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russk2txb
Joined: 30 Mar 2008
Posts: 109
Location: New Jersey
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| Extra clock cycle |
 Posted: Fri Dec 22, 2017 9:56 am |
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I am using a USB Master dev kit (18F67J10) to generate a fast clock. The clock must run for 68 microseconds. Here is the code: | Code: | .................... // set up timer 1 to interrupt in 68 us. At 40 Mhz, timer1 tick is .8 us, so it takes 85 ticks for 68 us.
.................... // T1 is a 16 bit timer so the preload value is 65536 - 85 = 65451
.................... set_timer1 (65451);
00E10: SETF FCF
00E12: MOVLW AB
00E14: MOVWF FCE
.................... gb68us = 0;
00E16: MOVLB 3
00E18: CLRF xA2
.................... do {
.................... output_low (PIN_F4);
00E1A: BCF F97.4
00E1C: BCF F8E.4
.................... output_high (PIN_F4);
00E1E: BCF F97.4
00E20: BSF F8E.4
.................... }
.................... while (gb68us == 0);
00E22: MOVF xA2,F
00E24: BZ 0E1A
.................... disable_interrupts (int_timer1);
00E26: BCF F9D.0
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The global variable gb68us is an int8 and is set to one by the timer1 interrupt handler.
It works very well except for one thing. After it is done, a single extra clock pulse is generated, 11 microseconds after the last normal cycle. Or it may be that the last pulse is delayed for 11 us. I set a breakpoint on the disable_interrupts line and the extra pulse is still generated (or delayed). I cannot figure out why.
Can anyone give me a clue as to what could cause this?
Thanks, Russ |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19545
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| Posted: Fri Dec 22, 2017 12:16 pm |
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That's about the time it'd take to get into and out of the interrupt handler. Probably the delay....
Leave the interrupt disabled.
| Code: |
disable_interrupts(INT_TIMER1);
clear_interrupts(INT_TIMER1);
do
{
output_low (PIN_F4);
output_high (PIN_F4);
}
while (!interrupt_active(INT_TIMER1));
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Interrupts are _slow_. They give you a response, but not for a significant time. Testing the flag tells you when it triggers
A disabled interrupt can still set, it just can't call a handler. |
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russk2txb
Joined: 30 Mar 2008
Posts: 109
Location: New Jersey
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| Posted: Fri Dec 22, 2017 4:33 pm |
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Thanks Ttelmah. You always have great answers. That worked just right. I am really not sure exactly how the interrupt delay got in the way of processing that last pulse on time, but it is obvious that it did. I had already tried not turning off the interrupt and it made no difference. But success is what is important here.
Thanks again, Russ |
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