Add ADC code to re-route P0 channel to Vdd.

README.md

@@ -3,6 +3,10 @@
 A C++ example project showing how to measure the BBC micro:bit V2 battery
 voltage.
 
+All the ADC channels are currently used for the Analogue pins and the on-board
+microphone, so for this project we hijack the ADC channel used for P0 and
+re-route it to the internal Vdd (voltage from the microcontroller power input).
+
 ## Building the project
 
 ### Dependencies

source/main.cpp

@@ -2,10 +2,85 @@
 
 MicroBit uBit;
 
+
+// The ADC channel to be used for the battery voltage measurement
+static int BATTERY_ADC_CHANNEL = -1;
+
+/**
+ * Measure the VDD voltage using the ADC channel for pin P0.
+ *
+ * @return int The VDD voltage in millivolts.
+ */
+static int get_vdd_millivolts() {
+    if (BATTERY_ADC_CHANNEL == -1) {
+        // Not initilised
+        return 0;
+    }
+    // Second arg ensures channel is activated before we do an analog read,
+    // so that it configures CH[n].CONFIG before we apply our changes
+    (void)uBit.adc.getChannel(uBit.io.P0, true);
+
+    // Configure CH[n].CONFIG with 0.6V internal reference and 1/6 gain
+    // so max Vin detection is 0-3.6V
+    NRF_SAADC->CH[BATTERY_ADC_CHANNEL].CONFIG =
+        // Positive channel resistor control set to bypass resistor ladder
+        (SAADC_CH_CONFIG_RESP_Bypass << SAADC_CH_CONFIG_RESP_Pos) |
+        // Negative channel resistor control set to bypass resistor ladder
+        (SAADC_CH_CONFIG_RESN_Bypass << SAADC_CH_CONFIG_RESN_Pos) |
+        // Gain control set to 1/6
+        (SAADC_CH_CONFIG_GAIN_Gain1_6 << SAADC_CH_CONFIG_GAIN_Pos) |
+        // Reference control set to internal 0.6V reference
+        (SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
+        // Acquisition time set to 3us
+        (SAADC_CH_CONFIG_TACQ_3us << SAADC_CH_CONFIG_TACQ_Pos) |
+        // Differential mode set to single ended
+        (SAADC_CH_CONFIG_MODE_SE << SAADC_CH_CONFIG_MODE_Pos) |
+        // Burst mode set to disabled
+        (SAADC_CH_CONFIG_BURST_Disabled << SAADC_CH_CONFIG_BURST_Pos);
+
+    // And set the positive input to VDD
+    NRF_SAADC->CH[BATTERY_ADC_CHANNEL].PSELP = SAADC_CH_PSELP_PSELP_VDD << SAADC_CH_PSELP_PSELP_Pos;
+
+    int vin_millivolts = (1000 * 0.6 * 6 * uBit.io.P0.getAnalogValue()) / 1024;
+
+    return vin_millivolts;
+}
+
+/**
+ * Initialize the ADC to be able to measure Vdd voltage.
+ *
+ * To do this it initialises and finds the ADC channel for pin P0,
+ * which is hijacked by get_vdd_millivolts() to measure Vdd instead.
+ */
+static void vdd_adc_init() {
+    // Ensure CODAL has configured the P0 ADC channel
+    (void)uBit.io.P0.getAnalogValue();
+    (void)uBit.adc.getChannel(uBit.io.P0, true);
+    (void)uBit.io.P0.getAnalogValue();
+
+    // Now look in the MCU ADC channels for a channel configured for P0 (P0.2 AIN0)
+    for (size_t i = 0; i < NRF52_ADC_CHANNELS; i++) {
+        if (NRF_SAADC->CH[i].PSELP == (SAADC_CH_PSELP_PSELP_AnalogInput0 << SAADC_CH_PSELP_PSELP_Pos)) {
+            BATTERY_ADC_CHANNEL = i;
+            break;
+        }
+    }
+
+    // Do one measurement and throw away the result, as the first is always a bit off
+    (void)get_vdd_millivolts();
+}
+
 int main() {
     uBit.init();
 
+    vdd_adc_init();
+
     while (true) {
-        uBit.display.scroll("Hello world!");
+        ManagedString battery_mv = ManagedString(get_vdd_millivolts()) + " mV";
+
+        uBit.serial.send(battery_mv + "\r\n");
+        uBit.display.scroll(battery_mv);
+
+        uBit.sleep(1000);
     }
 }