TMR1 How it works?

[ngeronikolos]

Thanks Taylor and Happy New Year,

I manage to make my RTC with resolution in 1m, following your instructions.
I test it for 7 hours and I have lost 1sec.That is ok for my project.

Continioing my project I wonder if I can check all the ports 12 TOTAL (PORTA&PORTB) of the PIC16F628 in a time aproximatelly 500μsec?
I am going to use 20Mhz osc do the pic faster.

Please advice
Nikos

[Darrel Taylor]

In 500μs, you can do all kinds of stuff.

Checking 2 ports at 20mhz shouldn't take more than 1-2μs. but I guess it depends on what you want to do with it after that.
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[ngeronikolos]

Taylor,

Thanks for your interested.
So, you said that the following code running in 20mhz take 2μsec:
----------------------------
START:
If PORTA.1 = 1 THEN
GO TO SOMEWHERE1
ENDIF
If PORTA.2 = 1 THEN
GO TO SOMEWHERE2
ENDIF

GOTO START
--------------------------

I need to check all the ports of A&B in a loop.
1 LOOP SHOULD TAKE MAXIM 500μsec.

Please advice
Nikos

[skimask]

Taylor,

Thanks for your interested.
So, you said that the following code running in 20mhz take 2μsec:
----------------------------
START:
If PORTA.1 = 1 THEN
GO TO SOMEWHERE1
ENDIF
If PORTA.2 = 1 THEN
GO TO SOMEWHERE2
ENDIF

GOTO START
--------------------------

I need to check all the ports of A&B in a loop.
1 LOOP SHOULD TAKE MAXIM 500μsec.

Please advice
Nikos

MPLab has a fine, working simulator in it that will count cycles/time for whatever you want. You just have to set breakpoints at the correct places and keep track of the count in the window. And you can check your .lst file to actually count the number of instruction words used by a particular statement.

At any rate,

Clk = 20mhz = 5mhz instruction rate = .2us per instruction

START: = 0 cycles

If PORTA.1 = 1 THEN
GO TO SOMEWHERE1
ENDIF

is roughly equivalent to a BTFSS instruction, 2 cycles

depending on the PIC and options used, checking all bits in Port A and B is 16 checks. Total of 32 cycles just for the checking. Add in 2 cycles for the jump back to the start, 34 cycles.
So, this loop, not including the GOTO SOMEWHEREs, takes about 34 cycles.

But, we haven't including any PBP interrupt checking or CLRWDTs yet. So, assume that you have an ON INTERRUPT statement somewhere, add in another 8 cycles (I think, I'll check when I get home) for each interrupt check, which happens before each PBP statement. Same thing for a CLRWDT.

Total so far = 34 + ( 17 * 8, interrupt check) + ( 17 x 1, CLRWDT ) = 187 cycles. .2us per cycle * 187 cycles = 37.4us per loop not including any goto's. 26738hz loop rate.

Again, if you have any doubts, check your .LST file generated by the assembler and manually count the number of instructions between the start and end of the loop.