Instant Interrupts - Revisited

[Darrel Taylor]

I want to augment the speed of blink light by loading a value
in TMR1H and TMR1L but the speed does not change

code:

'********************
TMR1H = %11111111
TMR1L = %11111110
'********************

Loading that value %1111111111111110 = $FFFE = 65534
Means the timer will overflow after only 2 ticks.
A smaller number would help.

Also, it has to be loaded after every interrupt, not just at the beginning of the program.
<br>

[debutpic]

thank's Darrel

Remark:

In my next code if I load TMR1H with a high value code work fine

Main:

TMR1H = %11111111
pause 1

----------------------------

but if I reduce the value in TMR1H code don't work
display is OFF

Main:

TMR1H = %10111111
pause 1

-------------------------------
if I want use this value "TMR1H = %10111111"
I must write "pause 20" and not "pause 1"

an idea ???????

[Darrel Taylor]

No, I have no idea what you are doing.
If you're using a timer, why do you need pauses?

What frequency are you trying to run the timer at?
What frequency is your primary oscillator?
What PIC are you using?
<br>

[debutpic]

I use one 16F877 MCU to 20 Mhz

TMR1 produce an interrupt every 0,4 µSec
for to drive four 7 segment displays (prescaler:1)

My board developpement is EasyPic4

If I write not a Pause after TMR1H %11111111
then the displays don't work

[skimask]

I use one 16F877 MCU to 20 Mhz
TMR1 produce an interrupt every 0,4 µSec

At 4Mhz, each instruction takes 1 uSec to execute.
At 20Mhz, each instruciton takes .2 uSec to execute.
When you take in to account the fact that each jump/return to/from the interrupt handler takes at minimum 4 instructions cycles (2 for the jump into the int handler, 2 for the RETFIE after the handler), which happens to be .8uSec, you have less than ZERO time to do anything.
Even at 40Mhz, you're still at ZERO time left over.
At 48Mhz, you might get one instruction in here and there...
And even at 64Mhz, you probably won't get more than one or two instructions done between interrupts.

[Ioannis]

I think it is better to post the details of your project so that you could get a decent help. And the .4usec to drive a LED display, to me seems too little time.

Ioannis

[skimask]

And the .4usec to drive a LED display, to me seems too little time.

Maybe it's .4uSec for each update and not so much to display...
but like you said, can only help as much as you know in the first place...

[debutpic]

Thank's for your explanation it's clear now
I reduced the value preloaded in TMR1
and he works fine without "pause"