fw: Implement battery voltage readout

firmware/Makefile

@@ -3,7 +3,7 @@ CFLAGS=-g -Wall -Wno-multichar -Wno-overflow -mcall-prologues -mmcu=$(MCU) -O2 -
 LDFLAGS=-Wl,-gc-sections -Wl,-relax
 CC=avr-gcc
 TARGET=main
-OBJECT_FILES=main.o input.o systick.o oled-display/font.o oled-display/i2c.o oled-display/lcd.o
+OBJECT_FILES=main.o batmon.o input.o systick.o oled-display/font.o oled-display/i2c.o oled-display/lcd.o
 
 all: $(TARGET).hex
 

firmware/batmon.c

@@ -0,0 +1,25 @@
+#include <avr/io.h>
+#include "batmon.h"
+
+void batmon_init()
+{
+	ADCSRA = (1 << ADPS2) | (1 << ADPS1); // 125 kHz ADC clock
+	ADMUX = ADC_CHANNEL | (1 << REFS0) | (1 << REFS1) | (1 << ADLAR); // Internal 1.2V bandgap, left adj.
+	ADCSRB = 0; // Manual start
+#if ADC_CHANNEL < 7
+	DIDR0 = (1 << ADC_CHANNEL); // Disconnect digital channel
+#endif
+
+	// Enable voltage divider
+	PORT_ADCC(DDR) |= PIN_ADCC;
+	PORT_ADCC(PORT) &= ~PIN_ADCC;
+
+	// Enable ADC
+	ADCSRA |= (1 << ADEN);
+}
+
+inline uint8_t batmon_get_voltage()
+{
+	ADCSRA |= (1 << ADSC); // TODO: Check whether this increases the power consumption of the chip by much
+	return ADCH;
+}

firmware/batmon.h

@@ -0,0 +1,12 @@
+#ifndef BATMON_H
+#define BATMON_H
+#include <avr/io.h>
+
+#define ADC_CHANNEL 0
+#define PORT_ADCC(t) t##D
+#define PIN_ADCC (1 << 1)
+
+void batmon_init();
+uint8_t batmon_get_voltage();
+
+#endif // BATMON_H

firmware/main.c

@@ -5,6 +5,7 @@
 #include "oled-display/lcd.h"
 #include "input.h"
 #include "systick.h"
+#include "batmon.h"
 
 #define PORT_SPI(t) t##B
 #define PIN_SPI_CS (1 << 2)
@@ -33,7 +34,7 @@ uint8_t spi_proc()
 		if (SPSR & (1 << SPIF)) {
 			switch (y) {
 			case Y_MAX - 1:
-				spdr_next = 55; // TODO: Vbat
+				spdr_next = batmon_get_voltage();
 				break;
 			}
 			SPDR = spdr_next;
@@ -86,6 +87,7 @@ int main()
 	spi_init();
 	systick_init();
 	input_init();
+	batmon_init();
 
 	_delay_ms(10);