timer: Replace screen timeout with SysTick

This provides an incredibly accurate screen timeout and removes the
superfluous screen timeout counter

firmware/layout2.c

@@ -28,6 +28,7 @@
 #include "string.h"
 #include "util.h"
 #include "qr_encode.h"
+#include "timer.h"
 
 void *layoutLast = layoutHome;
 
@@ -81,6 +82,9 @@ void layoutHome(void)
 		}
 	}
 	oledRefresh();
+
+	// Reset lock screen timeout
+	system_millis_lock = system_millis + SCREEN_TIMEOUT_MILLIS;
 }
 
 const char *str_amount(uint64_t amnt, const char *abbr, char *buf, int len)

firmware/trezor.c

@@ -38,15 +38,12 @@ void __attribute__((noreturn)) __stack_chk_fail(void)
 	for (;;) {} // loop forever
 }
 
-static uint32_t saver_counter = 0;
-
 void check_lock_screen(void)
 {
 	buttonUpdate();
 
 	// wake from screensaver on any button
 	if (layoutLast == layoutScreensaver && (button.NoUp || button.YesUp)) {
-		saver_counter = 0;
 		layoutHome();
 		return;
 	}
@@ -82,15 +79,11 @@ void check_lock_screen(void)
 
 	// if homescreen is shown for longer than 10 minutes, lock too
 	if (layoutLast == layoutHome) {
-		saver_counter++;
+		if (system_millis >= system_millis_lock) {
-		if (saver_counter > 285000 * 60 * 10) {
 			// lock the screen
 			session_clear(true);
 			layoutScreensaver();
-			saver_counter = 0;
 		}
-	} else {
-		saver_counter = 0;
 	}
 }
 

timer.c

@@ -26,6 +26,9 @@
 /* 1 tick = 1 ms */
 volatile uint32_t system_millis;
 
+/* Screen timeout */
+uint32_t system_millis_lock;
+
 /*
  * Initialise the Cortex-M3 SysTick timer
  */

timer.h

@@ -23,6 +23,11 @@
 /* 1 tick = 1 ms */
 extern volatile uint32_t system_millis;
 
+/* Screen timeout */
+extern uint32_t system_millis_lock;
+
+#define SCREEN_TIMEOUT_MILLIS (1000 * 60 * 10) /* 10 minutes */
+
 void timer_init(void);
 
 void sys_tick_handler(void);