU Lesser General Public License
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## along with this library. If not, see <http://www.gnu.org/licenses/>.
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##
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BINARY = usart-stdio
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LDSCRIPT = ../nucleo-f411re.ld
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include ../../Makefile.include
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‎examples/stm32/f4/nucleo-f411re/usart-stdio/README.md

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# README
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This program demonstrates using the standard I/O and library
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functions built into newlib on the Nucleo F411RE eval board.
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Normally the functions would seem to be extravagant for an
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embedded application but the F4 series has a lot of flash
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(512K in the case of the STM32F411RE) and so you can use this
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to make it easier to work with.
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This example asks you to enter a number, 1 or greater, and
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it uses that number as the delay constant when blinking the LED
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on the Nucleo board. It blinks the LED 1000 times so if you
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choose a large number it will take a while to finish.
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100000 is reasonably fast, 500000 is quite slow.
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There is also a simple buffered input routine in the code so
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you can edit the number as you are entering it. The ^H or DEL
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keys will delete a character, ^U will erase the line, and
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^W will delete the last word (defined by space characters).

‎examples/stm32/f4/nucleo-f411re/usart-stdio/usart-stdio.c

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/*
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* This file is part of the libopencm3 project.
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*
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* Copyright (C) 2015 Chuck McManis <[email protected]>
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*
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* This library is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public License
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* along with this library. If not, see <http://www.gnu.org/licenses/>.
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*
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <ctype.h>
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#include <libopencm3/stm32/rcc.h>
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#include <libopencm3/stm32/gpio.h>
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#include <libopencm3/stm32/usart.h>
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/*
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* To implement the STDIO functions you need to create
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* the _read and _write functions and hook them to the
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* USART you are using. This example also has a buffered
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* read function for basic line editing.
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*/
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int _write(int fd, char *ptr, int len);
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int _read(int fd, char *ptr, int len);
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void get_buffered_line(void);
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/*
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* This is a pretty classic ring buffer for characters
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*/
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#define BUFLEN 127
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static uint16_t start_ndx;
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static uint16_t end_ndx;
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static char buf[BUFLEN+1];
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#define buf_len ((end_ndx - start_ndx) % BUFLEN)
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static inline int inc_ndx(int n) { return ((n + 1) % BUFLEN); }
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static inline int dec_ndx(int n) { return (((n + BUFLEN) - 1) % BUFLEN); }
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static void clock_setup(void)
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{
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/* Enable GPIOD clock for LED & USARTs. */
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rcc_periph_clock_enable(RCC_GPIOA);
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/* Enable clocks for USART2. */
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rcc_periph_clock_enable(RCC_USART2);
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}
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static void usart_setup(void)
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{
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/* Setup USART2 parameters. */
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usart_set_baudrate(USART2, 115200);
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usart_set_databits(USART2, 8);
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usart_set_stopbits(USART2, USART_STOPBITS_1);
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usart_set_mode(USART2, USART_MODE_TX_RX);
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usart_set_parity(USART2, USART_PARITY_NONE);
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usart_set_flow_control(USART2, USART_FLOWCONTROL_NONE);
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/* Finally enable the USART. */
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usart_enable(USART2);
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}
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static void gpio_setup(void)
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{
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/* Setup GPIO pin GPIO5 on GPIO port A for LED. */
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gpio_mode_setup(GPIOA, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, GPIO5);
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/* Setup GPIO pins for USART2 transmit and receive. */
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gpio_mode_setup(GPIOA, GPIO_MODE_AF, GPIO_PUPD_NONE, GPIO2 | GPIO3);
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/* Setup USART2 TX pin as alternate function. */
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gpio_set_af(GPIOA, GPIO_AF7, GPIO2 | GPIO3);
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}
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int main(void)
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{
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int i, j;
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clock_setup();
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gpio_setup();
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usart_setup();
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printf("\nStandard I/O Example.\n");
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/* Blink the LED (PD12) on the board with every transmitted byte. */
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while (1) {
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int delay = 0;
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char local_buf[32];
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gpio_toggle(GPIOA, GPIO5); /* LED on/off */
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do {
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printf("Enter the delay constant for blink : ");
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fflush(stdout);
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fgets(local_buf, 32, stdin);
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delay = atoi(local_buf);
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if (delay <= 0) {
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printf("Error: expected a delay > 0\n");
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}
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} while (delay <= 0);
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printf("Blinking with a delay of %d\n", delay);
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for (j = 0; j < 1000; j++) {
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gpio_toggle(GPIOA, GPIO5);
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for (i = 0; i < delay; i++) { /* Wait a bit. */
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__asm__("NOP");
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}
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}
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}
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return 0;
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}
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/* back up the cursor one space */
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static inline void back_up(void)
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{
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end_ndx = dec_ndx(end_ndx);
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usart_send_blocking(USART2, '\010');
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usart_send_blocking(USART2, ' ');
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usart_send_blocking(USART2, '\010');
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}
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/*
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* A buffered line editing function.
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*/
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void
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get_buffered_line(void) {
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char c;
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if (start_ndx != end_ndx) {
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return;
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}
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while (1) {
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c = usart_recv_blocking(USART2);
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if (c == '\r') {
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buf[end_ndx] = '\n';
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end_ndx = inc_ndx(end_ndx);
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buf[end_ndx] = '\0';
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usart_send_blocking(USART2, '\r');
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usart_send_blocking(USART2, '\n');
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return;
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}
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/* ^H or DEL erase a character */
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if ((c == '\010') || (c == '\177')) {
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if (buf_len == 0) {
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usart_send_blocking(USART2, '\a');
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} else {
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back_up();
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}
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/* ^W erases a word */
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} else if (c == 0x17) {
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while ((buf_len > 0) &&
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(!(isspace((int) buf[end_ndx])))) {
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back_up();
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}
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/* ^U erases the line */
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} else if (c == 0x15) {
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while (buf_len > 0) {
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back_up();
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}
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/* Non-editing character so insert it */
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} else {
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if (buf_len == (BUFLEN - 1)) {
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usart_send_blocking(USART2, '\a');
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} else {
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buf[end_ndx] = c;
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end_ndx = inc_ndx(end_ndx);
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usart_send_blocking(USART2, c);
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}
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}
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}
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}
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/*
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* Called by libc stdio fwrite functions
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*/
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int
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_write(int fd, char *ptr, int len)
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{
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int i = 0;
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/*
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* Write "len" of char from "ptr" to file id "fd"
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* Return number of char written.
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*
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* Only work for STDOUT, STDIN, and STDERR
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*/
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if (fd > 2) {
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return -1;
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}
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while (*ptr && (i < len)) {
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usart_send_blocking(USART2, *ptr);
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if (*ptr == '\n') {
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usart_send_blocking(USART2, '\r');
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}
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i++;
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ptr++;
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}
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return i;
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}
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/*
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* Called by the libc stdio fread fucntions
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*
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* Implements a buffered read with line editing.
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*/
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int
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_read(int fd, char *ptr, int len)
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{
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int my_len;
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if (fd > 2) {
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return -1;
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}
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get_buffered_line();
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my_len = 0;
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while ((buf_len > 0) && (len > 0)) {
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*ptr++ = buf[start_ndx];
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start_ndx = inc_ndx(start_ndx);
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my_len++;
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len--;
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}
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return my_len; /* return the length we got */
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}

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