Hi, I need some clarity with reading PortB on say a pic 16F873. The PortB is set up with RB0-3 set as inputs and RB4-7 as outputs. PortB light pull up resistors are enabled. A command is issued: clrf PortB nop movf PortB,W. What is the value now held in W?

 

Bits 0-3 will be the value of the input pins. Bits 4-7 will be zero. If there is nothing connected to the inputs, PORTB will read 0x07.

 

Bits 0-3 will be the value of the input pins. Bits 4-7 will be zero. If there is nothing connected to the inputs, PORTB will read 0x07.

You meant 0x0F of course for the 4 LS input bits.

 

You meant 0x0F of course for the 4 LS input bits.

Yes, I did. My only excuse is that it past my bedtime

 

Thanks guys, so if I understand correctly, the output pins will read lo because they are cleared by the clrf PortB command - the pull-ups won't effect the result, while the other pins will read the hi of the pull-up resistors?

 

Yep

 

Hi guys, some more questions, if I may. I'm battling to understand the relationship between a port that has half it's pins set as output with the other half as input and the reading of the port pins. The circuit has Port B connected to a 4x3 keypad, RB0-2 for columns and RB4-7 connected to rows. Again using the example in my first question: The PortB is set up with RB0-3 set as inputs and RB4-7 as outputs. PortB light pull up resistors are enabled. A command is now issued: movlw %1110 0000 movwf PortB nop movf PortB,W. What is the value in W? Let's say that RB0 is low, i.e. a key was pressed.

 

%11101110
Bits 4-7 will read whatever has been output on those pins.

 

Ok great, and by ANDing d15 with W, will mask off RB4-7.

 

Yup!