Richard Seriani, Sr. wrote:
> ----- Original Message ----- 
> From: "Moses McKnight" <moses@skytex.net>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Thursday, August 30, 2007 10:15 PM
> Subject: [PIC] - I/O question on PIC16F818
> 
> 
>> Howdy,
>>
>>    I've got a question about the operation of an I/O pin that seems a
>> little funny to me.  For background, I'm using BoostC
>> (www.sourceboost.com), and a PIC16F818 running at 8Mhz on the internal
>> oscillator.
>>    First I'm using pin B5 as an output, pulling it low to turn on a
>> segment in a 7-segment display.  Then I change the pin to an output and
>> look for a high which would indicate that a switch connected to the same
>> line has been pressed.  Here's the code:
>>
>> //portb = 0b10011110;
>> trisb = 0b01100001;
>>
>> nop();
>> nop();
>> if (1 == portb.5) // SW1 - button 1
>> {
>> button_pressed = 1;
>> }
>>
>> I removed the debounce code for debugging.  What this does is read a
>> high every time unless I add/uncomment the first line pulling the pin
>> low first.  Why is this?  There are no caps on the line coming in to the
>> pin to store a charge - is there something else I'm missing on how the
>> PIC works?
>>
>> Thanks,
>> Moses
>> -- 
> 
> Moses,
> 
> Something my professor tried to get through my old, thick skull: You WRITE 
> to the LATCH and READ from the PIN.
> 
> You wrote a high to the LATCH on RB5, which was then latched to the RB5 pin.
> You next read the PIN, which is still latched high.
> 
> It seems as though you are correct that you have to set that pin low (clear 
> it) before making it a switch input if you expect it to be low when you read 
> it (switch not pressed).
> 
> Please see section 9, specifically section 9.10.2, Successive Operations on 
> an I/O Port, in the PICMicro Mid-Range MCU Family Reference Manual for more 
> information.

Thanks for the information!  It's making more sense to me now.  I guess
I expected the charge on the pin/line to magically go away when I
changed the pin to high impedance.  Duh!

I'll read that section of the manual again.

Thanks,
Moses

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