Inputs are "sinking" voltages?

[kevj]

Hey all - I'm moving right along figuring this stuff out thanks to all your help but I hit another unknown today.

I set a pin as an input, but when I feed it an input (say a +5v square wave pulse) it is "sinking" it. I view it with the scope and it just reads ground, but if I break the connection to the pin on the PIC, the line is showing the square wave. Touch the PIC pin again and it goes flat again on the scope.

The pin I'm using is RA1 on a 16F688. I was originally trying to use RA2, saw this problem, then saw in the data sheet in the pinout description that RA2 is listed as a schmitt trigger, and not TTL. So I tried to use RA1 instead as it is specifically listed as a TTL input.

I've also tried running a simple code to toggle a probe pin up and down and that's not working either - as if for some reason that pin just won't see my input at all.

Here is my code:

define OSC 20             'Set PBP Osc clock to 20Mhz
TRISA = 1                   'All A port I/O's as inputs
ANSEL = %00000000     'Turn off analog input for ports, set digital I/O
CMCON0 = %00000111   'Turn off comparators (CMCON0.0:2 = 111)
TRISC = 0           'All C port I/O's as outputs

mysignal VAR PORTA.1     'signal on RA1

probe VAR PORTC.3         'o-scope probe RC3 
LOW probe

mainloop:
  if mysignal = 1 THEN
    HIGH probe
    ENDIF
  IF mysignal = 0 THEN
    LOW probe
    ENDIF
  GOTO mainloop
END

This same sort of example has seemed to work on other PIC's but it just won't work here. I'm confused and assuming there is some additional register I need to configure that I'm missing.

Thanks!

[sinoteq]

Your A.1 in-port is not set as input. You use PortA.1 right?? Look at this:

TRISA=%00000 'This is all A-pins to output == Decimal 0
TRISA=%00001 This makes A.0 an INPUT == Decimal 1
TRISA=%00010' This will make A.1 an INPUT == Decimal 2
TRISA=%11111 'All input== Decimal 31


Get the point?

Sometimes it is better to use Binary when setting the registers because then you "see" the number better when you compare with the datasheet.

M

[mackrackit]

No ANSEL on this chip. The ADC is controlled with ADCON0.

ADCON0=7 ' NO ANALOG

[sinoteq]

The code is for 16F688 right? Then ANSEL is the way to go and no other way.
Datasheet page 36
M

[mackrackit]

My mistake Been a long night.

[kevj]

Your A.1 in-port is not set as input. You use PortA.1 right?? Look at this:

TRISA=%00000 'This is all A-pins to output == Decimal 0
TRISA=%00001 This makes A.0 an INPUT == Decimal 1
TRISA=%00010' This will make A.1 an INPUT == Decimal 2
TRISA=%11111 'All input== Decimal 31

Doh! Yes, I think I see it now. In my mind I was thinking just assigning dec 1 would make all the registers 1 - a short way of saying %11111111 which is of course not correct.

I get it now. Knew it had to be some silly oversight like that.

The code is for 16F688 right? Then ANSEL is the way to go and no other way.
Datasheet page 36
M

So then my code on the ANSEL is correct?

ANSEL = %00000000     'Turn off analog input for ports, set digital I/O

I thought that was pretty self explanatory in the data sheet but I could still have it wrong.

Thank you!