diff --git a/ispprog.c b/ispprog.c
index 8eb8713..5433941 100644
--- a/ispprog.c
+++ b/ispprog.c
@@ -28,6 +28,12 @@
 #include "target.h"
 #include "uart.h"
 
+#define TIMER_IRQFREQ_MS        10
+
+/* convert milliseconds to timer ticks */
+#define TIMER_MSEC2TICKS(x)     ((x * F_CPU) / (TIMER_DIVISOR * 1000ULL))
+#define TIMER_MSEC2IRQCNT(x)    (x / TIMER_IRQFREQ_MS)
+
 #define EV_NONE                 0x00
 #define EV_STATE_ENTER          0x01
 #define EV_BUTTON_PRESSED       0x02
@@ -95,7 +101,8 @@ static void reset_statemachine(uint8_t events)
                     /* remove all events */
                     events = EV_NONE;
 
-                    timer = 0; /* stop timer */
+                    /* stop timer */
+                    timer = TIMER_MSEC2IRQCNT(0);
 
                     spi_init(0);
 
@@ -122,7 +129,7 @@ static void reset_statemachine(uint8_t events)
                 {
                     events &= ~(EV_STATE_ENTER);
 
-                    timer = 1; /* timeout 10ms */
+                    timer = TIMER_MSEC2IRQCNT(10);
 
                     /* put device in ISP mode */
                     RESET_ACTIVE();
@@ -153,7 +160,7 @@ static void reset_statemachine(uint8_t events)
                 {
                     events &= ~(EV_STATE_ENTER);
 
-                    timer = 5; /* timeout 50ms */
+                    timer = TIMER_MSEC2IRQCNT(50);
 
                     /* put device in RUN mode */
                     RESET_INACTIVE();
@@ -598,7 +605,7 @@ static void cmdloop(void)
 ISR(TIMER0_OVF_vect)
 {
     /* restart timer */
-    TCNT0 = TIMER_RELOAD;
+    TCNT0 = 0xFF - TIMER_MSEC2TICKS(TIMER_IRQFREQ_MS);
 
     static uint8_t prev_pressed;
     if (ISP_CHECK())
diff --git a/target.h b/target.h
index 2012402..ee1269c 100644
--- a/target.h
+++ b/target.h
@@ -8,9 +8,8 @@
  * Fuse H: 0xDA (512 words bootloader, jtag disabled)
  * Fuse L: 0xFF (ext. Crystal)
  */
-#define F_CPU           7372800
+#define F_CPU           7372800ULL
 #define BAUDRATE        115200
-#define TIMER_RELOAD    (0xFF - 72)    /* 10ms @7.3728MHz */
 
 #define ISP_RESET       PORTB1        /* to target */
 #define ISP_LED         PORTB3        /* low active */
@@ -36,6 +35,7 @@
 
 #define USE_DISPLAY     0
 
+#define TIMER_DIVISOR   1024
 #define TIMER_INIT()    {   /* timer0, FCPU/1024, overflow interrupt */ \
                             TCCR0 = (1<<CS02) | (1<<CS00); \
                             TIMSK = (1<<TOIE0); \
@@ -54,9 +54,8 @@
  * Fuse H: 0xDC (512 words bootloader)
  * Fuse L: 0xE2 (internal osc)
  */
-#define F_CPU           8000000
+#define F_CPU           8000000ULL
 #define BAUDRATE        115200
-#define TIMER_RELOAD    (0xFF - 78)     /* 10ms @8MHz */
 
 /* trim internal oscillator to get "good" baudrate */
 #define OSCCAL_VALUE    0x80
@@ -96,6 +95,7 @@
 #define DISP_D5         PORTD6
 #define DISP_D6         PORTD7
 
+#define TIMER_DIVISOR   1024
 #define TIMER_INIT()    {   /* timer0, FCPU/1024, overflow interrupt */ \
                             TCCR0B = (1<<CS02) | (1<<CS00); \
                             TIMSK0 = (1<<TOIE0); \