Implement voltage to dBm conversion in MDx

MDx radios get an RSSI voltage in mV, to be converted into dBm, we used
and empirically derived linear relation to compute the dBm value.

openrtx/include/interfaces/rtx.h

@@ -110,8 +110,8 @@ rtxStatus_t rtx_getCurrentStatus();
 void rtx_taskFunc();
 
 /**
- * Get current RSSI voltage.
- * @return RSSI voltage.
+ * Get current RSSI in dBm.
+ * @return RSSI value in dBm.
  */
 float rtx_getRssi();
 

openrtx/include/state.h

@@ -37,6 +37,7 @@ typedef struct
     curTime_t time;
     float v_bat;
     float charge;
+    float rssi;
 
     uint8_t ui_screen;
     uint8_t backlight_level;

openrtx/src/state.c

@@ -37,6 +37,7 @@ void state_init()
 #endif
     state.v_bat = platform_getVbat();
     state.charge = battery_getCharge(state.v_bat);
+    state.rssi = rtx_getRssi();
 
     state.backlight_level = 255;
     state.channelInfoUpdated = true;

openrtx/src/threads.c

@@ -251,6 +251,7 @@ static void dev_task(void *arg)
 #endif
         state.v_bat = platform_getVbat();
         state.charge = battery_getCharge(state.v_bat);
+        state.rssi = rtx_getRssi();
 
         OSMutexPost(&state_mutex, OS_OPT_POST_NONE, &os_err);
 

platform/drivers/baseband/rtx_MD3x0.c

@@ -44,6 +44,13 @@ rtxStatus_t rtxStatus;            /* RTX driver status                  */
 uint8_t sqlOpenTsh;               /* Squelch opening threshold          */
 uint8_t sqlCloseTsh;              /* Squelch closing threshold          */
 
+// RSSI mV to dBm conversion function, gain is constant, while offset values
+// are aligned to calibration frequency test points
+float rssi_gain = 22.0f;
+float rssi_offset[] = {3277.618f, 3654.755f, 3808.191f,
+                       3811.318f, 3804.936f, 3806.591f,
+                       3723.882f, 3621.373f, 3559.782f};
+
 /*
  * Helper functions to reduce code mess. They all access 'rtxStatus'
  * internally.
@@ -443,5 +450,17 @@ void rtx_taskFunc()
 
 float rtx_getRssi()
 {
-    return adc1_getMeasurement(1);
+    // ADC1 returns the RSSI value in mV, we convert it to dBm trough
+    // their derived linear relation
+    float rssi_mv = adc1_getMeasurement(1);
+    float freq = rtxStatus.rxFrequency;
+    int index = 0;
+    if (freq < 401035000.0f)
+        index = 0;
+    else if (freq > 479995000.0f)
+        index = 8;
+    else
+        index = (int)(freq - 400035000.0f)/10000000;
+    float rssi_dbm = (rssi_mv - rssi_offset[index]) / rssi_gain;
+    return rssi_dbm;
 }