gps test works

chip_test_example/gps.hpp

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+// gps.hpp — u-blox NMEA GPS reader over UART
+// No external libs; uses POSIX termios + read().
+//
+// Hardware pins (THE TRUTH):
+//   GPIO14/TXD0 → UBLOX_TX line  (/dev/serial0)
+//   GPIO15/RXD0 → UBLOX_RX line
+//   Default baud: 9600
+//
+// Enable UART: sudo raspi-config → Interfaces → Serial Port
+//   "login shell over serial" = No, "serial port hardware" = Yes
+#pragma once
+
+#include <cstdio>
+#include <cstring>
+#include <cmath>
+#include <cstdlib>
+
+#include <fcntl.h>
+#include <termios.h>
+#include <unistd.h>
+
+namespace gps {
+
+struct Fix {
+    bool   valid = false;
+    double lat   = 0.0; // degrees, positive=N
+    double lon   = 0.0; // degrees, positive=E
+    float  speed_knots = 0.0f;
+    char   time[12]{}; // HHMMSS.ss (raw NMEA)
+    char   date[7]{};  // DDMMYY
+};
+
+class Reader {
+public:
+    bool begin(const char *dev = "/dev/serial0", int baud = 9600);
+    void end();
+
+    // Read one NMEA sentence into buf (null-terminated). Returns false on error.
+    bool readLine(char *buf, size_t cap);
+
+    // Parse $GPRMC or $GNRMC from a sentence. Returns false if sentence is
+    // not RMC or position is invalid.
+    static bool parseRmc(const char *sentence, Fix &fix);
+
+private:
+    int fd_ = -1;
+};
+
+inline bool Reader::begin(const char *dev, int baud)
+{
+    fd_ = ::open(dev, O_RDWR | O_NOCTTY | O_NDELAY);
+    if (fd_ < 0) return false;
+    ::fcntl(fd_, F_SETFL, 0); // blocking
+
+    termios t{};
+    if (tcgetattr(fd_, &t) < 0) return false;
+
+    speed_t spd;
+    switch (baud) {
+        case 4800:  spd = B4800;  break;
+        case 9600:  spd = B9600;  break;
+        case 19200: spd = B19200; break;
+        case 38400: spd = B38400; break;
+        case 57600: spd = B57600; break;
+        default:    spd = B9600;
+    }
+    cfsetispeed(&t, spd);
+    cfsetospeed(&t, spd);
+    cfmakeraw(&t);
+    t.c_cc[VMIN]  = 1;
+    t.c_cc[VTIME] = 0;
+    tcsetattr(fd_, TCSANOW, &t);
+    return true;
+}
+
+inline void Reader::end()
+{
+    if (fd_ >= 0) { ::close(fd_); fd_ = -1; }
+}
+
+inline bool Reader::readLine(char *buf, size_t cap)
+{
+    size_t n = 0;
+    char   c;
+    // Wait for '$' (start of NMEA sentence)
+    do {
+        if (::read(fd_, &c, 1) != 1) return false;
+    } while (c != '$');
+    buf[n++] = '$';
+
+    while (n < cap - 1) {
+        if (::read(fd_, &c, 1) != 1) return false;
+        buf[n++] = c;
+        if (c == '\n') break;
+    }
+    buf[n] = '\0';
+    return true;
+}
+
+// Parse $GPRMC or $GNRMC
+// Format: $GNRMC,HHMMSS.ss,A,DDMM.MMMM,N,DDDMM.MMMM,E,spd,crs,DDMMYY,...
+inline bool Reader::parseRmc(const char *sentence, Fix &fix)
+{
+    if (!sentence) return false;
+
+    // Check sentence type
+    if (std::strncmp(sentence, "$GPRMC", 6) != 0 &&
+        std::strncmp(sentence, "$GNRMC", 6) != 0)
+        return false;
+
+    // Make a mutable copy
+    char buf[128];
+    std::strncpy(buf, sentence, sizeof(buf) - 1);
+    buf[sizeof(buf) - 1] = '\0';
+
+    // Tokenize on commas
+    const char *fields[13]{};
+    int         nf = 0;
+    char       *p  = buf;
+    while (*p && nf < 13) {
+        fields[nf++] = p;
+        char *next = std::strchr(p, ',');
+        if (!next) break;
+        *next = '\0';
+        p     = next + 1;
+    }
+    if (nf < 10) return false;
+
+    // field[2] = status ('A'=valid, 'V'=void)
+    if (fields[2][0] != 'A') { fix.valid = false; return false; }
+
+    // field[1] = time
+    std::strncpy(fix.time, fields[1], sizeof(fix.time) - 1);
+
+    // field[3] = lat DDMM.MMMM, field[4] = N/S
+    double raw_lat = std::atof(fields[3]);
+    int    lat_deg = (int)(raw_lat / 100.0);
+    double lat_min = raw_lat - lat_deg * 100.0;
+    fix.lat = lat_deg + lat_min / 60.0;
+    if (fields[4][0] == 'S') fix.lat = -fix.lat;
+
+    // field[5] = lon DDDMM.MMMM, field[6] = E/W
+    double raw_lon = std::atof(fields[5]);
+    int    lon_deg = (int)(raw_lon / 100.0);
+    double lon_min = raw_lon - lon_deg * 100.0;
+    fix.lon = lon_deg + lon_min / 60.0;
+    if (fields[6][0] == 'W') fix.lon = -fix.lon;
+
+    fix.speed_knots = (float)std::atof(fields[7]);
+
+    // field[9] = date DDMMYY
+    std::strncpy(fix.date, fields[9], sizeof(fix.date) - 1);
+
+    fix.valid = true;
+    return true;
+}
+
+} // namespace gps

chip_test_example/gps_test.cpp

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+#include <cstdio>
+
+#include "gps.hpp"
+
+int main()
+{
+    gps::Reader reader;
+
+    if (!reader.begin()) {
+        std::fprintf(stderr, "gps open failed\n"
+            "  check /dev/serial0 (UART)\n"
+            "  enable serial: sudo raspi-config -> Interfaces -> Serial Port\n"
+            "    login shell = No, hardware = Yes\n");
+        return 1;
+    }
+    std::fprintf(stderr, "gps reading /dev/serial0 at 9600...\n");
+
+    char    line[128];
+    gps::Fix fix;
+    for (;;) {
+        if (!reader.readLine(line, sizeof(line))) {
+            std::fprintf(stderr, "uart read error\n");
+            break;
+        }
+        if (!gps::Reader::parseRmc(line, fix)) {
+            // Not an RMC sentence or no fix — print raw line for debug
+            // std::fprintf(stderr, "raw: %s", line);
+            continue;
+        }
+        if (fix.valid) {
+            std::fprintf(stderr, "fix  lat=%+.6f  lon=%+.6f  spd=%.1f kn  "
+                         "time=%s  date=%s\n",
+                         fix.lat, fix.lon, fix.speed_knots,
+                         fix.time, fix.date);
+        } else {
+            std::fprintf(stderr, "no fix\n");
+        }
+    }
+}