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 # arduino_pps_gprmc_time_sync
-Use Arduino Uno to simulate a GPS module, sending PPS pulse and GPRMC to Lidar
+
+Use Arduino Uno to simulate a GPS module, sending PPS pulse and GPRMC message to Lidar.
+
+## Verified Platform
+- Arduino Uno
+- Velodyne Lidar VLP-16 
+- ROS 2 Foxy with VLP-16 driver https://github.com/ros-drivers
+
+## Wiring Connection
+
+|  VLP-16   | Arduino  |
+|  ----  | ----  |
+| Ground  | GND |
+| GPS PULSE  | Pin#8 |
+| GPS RECEIVE  | Pin#11 |
+
+## VLP-16 Configuraition
+
+Make sure the parameter `gps_time` in yaml file is set to **true** to enable time sync.
+
+![](resource/vlp16_gps_time.png)
+
+## Test Result
+
+- VLP-16 web interface
+
+It is successfully synchronized if you see the coordinates of **GPS Position**  and locked **PPS** status.
+
+![](resource/vlp16_pps_locked.jpg)
+
+- VLP-16 ROS 2 timestamp
+
+If the `gps_time` parameter is set, then you will see the time jump after time synchronized.
+
+![](resource/vlp16_time_sync_stamp.jpg)
+
+- GPRMC debug message
+
+You can enable the debug messages in Arduino code to monitor the GPRMC.
+
+![](resource/arduino_serial_monitor.png)
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diff --git a/src/pps_gprmc_time_sync/pps_gprmc_time_sync.ino b/src/pps_gprmc_time_sync/pps_gprmc_time_sync.ino
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+#include <SoftwareSerial.h>
+
+const byte pps_pin = 8;
+const byte msg_pin = 9;
+const byte rx_pin = 10;
+const byte tx_pin = 11;
+const bool inverse_logic = true;
+SoftwareSerial gprmc_conn(rx_pin, tx_pin, inverse_logic);
+
+const unsigned int trigger_freq = 1;
+const unsigned long dt = 1000000 / trigger_freq; // => dt = 1 sec
+const unsigned long dt_pps_pull_low = dt + 100000; // => dt + 100ms
+const unsigned long dt_sent_gprmc = dt_pps_pull_low + 250000; // => dt_pps_pull_low + 100ms
+unsigned long timestamp;
+unsigned long trigger_start_time;
+unsigned long ts_pps_high;
+unsigned long ts_pps_low;
+unsigned long ts_msg_high;
+unsigned long ts_msg_low;
+unsigned long i;
+
+// Start timestamp: HH:MM:SS
+int hh = 00;
+int mm = 01;
+int ss = 02;
+// GPS coordinates from Taipei
+char pos_latitude[] = "25.04776";
+char pos_longitude[] = "121.53185";
+
+void setup() {
+  pinMode(rx_pin, INPUT);
+  pinMode(tx_pin, OUTPUT);
+  pinMode(pps_pin, OUTPUT); // PPS
+  pinMode(msg_pin, OUTPUT); // Indicator of msg_sent
+  Serial.begin(9600);
+  gprmc_conn.begin(9600);
+  // gprmc_conn.println("Hello World!");
+  trigger_start_time = micros();
+}
+
+void loop() {
+  char buffer[128];
+  byte CRC = 0;
+  
+  timestamp = micros() - trigger_start_time;
+  if (timestamp >= dt*i + dt_sent_gprmc)
+  {
+    digitalWrite(msg_pin, HIGH);
+    i += 1;
+    if (ss == 59)
+    {
+      ss = 0;
+      if (mm == 59)
+      {
+        mm = 0;
+        hh += 1;
+      }
+      else mm += 1;
+    }
+    else ss += 1;
+
+    ts_msg_high = micros();
+
+    // Prepare GPRMC msg
+    sprintf(buffer, "GPRMC,%02d%02d%02d,A,%s,N,%s,E,022.4,084.4,070423,,A", hh, mm, ss, pos_latitude, pos_longitude);
+    for (byte x = 0; x < strlen(buffer); x++) {
+      // XOR every character in between '$' and '*'
+      CRC = CRC ^ buffer[x];
+    }
+    
+    // Send GPRMC msg
+    gprmc_conn.print("$");
+    gprmc_conn.print(buffer);
+    gprmc_conn.print("*");
+    gprmc_conn.print(CRC, HEX);
+    gprmc_conn.println();
+  
+    digitalWrite(msg_pin, LOW);
+    ts_msg_low = micros();
+
+#if 0
+    // Print Debug Messages
+    Serial.print("PPS at: ");
+    Serial.print(ts_pps_high);
+    Serial.print("  PPS low after: ");
+    Serial.print(ts_pps_low-ts_pps_high);
+    Serial.print("  MSG high after: ");
+    Serial.print(ts_msg_high-ts_pps_high);
+    Serial.print("  MSG low after: ");
+    Serial.print(ts_msg_low-ts_pps_high);
+    Serial.print("  $");
+    Serial.print(buffer);
+    Serial.print("*");
+    Serial.print(CRC, HEX);
+    Serial.println();
+#endif
+
+  }
+  else if (timestamp >= dt*i + dt_pps_pull_low)
+  {
+    // Pull pps to low
+    digitalWrite(pps_pin, LOW);
+    ts_pps_low = micros();
+  }
+  else if (timestamp >= dt*i)
+  {
+    // Pull pps to high
+    digitalWrite(pps_pin, HIGH);  
+    ts_pps_high = micros();
+  }
+
+}