From 5090774fd8ca593d3b247b099b3b6a40e837475e Mon Sep 17 00:00:00 2001
From: "Jan \"Yenya\" Kasprzak" <kas@fi.muni.cz>
Date: Wed, 21 Nov 2012 23:07:17 +0100
Subject: [PATCH] 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.
---
 firmware/adc.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/firmware/adc.c b/firmware/adc.c
index 38f3677..d0ee1c4 100644
--- 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)
-- 
2.39.3