read battery voltage

main.c

@@ -38,6 +38,7 @@
 #include <libopencm3/cm3/nvic.h> // interrupt utilities
 #include <libopencm3/stm32/exti.h> // external interrupt utilities
 #include <libopencm3/stm32/timer.h> // timer utilities
+#include <libopencm3/stm32/adc.h> // ADC utilities
 
 /* own libraries */
 #include "global.h" // board definitions
@@ -85,6 +86,17 @@ const uint32_t ticks_midday = 12*60*60*TICKS_PER_SECOND;
 #define SQUARE_WAVE_GPIO_PIN GPIO_TIM2_CH1_ETR /**< timer pin input, connect to RTC IC square wave output (TIM2_CH1 on PA0) */
 /** @} */
 
+/** @defgroup battery_adc ADC used to measure battery voltage
+ *  @{
+ */
+#define BATTERY_ADC ADC1 /**< ADC used to measure battery voltage */
+#define BATTERY_ADC_RCC RCC_ADC1 /**< ADC clock peripheral */
+#define BATTERY_ADC_CHANNEL ADC_CHANNEL1 /**< ADC channel */
+#define BATTERY_PORT GPIOA /**< port on which the battery is connected */
+#define BATTERY_PORT_RCC RCC_GPIOA /**< timer port peripheral clock */
+#define BATTERY_PIN GPIO1 /**< pin of the port on which the battery is connected */
+/** @} */
+
 /** RGB values for the WS2812b clock LEDs */
 uint8_t clock_leds[WS2812B_LEDS*3] = {0};
 /** current time in tick */
@@ -423,6 +435,23 @@ int main(void)
 	nvic_enable_irq(SQUARE_WAVE_TIMER_IRQ); // allow interrupt for timer
 	timer_enable_counter(SQUARE_WAVE_TIMER); // enable timer to count ticks
 
+	// setup ADC to ready battery voltage (single conversion of a regular channel without interrupt)
+	rcc_periph_clock_enable(BATTERY_PORT_RCC); // enable clock for GPIO peripheral
+	gpio_set_mode(BATTERY_PORT, GPIO_MODE_INPUT, GPIO_CNF_INPUT_ANALOG, BATTERY_PIN); // set GPIO as analogue input for the ADC
+	rcc_periph_clock_enable(BATTERY_ADC_RCC); // enable clock for ADC peripheral
+	adc_off(BATTERY_ADC); // switch off ADC while configuring it
+	// configuration is correct per default
+	adc_set_single_conversion_mode(BATTERY_ADC); // we just want one measurement
+	adc_set_sample_time(BATTERY_ADC, BATTERY_ADC_CHANNEL, ADC_SMPR_SMP_28DOT5CYC); // use 28.5 cycles to sample (long enough to be stable)
+	adc_enable_temperature_sensor(BATTERY_ADC); // enable internal voltage reference
+	adc_set_sample_time(BATTERY_ADC, ADC_CHANNEL17, ADC_SMPR_SMP_28DOT5CYC); // use 28.5 cycles to sample internal voltage reference
+	adc_power_on(BATTERY_ADC); // switch on ADC
+	for (uint32_t i = 0; i < 800000; i++) { // wait t_stab for the ADC to stabilize
+		__asm__("nop");
+	}
+	adc_reset_calibration(BATTERY_ADC); // remove previous non-calibration
+	adc_calibration(BATTERY_ADC); // calibrate ADC for less accuracy errors
+
 	printf("welcome to the CuVoodoo LED clock\n"); // print welcome message
 	led_on(); // switch on LED to indicate setup completed
 
@@ -431,6 +460,24 @@ int main(void)
 		printf("/!\\ RTC oscillator is disabled\n");
 	}
 
+	// read internal reference 1.2V voltage
+	uint8_t channels[] = {ADC_CHANNEL17}; // voltages to convert
+	adc_set_regular_sequence(BATTERY_ADC, LENGTH(channels), channels); // set channels to convert
+	adc_start_conversion_direct(BATTERY_ADC); // start conversion to get voltages
+	while (!adc_eoc(BATTERY_ADC)); // wait until conversion finished
+	uint16_t ref_value = adc_read_regular(BATTERY_ADC); // read internal reference 1.2V voltage value
+	// check RTC battery voltage
+	channels[0] = BATTERY_ADC_CHANNEL; // voltage to convert
+	adc_set_regular_sequence(BATTERY_ADC, LENGTH(channels), channels); // set channels to convert
+	adc_start_conversion_direct(BATTERY_ADC); // start conversion to get voltages
+	while (!adc_eoc(BATTERY_ADC)); // wait until conversion finished
+	uint16_t battery_value = adc_read_regular(BATTERY_ADC); // read converted battery voltage
+	float battery_voltage = battery_value*1.2/ref_value; // calculate battery voltage
+	if (battery_voltage<2.4) {
+		printf("/!\\ low ");
+	}
+	printf("battery voltage: %.2fV\n", battery_voltage);
+
 	// get date and time
 	uint16_t* rtc_time = rtc_read_time(); // get RTC time/date
 	current_time = rtc_time[2]*ticks_hour+rtc_time[1]*ticks_minute+rtc_time[0]*ticks_second; // the current time