stm32f1/lib/radio_gps.c

/* This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
/** library for receiving NMEA-0183 GPS messages (code)
 *  @file radio_gps.c
 *  @author King Kévin <kingkevin@cuvoodoo.info>
 *  @date 2017
 *  @note peripherals used: USART @ref radio_gps_usart
 */

/* standard libraries */
#include <stdint.h> // standard integer types
#include <stdio.h> // standard I/O facilities
#include <stdlib.h> // general utilities

/* STM32 (including CM3) libraries */
#include <libopencm3/stm32/rcc.h> // real-time control clock library
#include <libopencm3/stm32/gpio.h> // general purpose input output library
#include <libopencm3/stm32/usart.h> // universal synchronous asynchronous receiver transmitter library
#include <libopencm3/cm3/nvic.h> // interrupt handler
#include <libopencmsis/core_cm3.h> // Cortex M3 utilities

#include "radio_gps.h" // GPS header and definitions
#include "global.h" // common methods

/** @defgroup radio_gps_usart USART peripheral used for communication with the GPS
 *  @{
 */
#define RADIO_GPS_USART 3 /**< USART peripheral */
/** @} */

#define RADIO_GPS_BAUDRATE 9600 /**< USART baudrate for the GPS communication, in bits per second (with 8N1 8 bits, no parity bit, 1 stop bit settings) */

static volatile char radio_gps_buffer[82] = {0}; /**< buffer for received data (82 chars is the maximum NMEA-0183 message length) */
static volatile uint8_t radio_gps_buffer_i = 0; /**< current position of read received data */

volatile bool radio_gps_received = false;
volatile char radio_gps_message[82+1] = {0};

void radio_gps_setup(void)
{
	/* enable USART I/O peripheral */
	rcc_periph_clock_enable(USART_PORT_RCC(RADIO_GPS_USART)); // enable clock for USART port peripheral
	rcc_periph_clock_enable(USART_RCC(RADIO_GPS_USART)); // enable clock for USART peripheral
	rcc_periph_clock_enable(RCC_AFIO); // enable pin alternate function (USART)
	gpio_set_mode(USART_PORT(RADIO_GPS_USART), GPIO_MODE_INPUT, GPIO_CNF_INPUT_PULL_UPDOWN, USART_PIN_RX(RADIO_GPS_USART)); // setup GPIO pin USART receive
	gpio_set(USART_PORT(RADIO_GPS_USART), USART_PIN_RX(RADIO_GPS_USART)); // pull up to avoid noise when not connected

	/* setup USART parameters */
	usart_set_baudrate(USART(RADIO_GPS_USART), RADIO_GPS_BAUDRATE);
	usart_set_databits(USART(RADIO_GPS_USART), 8);
	usart_set_stopbits(USART(RADIO_GPS_USART), USART_STOPBITS_1);
	usart_set_mode(USART(RADIO_GPS_USART), USART_MODE_RX); // we only need to receive messages
	usart_set_parity(USART(RADIO_GPS_USART), USART_PARITY_NONE);
	usart_set_flow_control(USART(RADIO_GPS_USART), USART_FLOWCONTROL_NONE);

	nvic_enable_irq(USART_IRQ(RADIO_GPS_USART)); // enable the USART interrupt
	usart_enable_rx_interrupt(USART(RADIO_GPS_USART)); // enable receive interrupt
	usart_enable(USART(RADIO_GPS_USART)); // enable USART

	/* reset buffer states */
	radio_gps_received = false;
	radio_gps_buffer_i = 0;
}

/** USART interrupt service routine called when data has been received */
void USART_ISR(RADIO_GPS_USART)(void)
{
	if (usart_get_flag(USART(RADIO_GPS_USART), USART_SR_RXNE)) { // data has been received
		char c =  usart_recv(USART(RADIO_GPS_USART)); // put character in buffer (and clear flag)
		// only save data if there is space in the buffer
		if (radio_gps_buffer_i<LENGTH(radio_gps_buffer)) {
			radio_gps_buffer[radio_gps_buffer_i++] = c;
		}
		if ('\n'==c) { // end of message received
			// verify if the message is correctly formatted
			if (radio_gps_buffer_i<1 || '$'!=radio_gps_buffer[0]) {
				goto malformatted;
			} else if (radio_gps_buffer_i<2 || '\r'!=radio_gps_buffer[radio_gps_buffer_i-2]) {
				goto malformatted;
			} else { // verify checksum when available
				uint8_t checksum_calc = 0;
				uint8_t checksum_i = 0;
				bool checkum_present = false;
				for (checksum_i = 1; checksum_i < radio_gps_buffer_i-2; checksum_i++) {
					if ('*'==radio_gps_buffer[checksum_i]) {
						checkum_present = true;
						break;
					} else {
						checksum_calc ^= radio_gps_buffer[checksum_i];
					}
				}
				if (checkum_present) {
					if (checksum_i!=radio_gps_buffer_i-5) { // only 2 checksum character should be remaining
						goto malformatted;
					} else if (checksum_calc-(radio_gps_buffer[checksum_i+1]-'0')*0x10-(radio_gps_buffer[checksum_i+2]-'0')) { // checksum does not match
						goto malformatted;
					}
				}
			}
			// copy message to user
			for (uint8_t i = 0; i < radio_gps_buffer_i && i < LENGTH(radio_gps_message); i++) {
				radio_gps_message[i] = radio_gps_buffer[i];
			}
			// terminate string
			if (radio_gps_buffer_i<LENGTH(radio_gps_message)) {
				radio_gps_message[radio_gps_buffer_i] = '\0';
			} else {
				radio_gps_message[LENGTH(radio_gps_message)-1]  = '\0';
			}
			radio_gps_received = true; // notify user a message is available
malformatted:
			radio_gps_buffer_i = 0; // reset buffer
		}
	}
}