> From the diagram in the datasheet, if the bit is set as an input, the output
> from the latch would be 1.

The TRIS latch, yes. But it would put the ouput driver (the P/N pair)
in Hi-Z, exactly as is stated in the data sheet:

"Setting a TRISA bit (=1) will make the corresponding
PORTA pin an input, i.e., put the corresponding output
driver in a hi-impedance mode."

The upper transistor drawn in the diagram is a "P" type. It won't
act as a weak pull-up if the TRIS bit is 1, as you seem to mean.
Port B pins have weak pull-ups, see PORTB description to see the
difference.

> If nothing is driving the input pin high or low, I
> would think that is the value that would be read. What would be a reason for
> the value to be 0? Doesn't something need to pull the signal low to make it
> read 0?

No reason for it to be 1 either. See above. In input mode, the pin is
simply left "floating". A floating pin is a no-no in the "real world".
It could read anything, and possibly even oscillate in some cases,
with the side effect of consuming more current. That's why it's
important that you connect unused inputs either to ground or Vcc,
depending on the circuit.

In a simulator, they obviously have to choose a fixed state in this
case, since they probably won't simulate a real circuit with all its
complexity. That's why you have to use those simulators with a bit
of caution.

Gary Kato <garykato@aol.com> wrote:
>>gpsim _will not_ let me turn
>>the appropriate bit on in PORTA register  (I can turn on the other bits
>>though, when the appropriate TRISA bits are set to 0)
> 
> Right. If TRISA,0 is set to 1 then MPSIM won't let me set PORTA,0. If TRISA,0
> is 0, then MPSIM allows me to set the value of PORTA,0.
> 
> So, why not just clear TRISA,0 so you can set the value or PORTA,0? You should
> still be able to read  PORTA,0. You just have to remember to change your code
> once you get real hardware.

Hi Gary,

This is precisely what I have done for now.  As well as put in comments to
remind myself later to set the bits.   I suppose I wasn't so confuzzled
after all...  Now if I can get some kind of stimulus system working to
simulate voltage on the pins I'll be gtg :)

Thanks
-v

>> If this were a real PIC and RA0 was not connected to anything, would it
>read 1
>> when RA0 is set as an input?
>
>It would read an undefined value. Might be 1 or 0, who knows?
>

From the diagram in the datasheet, if the bit is set as an input, the output
from the latch would be 1. If nothing is driving the input pin high or low, I
would think that is the value that would be read. What would be a reason for
the value to be 0? Doesn't something need to pull the signal low to make it
read 0?

>gpsim _will not_ let me turn
>the appropriate bit on in PORTA register  (I can turn on the other bits
>though, when the appropriate TRISA bits are set to 0)

Right. If TRISA,0 is set to 1 then MPSIM won't let me set PORTA,0. If TRISA,0
is 0, then MPSIM allows me to set the value of PORTA,0.

So, why not just clear TRISA,0 so you can set the value or PORTA,0? You should
still be able to read  PORTA,0. You just have to remember to change your code
once you get real hardware.

Gary Kato <garykato@aol.com> wrote:
> I don't use gpsim, but the standard MPLAB SIM. MPLAB SIM seems to assume that
> input pins are tied low unless told otherwise by the Pin Stimulator. I tried
> setting RA0 as an output and setting the latch, then setting RA0 as an input
> and RA0 turned off at that point.
> 
> I don't know if gpsim has some way for you to force the value of RA0 to what
> you want when it's configured as an input. MPLAB SIM refuses to do that, but
> then it has the Pin Stimulator.

Hello Gary, 

gpsim allows me to turn bits on and off on various registers, when I
set certain bits on TRISA to 1 (output), gpsim _will not_ let me turn
the appropriate bit on in PORTA register  (I can turn on the other bits
though, when the appropriate TRISA bits are set to 0)

Is this what is happening to you in MPLAB SIM?
-v

goose <ruse@webmail.co.za> wrote:
>> init
>>       bsf STATUS,RP0          ;select bank 1
>>[1]         movlw 0x01              ;set PORTA RA0 pin to input
>>[2]         movwf TRISA
>         ^^^^^^^^^^^^^
> 
> here you set it to be an input. i.e. you can only intelligbly(sp?)
> *read* from it, writing to it will not have any effect.
> 
>>         bcf STATUS,RP0          ;select bank 0
>> 
>>         movlw 0x10              ;set counter to 10
>>         movwf tmp
>>[3]        bsf PORTA,0             ;simulate input on port RA0
>>                               ;this is where I believe I'm
>>                               ;having probelms
> 
> there is no problem. you cannot turn an input on and
> off (high and low) from *within* the chip, you need to
> do it from outside.
> 
> try this:
> 1. bridge RA0 and RA1.
> 2. replace lines [1] and [2] above with this:
>      bsf   TRISA, 1    ;  RA0 is now input
>      bcf   TRISA, 0    ;  RA1 is now output
> 3. replace line [3] with
>      bsf   PORTA, 1    ; this will turn on RA1, which will
>                        ; make RA0 high, which you can then read


Hi goose,

Thanks for the information.  I thought that might have been the cause
but I was uncertain, as I'm still somewhat new to PIC software development

I don't have breadboard components at the moment, and the simulator I am
using (gpsim) is still under development and its features are somewhat
limited.   As per your suggestion, is bridging RA0 and RA1 possible via
some instructions on the PIC?  Or were you thinking about doing this
via a simulator?

Thanks
-v

> main
>         btfsc PORTA,0	
>         decfsz tmp,1            ;countdown from 20 
> 				;this never gets executed as PORTA bit 0 is
> 				;always zero
>         goto main
> end
> 
>

Just for future reference, when 'goto main' is skipped, the 
microcontroller will *not* just stop. Instead it will keep executing the 
  rest of (unwritten) program memory and roll back to the beginning of 
the program. At the and, you should put a

stop
	goto stop

end


Al

> If this were a real PIC and RA0 was not connected to anything, would it read 1
> when RA0 is set as an input?

It would read an undefined value. Might be 1 or 0, who knows?

<foobear@nospam.foobear.net> wrote in message news:<0wnUb.151$FF5.41618@petpeeve.ziplink.net>...
> Hello folks,
> 
> I'm writing a simple program for the 16f84 pic to learn how it works. I
> would like to simulate a switchbutton on pin RA0, that when pressed,
> causes an internal counter to decrement.   My problem seems to be when I
> try to turn on the bits on PORTA RA0 to simulate the pin going high, it
> never gets done.
> 
> I am using the gpasm assembler version 0.10.0 and the gpsim simulator
> version 0.21.1
> 
> Here is the program 
> 
> include p16f84.inc
>         list    p=16f84
>         radix   hex
>         __fuses _CP_OFF & _WDT_OFF
> 
>         org     0x00            
>         goto    init
> 
>       cblock  0x0c
>               tmp
>       endc
> 
> init
>       bsf STATUS,RP0          ;select bank 1
>[1]         movlw 0x01              ;set PORTA RA0 pin to input
>[2]         movwf TRISA
         ^^^^^^^^^^^^^

here you set it to be an input. i.e. you can only intelligbly(sp?)
*read* from it, writing to it will not have any effect.

>         bcf STATUS,RP0          ;select bank 0
> 
>         movlw 0x10              ;set counter to 10
>         movwf tmp
>[3]        bsf PORTA,0             ;simulate input on port RA0
>                               ;this is where I believe I'm
>                               ;having probelms

there is no problem. you cannot turn an input on and
off (high and low) from *within* the chip, you need to
do it from outside.

try this:
1. bridge RA0 and RA1.
2. replace lines [1] and [2] above with this:
      bsf   TRISA, 1    ;  RA0 is now input
      bcf   TRISA, 0    ;  RA1 is now output
3. replace line [3] with
      bsf   PORTA, 1    ; this will turn on RA1, which will
                        ; make RA0 high, which you can then read

hth

goose,
   hand

I don't use gpsim, but the standard MPLAB SIM. MPLAB SIM seems to assume that
input pins are tied low unless told otherwise by the Pin Stimulator. I tried
setting RA0 as an output and setting the latch, then setting RA0 as an input
and RA0 turned off at that point.

I don't know if gpsim has some way for you to force the value of RA0 to what
you want when it's configured as an input. MPLAB SIM refuses to do that, but
then it has the Pin Stimulator.