This test code in SysTickHandler() should give you a rather
accurate clock with only a few seconds deviation per hour:
#include "serial.h"
#include "sersendf.h"
void SysTick_Handler(void) {
static uint32_t count = 0;
static uint8_t minutes = 0, seconds = 0;
count++;
if ( ! (count % 500)) { // A full second.
seconds++;
if ( ! (seconds % 60)) {
seconds = 0;
minutes++;
}
sersendf_P(PSTR("%su:"), minutes);
if (seconds < 10)
serial_writechar('0');
sersendf_P(PSTR("%su\n"), seconds);
}
[...]
Nico Tonnhofer