diff --git a/main.c b/main.c
index 3e71856..feb050a 100644
--- a/main.c
+++ b/main.c
@@ -49,9 +49,12 @@
 /** @defgroup main_flags flag set in interrupts to be processed in main task
  *  @{
  */
-volatile bool rtc_internal_tick_flag = false; /**< flag set when internal RTC ticked */
+volatile bool rtc_tick_flag = false; /**< flag set when RTC ticked */
 /** @} */
 
+#define SQUARE_WAVE_PORT B /**< port connected to RTC DS1307 square wave output */
+#define SQUARE_WAVE_PIN 0 /**< pin connected to RTC DS1307 square wave output */
+
 /** user input command */
 static char command[32] = {0};
 /** user input command index */
@@ -199,32 +202,42 @@ void main(void)
 	}
 #endif
 
-	// setup internal RTC
-	printf("setup internal RTC: ");
-	rtc_auto_awake(RCC_LSE, 32768-1); // ensure internal RTC is on, uses the 32.678 kHz LSE, and the prescale is set to our tick speed, else update backup registers accordingly (power off the micro-controller for the change to take effect)
-	rtc_interrupt_enable(RTC_SEC); // enable RTC interrupt on "seconds"
-	nvic_enable_irq(NVIC_RTC_IRQ); // allow the RTC to interrupt
+	// disable internal RTC
+	printf("disable internal RTC: ");
+	rcc_periph_clock_enable(RCC_PWR); // enable power domain clock
+	rcc_periph_clock_enable(RCC_BKP); // enable backup domain clock
+	pwr_disable_backup_domain_write_protect(); // enable write on backup domain (including RTC)
+	RCC_BDCR &= ~RCC_BDCR_RTCEN; // disable RTC
 	printf("OK\n");
-	// display uptime
-	uint32_t ticks_time = rtc_get_counter_val(); // get time from internal RTC (since first start/power up)
-	printf("uptime: %u.%02u:%02u:%02u\n", ticks_time/(60*60*24), (ticks_time/(60*60))%24, (ticks_time%(60*60))/60, (ticks_time%60)); // display uptime
 
 	// setup external RTC
 	printf("setup external RTC: ");
 	rtc_ds1307_setup(); // setup external RTC module
+	// enable square wave output and configure input interrupt
+	rtc_ds1307_write_square_wave(1); // enable 1 Hz square wave output to sync on seconds
+	rcc_periph_clock_enable(RCC_GPIO(SQUARE_WAVE_PORT)); // enable clock for GPIO
+	gpio_set_mode(GPIO(SQUARE_WAVE_PORT), GPIO_MODE_INPUT, GPIO_CNF_INPUT_FLOAT, GPIO(SQUARE_WAVE_PIN)); // set button pin to input
+	rcc_periph_clock_enable(RCC_AFIO); // enable alternate function clock for external interrupt
+	exti_select_source(EXTI(SQUARE_WAVE_PIN), GPIO(SQUARE_WAVE_PORT)); // mask external interrupt of this pin only for this port
+	exti_set_trigger(EXTI(SQUARE_WAVE_PIN), EXTI_TRIGGER_FALLING); // trigger on falling edge of square wave (this is also when the RTC register are updated)
+	exti_enable_request(EXTI(SQUARE_WAVE_PIN)); // enable external interrupt
+	nvic_enable_irq(NVIC_EXTI_IRQ(SQUARE_WAVE_PIN)); // enable interrupt
 	printf("OK\n");
+
+	// display date
+	uint8_t* rtc_ds1307_time = rtc_ds1307_read_time(); // get time/date from external RTC
+	uint8_t rtc_seconds = 0; // count the number of seconds passed and sync time on minute
+	if (rtc_ds1307_time==NULL) {
+		printf("could not get time from DS1307\n");
+	} else {
+		rtc_seconds = rtc_ds1307_time[0]; // remember seconds of minute
+		printf("current date: 20%02u-%02u-%02u %02u:%02u:%02u\n", rtc_ds1307_time[6], rtc_ds1307_time[5], rtc_ds1307_time[4], rtc_ds1307_time[2], rtc_ds1307_time[1], rtc_ds1307_time[0]);
+	}
 	// verify is external RTC is running
 	if (rtc_ds1307_oscillator_disabled()) {
 		printf("/!\\ RTC oscillator is disabled: the battery may be empty\n");
 		rtc_ds1307_oscillator_enable(); // enable oscillator again
 	}
-	// display date
-	uint8_t* rtc_ds1307_time = rtc_ds1307_read_time(); // get time/date from external RTC
-	if (rtc_ds1307_time==NULL) {
-		printf("could not get time from DS1307\n");
-	} else {
-		printf("current date: 20%02u-%02u-%02u %02u:%02u:%02u\n", rtc_ds1307_time[6], rtc_ds1307_time[5], rtc_ds1307_time[4], rtc_ds1307_time[2], rtc_ds1307_time[1], rtc_ds1307_time[0]);
-	}
 
 	// setup TM1637 and MAX7219 7-segments displays
 	printf("setup 7-segment displays: ");
@@ -306,26 +319,23 @@ void main(void)
 			}
 			button_flag = false; // reset flag
 		}
-		while (rtc_internal_tick_flag) { // the internal RTC ticked
-			rtc_internal_tick_flag = false; // reset flag
-			led_toggle(); // toggle LED (good to indicate if main function is stuck)
-			ticks_time = rtc_get_counter_val(); // copy time from internal RTC for processing
+		while (rtc_tick_flag) { // the external RTC ticked
+			rtc_tick_flag = false; // reset flag
 			action = true; // action has been performed
-			if ((ticks_time%(60))==0) { // one minute passed
-				printf("uptime: %u.%02u:%02u:%02u\n", ticks_time/(60*60*24), (ticks_time/(60*60))%24, (ticks_time%(60*60))/60, (ticks_time%60)); // display uptime
-			}
-			rtc_ds1307_time = rtc_ds1307_read_time(); // get time/date from external RTC
-			if (rtc_ds1307_time==NULL) {
-				printf("could not get time from DS1307: resetting\n");
-				rtc_ds1307_setup(); // resetting periph
-			} else {
-				if (!led_tm1637_time(rtc_ds1307_time[1], rtc_ds1307_time[0])) {
-					printf("could not set time on TM1637\n");
+			led_toggle(); // toggle LED (good to indicate if main function is stuck)
+			rtc_seconds++; // increment number of seconds passed
+			if (rtc_seconds>=60) { // one minute passed
+				rtc_ds1307_time = rtc_ds1307_read_time(); // get time/date from external RTC
+				if (rtc_ds1307_time==NULL) {
+					printf("could not get time from DS1307: resetting\n");
+					rtc_ds1307_setup(); // resetting periph
+				} else {
+					rtc_seconds = rtc_ds1307_time[0]; // get actual number of seconds
+					if (0==rtc_ds1307_time[1] && 0==rtc_ds1307_time[2]) { // new day arrived
+						led_max7219_number(20000000+rtc_ds1307_time[6]*10000+rtc_ds1307_time[5]*100+rtc_ds1307_time[4], 0x14, 1); // display date on 2nd display
+					}
 				}
 			}
-			if (!led_tm1637_on()) { // switch on display
-				printf("could not switch on TM1637\n");
-			}
 		}
 		if (action) { // go to sleep if nothing had to be done, else recheck for activity
 			action = false;
@@ -335,9 +345,9 @@ void main(void)
 	} // main loop
 }
 
-/** @brief interrupt service routine called when tick passed on RTC */
-void rtc_isr(void)
+void EXTI_ISR(SQUARE_WAVE_PIN)(void)
 {
-	rtc_clear_flag(RTC_SEC); // clear flag
-	rtc_internal_tick_flag = true; // notify to show new time
+    exti_reset_request(EXTI(SQUARE_WAVE_PIN)); // reset interrupt
+    rtc_tick_flag = true; // perform button action
 }
+