Calculate TMR0 reload value at compile time?

[Darrel Taylor]

Henrick,

The "method" looks OK, but I think you'll have some problems with the math due to integer truncation.

    #define nS_per_cycle #v((1000/(OSC/4)))

That line will work ok IF the OSC freq. is an exact multiple of 4mhz.
But if OSC is 10mhz then (OSC/4) is 2.5 and the .5 gets dropped. Then nS_per_cycle would be 500, which is wrong for 10mhz.

Same as with PBP, do the multiplication first to avoid the integer problem.

    #define nS_per_cycle #v(1000*4/OSC)
Of course, they are constants, so you could just (4000/OSC).

The same thing happens with ...
    #define MB_Frame_Timeout_ns #v((100000000/(MODBUS_BAUDRATE / 11)) * 35)

There are no banking issues to deal with in your code.

hth,

[HenrikOlsson]

Darrel,
Thank you!
I can't believe I didn't catch that one. I noticed the truncation, I even have it display the calculated results yet I never noticed the ns per cycle number being way off...

I don't think the error in the timeout calculation would have ill effect in real life but I changed it to:
#define MB_Frame_Timeout_ns #v((100000000 * 11 / MODBUS_BAUDRATE) * 35)

Thanks, as always it does help a lot.

/Henrik.