adc: measure the offset of the gain-20 stage

We measure the offset of the ADC1:ADC1 gain 20 stage and subtract it
from ADC0:ADC1 and ADC2:ADC1 differential readings.

diff --git a/firmware/adc.c b/firmware/adc.c
index 38f36772a7aa6be7fa138351affb77e79b7f77b2..d0ee1c40f3377f17e40bd1f7662d662f72f8034f 100644 (file)
--- a/firmware/adc.c
+++ b/firmware/adc.c
@@ -5,12 +5,14 @@
 
 #define AMBIENT_ADC N_PWMLEDS
 #define BATTERY_ADC (N_PWMLEDS + 1)
+#define ADC1_GAIN20 (N_PWMLEDS + 2)
 
-#define NUM_ADCS 5
+#define NUM_ADCS 6
 volatile static unsigned char current_adc;
 static uint16_t adc_sum;
 static unsigned char sum_shift;
 static unsigned char adc_vals;
+static uint16_t adc1_gain20_offset_x16;
 
 static void inline setup_mux(unsigned char n)
 {
@@ -38,6 +40,10 @@ static void inline setup_mux(unsigned char n)
                ADMUX = _BV(REFS1) | _BV(MUX2) | _BV(MUX1);
                sum_shift = 0; // 1 measurement
                break;
+       case 5: // gain stage offset: 1.1V, ADC1,1, gain 20
+               ADMUX = _BV(REFS1) | _BV(MUX3) | _BV(MUX2) | _BV(MUX0);
+               sum_shift = 3; // 8 measurements
+               break;
        }
 
        adc_sum = 0;
@@ -59,6 +65,7 @@ static void start_next_adc()
 
 void init_adc()
 {
+       unsigned char i;
        current_adc = NUM_ADCS;
 
        ADCSRA = _BV(ADEN)                      // enable
@@ -71,12 +78,27 @@ void init_adc()
        DIDR0 = _BV(ADC0D) | _BV(ADC1D) | _BV(ADC2D)
                | _BV(ADC4D) | _BV(ADC5D) | _BV(ADC6D);
 
+       // 1.1V, ADC1,1, gain 20
+       ADMUX = _BV(REFS1) | _BV(MUX3) | _BV(MUX2) | _BV(MUX0);
        ADCSRA |= _BV(ADSC);
 
        /* Do first conversion and drop the result */
        while ((ADCSRA & _BV(ADIF)) == 0)
                ;
        ADCSRA |= _BV(ADIF); // clear the IRQ flag
+
+       adc1_gain20_offset_x16 = 0;
+
+       for (i = 0; i < 16; i++) {
+               ADCSRA |= _BV(ADSC);
+
+               while ((ADCSRA & _BV(ADIF)) == 0)
+                       ;
+               adc1_gain20_offset_x16 += ADCW;
+
+               ADCSRA |= _BV(ADIF); // clear the IRQ flag
+       }
+
        ADCSRA |= _BV(ADIE); // enable IRQ
 
        ADCSRA &= ~_BV(ADEN); // disable until needed
@@ -110,6 +132,18 @@ ISR(ADC_vect) { // IRQ handler
 
        adcval = adc_sum >> sum_shift;
 
+       if (current_adc == ADC1_GAIN20) {
+               // running average
+               adc1_gain20_offset_x16 += adcval
+                       - (adc1_gain20_offset_x16 >> 4);
+       } else if (current_adc == 0 || current_adc == 1) {
+               uint16_t offset = adc1_gain20_offset_x16 >> 4;
+               if (adcval >= offset)
+                       adcval -= offset;
+               else
+                       adcval = 0;
+       }
+
        if (current_adc < N_PWMLEDS)
                pwmled_adc(current_adc, adcval);
        if (current_adc == AMBIENT_ADC)