feat: add GPS UART module with NMEA parsing and checksum validation

uart/gps_uart/CMakeLists.txt

@@ -0,0 +1,12 @@
+# Tell CMake where to find the executable source file
+add_executable(gps_uart gps_uart.c)
+
+# Link to pico_stdlib (gpio, time, etc. functions)
+target_link_libraries(gps_uart pico_stdlib hardware_uart)
+
+# Enable usb output, disable uart output
+pico_enable_stdio_usb(gps_uart 1)
+pico_enable_stdio_uart(gps_uart 1)
+
+# Create map/bin/hex/uf2 files
+pico_add_extra_outputs(gps_uart)

uart/gps_uart/gps_uart.c

@@ -0,0 +1,191 @@
+#include "hardware/uart.h"
+#include "pico/stdlib.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+
+// uart id and tx/rx pins could vary according to the microcontroller used
+// please refer to the datasheet
+#define UART_ID uart1
+#define BAUD_RATE 9600
+#define UART_TX_PIN 4
+#define UART_RX_PIN 5
+#define MAX_NMEA_LENGTH 256
+
+typedef struct
+{
+    float latitude;
+    float longitude;
+    bool is_valid;
+} GPSData;
+
+
+void uart_gps_init()
+{
+    uart_init(UART_ID, BAUD_RATE);
+
+    gpio_set_function(UART_TX_PIN, GPIO_FUNC_UART);
+    gpio_set_function(UART_RX_PIN, GPIO_FUNC_UART);
+
+    uart_set_hw_flow(UART_ID, false, false);
+    uart_set_format(UART_ID, 8, 1, UART_PARITY_NONE);
+    uart_set_fifo_enabled(UART_ID, true);
+}
+
+bool validate_nmea_checksum(char *nmea_string)
+{
+    int len = strlen(nmea_string);
+    printf("Checksum Validation - String Length: %d\n", len);
+    printf("Full NMEA String: %s", nmea_string);
+
+    if (len < 7)
+    {
+        printf("Invalid: Too short (< 7 chars)\n");
+        return false;
+    }
+
+    char *checksum_ptr = strchr(nmea_string, '*');
+    if (!checksum_ptr)
+    {
+        printf("Invalid: No checksum marker (*) found\n");
+        return false;
+    }
+
+    uint8_t calculated_checksum = 0;
+    for (char *p = nmea_string + 1; p < checksum_ptr; p++)
+    {
+        calculated_checksum ^= *p;
+    }
+
+    char hex_checksum[3];
+    snprintf(hex_checksum, sizeof(hex_checksum), "%02X", calculated_checksum);
+
+    printf("Calculated Checksum: %s\n", hex_checksum);
+    printf("Received Checksum: %s\n", checksum_ptr + 1);
+
+    bool is_valid = strncmp(hex_checksum, checksum_ptr + 1, 2) == 0;
+    printf("Checksum Validation: %s\n", is_valid ? "VALID" : "INVALID");
+
+    return is_valid;
+}
+
+void convert_nmea_to_decimal(char *nmea_coord, float *decimal_coord)
+{
+    float degrees = atof(nmea_coord) / 100.0;
+    int int_degrees = (int)degrees;
+    float minutes = (degrees - int_degrees) * 100.0;
+
+    *decimal_coord = int_degrees + (minutes / 60.0);
+}
+
+bool parse_nmea_gps(char *nmea_string, GPSData *gps_data)
+{
+    if (!validate_nmea_checksum(nmea_string))
+    {
+        printf("Invalid NMEA Checksum\n");
+        return false;
+    }
+
+    // Ignore all NMEA strings that are not of type GPRMC
+    if (strncmp(nmea_string, "$GPRMC", 6) != 0)
+    {
+        return false;
+    }
+
+    char *token;
+    char *tokens[12] = {0};
+    int token_count = 0;
+
+    // Tokenize the string
+    token = strtok(nmea_string, ",");
+    while (token != NULL && token_count < 12)
+    {
+        tokens[token_count++] = token;
+
+        token = strtok(NULL, ",");
+    }
+
+    // Check if we have enough tokens and data is valid
+    if (token_count >= 12 && strcmp(tokens[2], "A") == 0)
+    {
+        if (strlen(tokens[3]) > 0)
+        {
+            convert_nmea_to_decimal(tokens[3], &gps_data->latitude);
+            if (tokens[4][0] == 'S')
+                gps_data->latitude = -gps_data->latitude;
+        }
+
+        if (strlen(tokens[5]) > 0)
+        {
+            convert_nmea_to_decimal(tokens[5], &gps_data->longitude);
+            if (tokens[6][0] == 'W')
+                gps_data->longitude = -gps_data->longitude;
+        }
+
+        gps_data->is_valid = true;
+        return true;
+    }
+}
+
+void process_gps_data(GPSData *gps_data)
+{
+    char nmea_buffer[MAX_NMEA_LENGTH];
+    int chars_read = 0;
+
+    while (uart_is_readable(UART_ID) && chars_read < MAX_NMEA_LENGTH - 1)
+    {
+        char received_char = uart_getc(UART_ID);
+        nmea_buffer[chars_read] = received_char;
+
+        // NMEA strings are terminated by a newline character
+        if ((int)received_char == 10)
+        {
+            nmea_buffer[chars_read + 1] = '\0';
+
+            if (parse_nmea_gps(nmea_buffer, gps_data))
+            {
+                printf("Valid GPS Data Received\n");
+            }
+
+            chars_read = 0;
+            break;
+        }
+        else
+        {
+            chars_read++;
+        }
+    }
+}
+
+int main()
+{
+  stdio_init_all();
+
+  uart_gps_init();
+
+  // recommended cold start waiting time (could vary based on the gps module)
+  sleep_ms(7 * 60 * 1000);
+
+  GPSData gps_data = {0};
+
+  while (1)
+  {
+    process_gps_data(&gps_data);
+
+    if (gps_data.is_valid)
+    {
+      printf("GPS Location:\n");
+      printf("Latitude: %.6f\n", gps_data.latitude);
+      printf("Longitude: %.6f\n", gps_data.longitude);
+      gps_data.is_valid = false;
+    }
+
+    // sufficient waiting time required between each gps reading and the next one
+    sleep_ms(30 * 1000);
+    tight_loop_contents();
+  }
+
+  return 0;
+}