Value counter Timer

david guilleman

Intellectual 450 points

Hello,

I use example dmTimerCounter and I see in comment 700ms between two interrupt.

I don't see prescaler in program.

I see:

#define TIMER_INITIAL_COUNT (0xFF000000u)
#define TIMER_RLD_COUNT (0xFF000000u)

Can you explain me how to calculate 700ms with 0xFF000000 and generaly how to calculate interrupt time with count load value.

Thanks

over 12 years ago

0 qxc over 12 years ago

Genius 5820 points

The timer counts up from 0xFF000000 to 0xFFFFFFFF means it counts for 0x00FFFFFF (equal to 16777215). The resulting time can be calculated by dividing the counter clock by the count value 16777215.

Since you did not mention which hardware you are using I can't tell you what clock value you have to use for calculation. But for a clock of e.g. 20 MHz the resulting timer value would be about 1,2 sec.

0 qxc over 12 years ago in reply to qxc

Genius 5820 points

Sorry, my mistake: for a frequency of 20 MHz a timer tick is about 0,05 usecs long, multiplied with 16777215 you get a time of about 0,84 sec.

0 david guilleman over 12 years ago in reply to qxc

Intellectual 450 points

I'm on starter kit EVMSK335x so it's 720Mhz.

So for 16777215 I have 23 ms if I understand.

Thanks

0 qxc over 12 years ago in reply to david guilleman

Genius 5820 points

The clock is not necessarily the master clock of the CPU, M_CLK_OSC (which is used for DMTimer when no prescaler is active like in DMTimer example) normally is smaller.

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Value counter Timer