#define F_CPU 72000000UL
#define F_CPU_MHZ 72
#define SYSTEM_TICK_NS ((1 /F_CPU_MHZ) * 1000UL)

uint32_t GetPrescaleForDelay(uint32_t delay, char * units)
{
uint32_t prescale;
uint32_t multiplyer;
uint32_t ticks;

if(strcmp(units,"ns")==0)
{ multiplyer = 1; }
else if(strcmp(units,"us")==0)
{ multiplyer = 1000; }
else if(strcmp(units,"ms")==0)
{ multiplyer = 1000000; }
//else
//{ return 0; }

//how many ticks needed for delay
// if(SYSTEM_TICK_NS > 0) //не помогает
ticks = (delay * multiplyer) / SYSTEM_TICK_NS;

prescale = F_CPU / ticks;
return prescale;
}