diff --git a/GPS/GPS.bin b/GPS/GPS.bin
index e2a9967..f688aec 100755
Binary files a/GPS/GPS.bin and b/GPS/GPS.bin differ
diff --git a/GPS/GPS.c b/GPS/GPS.c
index cecc433..e5991e8 100644
--- a/GPS/GPS.c
+++ b/GPS/GPS.c
@@ -26,6 +26,7 @@
 #define GPS_endline() do{GPS_send_string((uint8_t*)"\r\n");}while(0)
 #define U(arg)  ((uint8_t*)arg)
 
+gps_status GPS_status = GPS_WAIT;
 
 void GPS_send_string(uint8_t *str){
 	while(*str)
@@ -48,35 +49,43 @@ uint8_t *ustrchr(uint8_t *str, uint8_t symbol){
 	return NULL;
 }
 
-/*
-// Check checksum
-int checksum(uint8_t *buf){
+uint8_t hex(uint8_t n){
+	return ((n < 10) ? (n+'0') : (n+'A'-10));
+}
+
+/**
+ * Check checksum
+ */
+int checksum_true(uint8_t *buf){
 	uint8_t *eol;
-	char chs[3];
-	uint8_t checksum = 0;
-	if(*buf != '$' || !(eol = (uint8_t*)ustrchr((char*)buf, '*'))){
-		DBG("Wrong data: %s\n", buf);
+	uint8_t checksum = 0, cs[3];
+	if(*buf != '$' || !(eol = ustrchr(buf, '*'))){
+		DBG("Wrong data: ");
+		DBG(buf);
+		DBG("\n");
 		return 0;
 	}
 	while(++buf != eol)
 		checksum ^= *buf;
-	snprintf(chs, 3, "%02X", checksum);
-	if(strncmp(chs, (char*)++buf, 2)){
-		DBG("Wrong checksum: %s", chs);
-		return 0;
-	}
-	return 1;
-}*/
+	++buf;
+	cs[0] = hex(checksum >> 4);
+	cs[1] = hex(checksum & 0x0f);
+	if(buf[0] == cs[0] && buf[1] == cs[1])
+		return 1;
+#ifdef EBUG
+	cs[2] = 0;
+	P("CHS, get ");
+	P(buf);
+	P(" need ");
+	P(cs);
+	usb_send('\n');
+#endif
+	return 0;
+}
 
 void send_chksum(uint8_t chs){
-	void puts(uint8_t c){
-		if(c < 10)
-			fill_uart_buff(USART2, c + '0');
-		else
-			fill_uart_buff(USART2, c + 'A' - 10);
-	}
-	puts(chs >> 4);
-	puts(chs & 0x0f);
+	fill_uart_buff(USART2, hex(chs >> 4));
+	fill_uart_buff(USART2, hex(chs & 0x0f));
 }
 /**
  * Calculate checksum & write message to port
@@ -135,23 +144,51 @@ uint8_t *nextpos(uint8_t **buf, int pos){
 */
 /**
  * Parse answer from GPS module
+ *
+ * Recommended minimum specific GPS/Transit data
+ * $GPRMC,hhmmss,status,latitude,N,longitude,E,spd,cog,ddmmyy,mv,mvE,mode*cs
+ * 1    = UTC of position fix
+ * 2    = Data status (V=navigation receiver warning)
+ * 3    = Latitude of fix
+ * 4    = N or S
+ * 5    = Longitude of fix
+ * 6    = E or W
+ * 7    = Speed over ground in knots
+ * 8    = Cource over ground in degrees
+ * 9    = UT date
+ * 10   = Magnetic variation degrees (Easterly var. subtracts from true course)
+ * 11   = E or W
+ * 12   = Mode: N(bad), E(approx), A(auto), D(diff)
+ * 213457.00,A,4340.59415,N,04127.47560,E,2.494,,290615,,,A*7B
  */
 void GPS_parse_answer(uint8_t *buf){
 	uint8_t *ptr;
 	DBG(buf);
-	if(strncmp(buf+3, U("RMC"), 3)) return; // not RMC message
-	buf += 7; // skip header
-	P("time: ");
-	if(*buf != ','){
-		ptr = ustrchr(buf, ',');
-		*ptr++ = 0;
-		P(buf);
-		buf = ptr;
-		P(" ");
-	}else{
-		P("undefined ");
-		++buf;
+	if(strncmp(buf+3, U("RMC"), 3)){ // not RMC message
+		GPS_send_start_seq();
+		return;
 	}
+	if(!checksum_true(buf)){
+		DBG("Wrong chs\n");
+		return; // wrong checksum
+	}
+	buf += 7; // skip header
+	if(*buf == ','){ // time unknown
+		GPS_status = GPS_WAIT;
+		return;
+	}
+	//P("time: ");
+	ptr = ustrchr(buf, ',');
+	*ptr++ = 0;
+	//P(buf);
+	if(*ptr == 'A')
+		GPS_status = GPS_VALID;
+	else
+		GPS_status = GPS_NOT_VALID;
+	print_curtime();
+	set_time(buf);
+//	buf = ustrchr(ptr, ',');
+//	P(" ");
 	P("\n");
 }
 
diff --git a/GPS/GPS.geany b/GPS/GPS.geany
index d6ec8a5..6f2dce9 100644
--- a/GPS/GPS.geany
+++ b/GPS/GPS.geany
@@ -21,16 +21,17 @@ long_line_behaviour=1
 long_line_column=80
 
 [files]
-current_page=1
-FILE_NAME_0=4732;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuser_proto.c;0;4
-FILE_NAME_1=0;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuser_proto.h;0;4
-FILE_NAME_2=6683;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuart.c;0;4
-FILE_NAME_3=1297;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuart.h;0;4
-FILE_NAME_4=2072;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fmain.c;0;4
-FILE_NAME_5=1364;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fmain.h;0;4
-FILE_NAME_6=949;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2FGPS.c;0;4
-FILE_NAME_7=903;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2FGPS.h;0;4
+current_page=0
+FILE_NAME_0=2419;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuser_proto.c;0;4
+FILE_NAME_1=1153;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuser_proto.h;0;4
+FILE_NAME_2=5725;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuart.c;0;4
+FILE_NAME_3=1269;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fuart.h;0;4
+FILE_NAME_4=925;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fmain.c;0;4
+FILE_NAME_5=1384;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fmain.h;0;4
+FILE_NAME_6=1188;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2FGPS.c;0;4
+FILE_NAME_7=875;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2FGPS.h;0;4
 FILE_NAME_8=2128;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FSTM32%2Fc8t6%2FGPS%2Fhardware_ini.c;0;4
+FILE_NAME_9=2589;C;0;EUTF-8;1;1;0;%2Fhome%2Feddy%2FDocs%2FSAO%2FELECTRONICS%2FGPS%2Fsrc%2Fmain.c;0;4
 
 [VTE]
 last_dir=/home/eddy/Docs/SAO/Cameras/FLI_camera/my/Mytakepic
diff --git a/GPS/GPS.h b/GPS/GPS.h
index 6ce2330..d400864 100644
--- a/GPS/GPS.h
+++ b/GPS/GPS.h
@@ -23,6 +23,14 @@
 #ifndef __GPS_H__
 #define __GPS_H__
 
+typedef enum{
+	 GPS_WAIT       // wait for satellites
+	,GPS_NOT_VALID  // time known, but not valid
+	,GPS_VALID
+} gps_status;
+
+extern gps_status GPS_status;
+
 void GPS_parse_answer(uint8_t *string);
 void GPS_send_start_seq();
 
diff --git a/GPS/hardware_ini.c b/GPS/hardware_ini.c
index 887faab..2ca2dd3 100644
--- a/GPS/hardware_ini.c
+++ b/GPS/hardware_ini.c
@@ -32,9 +32,17 @@
  * GPIO initialisaion: clocking + pins setup
  */
 void GPIO_init(){
-/*	rcc_peripheral_enable_clock(&RCC_APB2ENR, RCC_APB2ENR_IOPAEN |
+	rcc_peripheral_enable_clock(&RCC_APB2ENR, RCC_APB2ENR_IOPAEN |
 			RCC_APB2ENR_IOPBEN | RCC_APB2ENR_IOPCEN | RCC_APB2ENR_IOPDEN |
 			RCC_APB2ENR_IOPEEN);
+	// Setup EXTI on PA4 (PPS input from GPS) - pull down
+	gpio_set_mode(GPIOA, GPIO_MODE_INPUT, GPIO_CNF_INPUT_PULL_UPDOWN, GPIO4);
+	//AFIO_EXTICR2 = 0;
+	exti_enable_request(EXTI4);
+	// trigger on rising edge
+	exti_set_trigger(EXTI4, EXTI_TRIGGER_RISING);
+	nvic_enable_irq(NVIC_EXTI4_IRQ);
+/*
 	// Buttons: pull-up input
 	gpio_set_mode(BTNS_PORT, GPIO_MODE_INPUT, GPIO_CNF_INPUT_PULL_UPDOWN,
 			BTN_S2_PIN | BTN_S3_PIN);
@@ -60,7 +68,7 @@ void GPIO_init(){
  */
 void SysTick_init(){
 	systick_set_clocksource(STK_CSR_CLKSOURCE_AHB_DIV8); // Systyck: 72/8=9MHz
-	systick_set_reload(899); // 900 pulses: 10kHz
+	STK_RVR = 8999; // 9000 pulses: 1kHz
 	systick_interrupt_enable();
 	systick_counter_enable();
 }
diff --git a/GPS/main.c b/GPS/main.c
index c1c2804..49d3fc4 100644
--- a/GPS/main.c
+++ b/GPS/main.c
@@ -25,12 +25,28 @@
 #include "uart.h"
 #include "GPS.h"
 
-volatile uint32_t Ticks = 0; // global ticks (10kHz)
-volatile uint32_t Timer = 0; // global timer (milliseconds)
+volatile uint32_t Timer = 0; // milliseconds
 usbd_device *usbd_dev;
+volatile uint32_t systick_val = 0;
+volatile uint32_t timer_val = 0;
+volatile int clear_ST_on_connect = 1;
+
+curtime current_time = {25,61,61};
+
+void time_increment(){
+	Timer = 0;
+	if(current_time.H == 25) return; // Time not initialized
+	if(++current_time.S == 60){
+		current_time.S = 0;
+		if(++current_time.M == 60){
+			current_time.M = 0;
+			if(++current_time.H == 24)
+				current_time.H = 0;
+		}
+	}
+}
 
 int main(){
-	uint32_t usb_timer = 0;
 	uint8_t *string;
 	// RCC clocking: 8MHz oscillator -> 72MHz system
 	rcc_clock_setup_in_hse_8mhz_out_72mhz();
@@ -59,13 +75,16 @@ int main(){
 		if(usbdatalen){ // there's something in USB buffer
 			usbdatalen = parce_incoming_buf(usbdatabuf, usbdatalen);
 		}
-		if((string = check_UART2()))
+		if((string = check_UART2())){
 			GPS_parse_answer(string);
-		if(Ticks - usb_timer > 9){ // 1ms cycle
-			usb_timer += 10;
-			//usb_send_buffer();
-		}else if(Ticks < usb_timer){ // Timer overflow
-			usb_timer = 0;
+		}
+		if(systick_val){
+			P("Systick differs by ");
+			print_int(systick_val);
+			systick_val = 0;
+			P(", timer value: ");
+			print_int(timer_val);
+			P("\n");
 		}
 	}
 }
@@ -75,15 +94,30 @@ int main(){
  * SysTick interrupt: increment global time & send data buffer through USB
  */
 void sys_tick_handler(){
-	static int T = 0; // 0..10 - for millisecond timer
-	++Ticks;
-	if(++T == 10){
-		usb_send_buffer();
-		++Timer;
-		T = 0;
+	if(++Timer == 1000){
+		time_increment();
+	}
+	usb_send_buffer();
+}
+// STK_CVR - current systick val
+// STK_RVR - ticks till interrupt - 1
+
+// PA4 interrupt
+void exti4_isr(){
+	if(EXTI_PR & EXTI4){
+		// correct
+		systick_val = STK_CVR;
+		timer_val = Timer;
+		if(clear_ST_on_connect){
+			STK_CVR = 0;
+			clear_ST_on_connect = 0;
+			time_increment();
+		}
+		EXTI_PR = EXTI4;
 	}
 }
 
+
 // pause function, delay in ms
 void Delay(uint16_t _U_ time){
 	uint32_t waitto = Timer + time;
@@ -91,8 +125,38 @@ void Delay(uint16_t _U_ time){
 }
 
 /**
- * print current time in milliseconds
+ * set current time by buffer hhmmss
  */
-void print_time(){
-	print_int(Timer);
+void set_time(uint8_t *buf){
+	inline uint8_t atou(uint8_t *b){
+		return (b[0]-'0')*10 + b[1]-'0';
+	}
+	uint8_t H = atou(buf) + 3;
+	if(H > 23) H -= 24;
+	current_time.H = H;
+	current_time.M = atou(&buf[2]);
+	current_time.S = atou(&buf[4]);
+}
+
+
+void print_curtime(){
+	int T = Timer;
+	newline();
+	if(current_time.H < 25 && GPS_status != GPS_WAIT){
+		P("Current time: ");
+		if(current_time.H < 10) usb_send('0');
+		print_int(current_time.H); usb_send(':');
+		if(current_time.M < 10) usb_send('0');
+		print_int(current_time.M); usb_send(':');
+		if(current_time.S < 10) usb_send('0');
+		print_int(current_time.S); usb_send('.');
+	/*	uint32_t millis = STK_CVR * 1000;
+		millis /= STK_RVR;*/
+		if(T < 100) usb_send('0');
+		if(T < 10) usb_send('0');
+		print_int(T);
+		if(GPS_status == GPS_NOT_VALID) P(" (not valid)");
+		newline();
+	}else
+		P("Waiting for satellites\n");
 }
diff --git a/GPS/main.h b/GPS/main.h
index 54c68ba..eda190b 100644
--- a/GPS/main.h
+++ b/GPS/main.h
@@ -25,6 +25,7 @@
 #define __MAIN_H__
 
 #include <stdlib.h>
+#include <libopencm3/stm32/exti.h>
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/usart.h>
 #include <libopencm3/cm3/nvic.h>
@@ -48,8 +49,20 @@ extern void *memcpy(void *dest, const void *src, int n);
 #define U16(x)  ((uint16_t) x)
 #define U32(x)  ((uint32_t) x)
 
+typedef struct{
+	uint8_t H;
+	uint8_t M;
+	uint8_t S;
+} curtime;
+
+extern curtime current_time;
 extern volatile uint32_t Timer; // global timer (milliseconds)
+extern volatile int clear_ST_on_connect; // flag for clearing Systick counter on next PPS
+
 void Delay(uint16_t time);
+void set_time(uint8_t *buf);
+
+void print_curtime();
 
 #endif // __MAIN_H__
 
diff --git a/GPS/uart.c b/GPS/uart.c
index 7e83fbe..0a528d7 100644
--- a/GPS/uart.c
+++ b/GPS/uart.c
@@ -23,7 +23,6 @@
 #include "uart.h"
 #include "cdcacm.h"
 #include "hardware_ini.h"
-#include <string.h> // for memcpy
 
 // Buffers for Tx
 static UART_buff TX_buffer[2]; // Tx buffers for all three ports
diff --git a/GPS/user_proto.c b/GPS/user_proto.c
index aa904ba..c9cfa99 100644
--- a/GPS/user_proto.c
+++ b/GPS/user_proto.c
@@ -40,8 +40,10 @@ intfun I = NULL; // function to process entered integer
 #define READINT() do{i += read_int(&buf[i+1], len-i-1);}while(0)
 
 void help(){
+	P("C\tclear SysTick on PPS\n");
 	P("H\tshow this help\n");
 //	P("I\ttest entering integer value\n");
+	P("S\tSend GPS starting sequence\n");
 	P("T\tshow current approx. time\n");
 }
 
@@ -76,6 +78,9 @@ int parce_incoming_buf(char *buf, int len){
 		command = buf[i];
 		if(!command) continue; // omit zero
 		switch (command){
+			case 'C':
+				clear_ST_on_connect = 1;
+			break;
 			case 'H': // show help
 				help();
 			break;
@@ -83,10 +88,11 @@ int parce_incoming_buf(char *buf, int len){
 				I = show_int;
 				READINT();
 			break;*/
+			case 'S':
+				GPS_send_start_seq();
+			break;
 			case 'T':
-				newline();
-				print_int(Timer); // be careful for Time >= 2^{31}!!!
-				newline();
+				print_curtime();
 			break;
 			case '\n': // show newline, space and tab as is
 			case '\r':