diff --git a/src/main.c b/src/main.c
index c15a0ef..eed5a6a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3,6 +3,9 @@
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/dma.h>
 #include <libopencm3/stm32/timer.h>
+#include <libopencm3/stm32/rtc.h>
+#include <libopencm3/stm32/pwr.h>
+#include <libopencm3/stm32/exti.h>
 #include <libopencm3/cm3/nvic.h>
 #include <libopencm3/cm3/systick.h>
 
@@ -33,6 +36,19 @@ volatile int wait_frame = 1;
 #define ADC_NUM_CHANNELS 3
 volatile uint16_t adc_values[ADC_NUM_CHANNELS];
 
+static void unlock_rtc_access(void)
+{
+	pwr_disable_backup_domain_write_protect();
+	RTC_WPR = 0xCA;
+	RTC_WPR = 0x53;
+}
+
+static void lock_rtc_access(void)
+{
+	RTC_WPR = 0xFF;
+	pwr_enable_backup_domain_write_protect();
+}
+
 static void init_gpio(void)
 {
 	// Set up UART TX on PB6 for debugging
@@ -79,6 +95,39 @@ static void init_clock(void)
 	rcc_periph_clock_enable(RCC_DMA);
 }
 
+static void init_rtc(void)
+{
+	// RTC clock setup
+	// see libopencm3-examples::examples/stm32/l1/stm32l-discovery/button-irq-printf-lowpower/main.c
+
+	/* turn on power block to enable unlocking */
+	rcc_periph_clock_enable(RCC_PWR);
+	pwr_disable_backup_domain_write_protect();
+
+	/* reset rtc */
+	RCC_BDCR |= RCC_BDCR_BDRST;
+	RCC_BDCR &= ~RCC_BDCR_BDRST;
+
+	// use LSI for RTC
+	rcc_osc_on(RCC_LSI);
+	rcc_wait_for_osc_ready(RCC_LSI);
+
+	/* Select the LSI as rtc clock */
+	RCC_BDCR |= RCC_BDCR_RTCSEL_LSI;
+
+	/* ?! Stdperiph examples don't turn this on until _afterwards_ which
+	 * simply doesn't work.  It must be on at least to be able to
+	 * configure it */
+	RCC_BDCR |= RCC_BDCR_RTCEN;
+
+	pwr_enable_backup_domain_write_protect();
+
+	nvic_enable_irq(NVIC_RTC_IRQ);
+
+	exti_set_trigger(EXTI17, EXTI_TRIGGER_RISING);
+	exti_enable_request(EXTI17);
+}
+
 static void init_timer(void)
 {
 	// *** TIM1 ***
@@ -183,6 +232,83 @@ static void init_adc(void)
 
 }
 
+static void deepsleep(uint32_t duration_secs)
+{
+	uint32_t tmp = 0;
+
+	// unlock RTC registers
+	unlock_rtc_access();
+
+	// enter initialization mode
+	RTC_ISR |= RTC_ISR_INIT;
+
+	// wait until initialization mode has been entered
+	while((RTC_ISR & RTC_ISR_INITF) != RTC_ISR_INITF) {
+		// do nothing
+	}
+
+	RTC_TR = 0; // 00:00:00
+	RTC_DR = // friday, 01.01.16
+		(1 << RTC_DR_YT_SHIFT) |
+		(6 << RTC_DR_YU_SHIFT) |
+		(5 << RTC_DR_WDU_SHIFT) |
+		(0 << RTC_DR_MT_SHIFT) |
+		(1 << RTC_DR_MU_SHIFT) |
+		(0 << RTC_DR_DT_SHIFT) |
+		(1 << RTC_DR_DU_SHIFT);
+
+	// disable Alarm A
+	RTC_CR &= ~RTC_CR_ALRAE;
+
+	// wait until register is writeable
+	while((RTC_ISR & RTC_ISR_ALRAWF) != RTC_ISR_ALRAWF) {
+		// do nothing
+	}
+
+	tmp |= (duration_secs % 10) << RTC_ALRMXR_SU_SHIFT;
+	duration_secs /= 10;
+
+	tmp |= (duration_secs % 6) << RTC_ALRMXR_ST_SHIFT;
+	duration_secs /= 6;
+
+	tmp |= (duration_secs % 10) << RTC_ALRMXR_MNU_SHIFT;
+	duration_secs /= 10;
+
+	tmp |= (duration_secs % 6) << RTC_ALRMXR_MNT_SHIFT;
+	duration_secs /= 6;
+
+	tmp |= (duration_secs % 10) << RTC_ALRMXR_HU_SHIFT;
+	duration_secs /= 10;
+
+	tmp |= (duration_secs % 2) << RTC_ALRMXR_HT_SHIFT;
+	// FIXME: >1d is not supported
+
+	tmp |= RTC_ALRMXR_MSK4; // ignore day/date
+
+	// set alarm register
+	RTC_ALRMAR = tmp;
+
+	// clear Alarm A flag
+	RTC_ISR &= ~RTC_ISR_ALRAF;
+
+	// enable RTC alarm interrupt for wakeup
+	RTC_CR |= RTC_CR_ALRAE | RTC_CR_ALRAIE;
+
+	// leave initialization mode
+	RTC_ISR &= ~RTC_ISR_INIT;
+
+	// lock registers again (using invalid key)
+	lock_rtc_access();
+
+	// enter deep sleep mode
+	SCB_SCR |= SCB_SCR_SLEEPDEEP;
+	PWR_CR |= PWR_CR_LPDS; // voltage regulator low-power mode
+	pwr_set_stop_mode();
+	__WFI();
+	SCB_SCR &= ~SCB_SCR_SLEEPDEEP; // no deepsleep except in this function
+	init_clock();
+}
+
 #if 0
 /* Set up timer to fire freq times per second */
 static void init_systick(int freq)
@@ -256,6 +382,7 @@ int main(void)
 	fxp_t CURRENT_OFFSET = fxp_from_float(0.049);
 
 	init_clock();
+	init_rtc();
 	init_gpio();
 	init_adc();
 	init_timer();
@@ -514,6 +641,14 @@ int main(void)
 						nextState = ConvMPP;
 					}
 
+					if(time_in_state > 10000) {
+						lcd_set_cursor_pos(0, 0);
+						lcd_send_string("Sleep mode        ");
+						while(lcd_process() == 0); // send everything immediately
+						deepsleep(10);
+						time_in_state = 9500; // run the voltage test again
+					}
+
 					break;
 
 				default:
@@ -598,6 +733,25 @@ int main(void)
 			sentSomething = 1;
 		}
 
+		if((timebase_ms % 500) == 300) {
+			debug_send_string("RTC seconds: ");
+
+			fxp_format_int((RTC_TR >> RTC_TR_SU_SHIFT) & RTC_TR_SU_MASK, msg);
+			debug_send_string(msg);
+
+			debug_send_string(" - ALRAF: ");
+
+			fxp_format_int(RTC_ISR & RTC_ISR_ALRAF, msg);
+			debug_send_string(msg);
+
+			debug_send_string(" - EXTI17: ");
+
+			fxp_format_int(exti_get_flag_status(EXTI17), msg);
+			debug_send_string(msg);
+
+			sentSomething = 1;
+		}
+
 		if(sentSomething) {
 			debug_send_string("\r\n");
 			sentSomething = 0;
@@ -633,6 +787,11 @@ void tim3_isr(void)
 	}
 }
 
+void rtc_isr(void)
+{
+	exti_reset_request(EXTI17);
+}
+
 void hard_fault_handler(void)
 {
 	while (1);