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I'm trying to use SMT1 on a PIC18F45K42 to count cycles of a square wave on pin RB0. I can't get the counter to increment, not sure what I'm doing wrong. If I understand correctly, SMT1TMR should be incrementing but it's not. (I also checked SMT1TMRL, etc, directly and it's not changing).

1) I am trying to do a normal counter, not gated, so I'm not using the Window signal at all (I don't want to have to use it, I just want to zero the counter from time to time then check to see how many square cycles have arrived).

2) I have registers set as follows (pic below) according to the paused debugger in MPLAB X. I am putting a scope probe directly on the pin and I can see the square wave is arriving. I can also pause the debugger occasionally to read PORTB and see PORTB.0 is changing between high and low, so I believe it is being received.

3) I'm playing with square waves from 20 Hz to around 400 Hz created from a function generator.

I have attached an image of the registers. Here are the values for reference:

SMT1SIGPPS  0x08 (should be RB0)
SMT1CON0  0x80
SMT1CON1  0xC8
SMT1STAT   0x05
SMT1SIG    0x00
TRISB   0xE3
PMD6   0x17  (SMT1MD is 0, which should be "not disabled")

Any suggestions much appreciated. This seems like it should be so simple and straightforward.

Thank you.

enter image description here

kjav
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1 Answers1

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I figured it out. The key is in data sheet 25.1.2 Period Match Interrupt. The Period register has to be set to longer than the counter will run. It defaults to 0, so the counter couldn't increment. Fixed it by manually loading the 3 period registers with max value... added the following to my ini code, seems to be working as expected now.

SMT1PRU = 0xFF;     //set max period for SMT1 so counter doesn't roll over
SMT1PRH = 0xFF;
SMT1PRL = 0xFF;
kjav
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