Shiftin on 16F628 changes input A.2 to output?

[RuudNL]

Hello,

I have a strange thing going on here...
I try to decode serial information (shift register) to 2 seven-segment displays.
This is the code:

 DEFINE OSC 8
 CMCON = 7
 TRISA = %11111100             ' PORTA.0 and PORTA.1 = Outputs (Anodes Displays)
 TRISB = %00000000             ' PORTB = Outputs (Cathodes Displays)

 W  Var Word                   ' Variable for ShiftIn 
 PORTB = 255                   ' All Cathodes High, display off
 PORTA.0 = 0                   ' Anode Low, display off 
 PORTA.1 = 0                   ' Anode Low, display off

' PORTA.2 = Clock in
' PORTA.3 = Data in
' PORTA.4 = Latch in
 
 MainLoop:
   ShiftIn PORTA.3, PORTA.2, 0, [W\16]  
 WaitLoop:                     ' Wait for the Latch to go High
   If PORTA.4 = 0 then Waitloop  
   PORTB = (W & 255) XOR 255  ' PORTB = Lowest 8 Bits
   PORTA.0 = (W >> 8) & 1     ' PORTA.0 = Anode display
   PORTA.1 = (W >> 8) & 2     ' PORTA.1 = Anode display
 Goto MainLoop

The strange thing is that PORTA.2, although configured as input, is changed to an output!
If a comment the 'ShiftIn' out, PORTA.2 stays an input.
I am sure I am overlooking something, but what????

[Acetronics2]

Hi,

Might be you are somewhat mixing "Shiftin" and "Serin" commands ...
just have a look to manual $ 5.74 to be convinced.

Alain

[RuudNL]

I have the manual in front of me and I definitely don't want SerIn!
I am not dealing with serial data (as with a COM-ports), but I am trying to emulate a shift register.
But even if there was confusion, there is still no explanation why an input would change into an output!
Should I leave the TRISA out and let ShiftIn handle the port status?

[HenrikOlsson]

Hi,
You are asking it to use PortA.2 as the clock output during the SHIFTIN

ShiftIn PORTA.3, PORTA.2, 0, [W\16]

See, PortA.3 is the data input, PortA.2 is the clock output, so it has to change from input to output or the SHIFTIN wouldn't work.

You can't use SHIFTIN to move data from an external device where that device provides the clock. With SHIFTIN the PIC provides the clock the external device.

/Henrik.

[RuudNL]

OK, so that is the problem!
I thought this was a way to emulate a shift register, but I didn't realise that ShiftIn was generating the clock, instead of accepting a clock!