Setting timer 1 with #asm code

delphy_boy · Joined: 15 Sep 2008 Posts: 8

Hi,

I am using a PIC18F6722 with compiler V4.008.

I am trying to use timer 1 to overflow at at half its normal count by preloading it with 0x8000 every time the timer 1 isr is triggered. So as not to introduce count errors i want to just set the msb of TMR1H using an asm instruction. The code is as follows but it does not load the value. Any ideas why the asm instruction does not work?

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

I suggest to consult the PIC18 manual. You have to disable 16 Bit accesses to read or write TMR1H allone. (RD16 = 0).

By the way, there is no need to use assembler code. You can perform the same operations in C code when defining the respective registers with #byte.

Regards,
Frank

Ttelmah · Guest

Generally, it is much easier to just use a bit define.
It is worth doing the define, even for the assembler version (define the timer high register, and use this by name), since then the assembler will automatically handle things like bank switching if needed (not applicable in your case).

delphy_boy · Joined: 15 Sep 2008 Posts: 8

Thanks guys, i will give this a go.

Much appreciated

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Compiler version 4.008 is bad and should never have been released. Save everybody a lot of time and revert to v3.249 or upgrade to a newer version (the v4 compiler started to work for the old features around the mid v4.03x versions). See the sticky 'V4 comments' thread for more info.

Why not use the C function setup_timer_1? I don't see why assembler would be preferred over this function. Am I missing something?

A much larger problem are the:
- non-constant delays introduced by other interrupts being active at the same time.
- The 40-something instruction time delay before the interrupt function is entered.
To work around these problems you could read the timer1 and add your desired offset:

Ttelmah · Guest

The 'idea' of just setting the top bit, is quite sensible, and potentially requires only one instruction, rather than having to read/modify/write the whole 16bit value, and involve quite a lot of instructions.
However the hardware in part 'intervenes', so in 8 bit mode, updating the top register, will still reset the counter prescaler, and the compiler by default uses the registers in 16bit mode, requiring the extra overhead of changing modes as well.
I must admit, if I was wanting the 'best' performance, I'd probably switch the counter to 8bit mode permanently, and not use the internal timer functions at all, once the clock was running.
Agree wholeheartedy with the comments about 4.008. The odds of getting any program to actually work 'right' with this, are low...

Best Wishes