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MOV PORT, A not working appropriately.

I'm running this code:

;; Test 8 operations
MOV R0,#08
LOOP: MOV A,P2
JB P1.1, L
RR A
JMP CONT
L: RL A
CONT: MOV P2,A
MOV P2,A
DJNZ R0,LOOP
END

And when I execute (MOV P2,A) it just moves to the latch, not to the pins. So that when I read it again I'll a wrong result.

Parents

0 ImPer Westermark over 11 years ago in reply to Eduardo Reis

A classic 8051 have no input/output configuration. The pins are always outputs but the pins have strong drive low, while only weak pull-up to hold them high.

So when using them as inputs you write a one to the bit position. Then external electronics can override the pull-up and force the pin low. So while a write of 0cFF makes all pins usable as inputs, the reading of the port will indicate the actual pin state after external electronics have had a chance to override.

But that also mean that when you write a zero to the port pin, it gets locked as low and will read back as low.

Do you now see an issue with having a loop that multiple times reads and writes the port and tries to shift the result?
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0 ImPer Westermark over 11 years ago in reply to Eduardo Reis

A classic 8051 have no input/output configuration. The pins are always outputs but the pins have strong drive low, while only weak pull-up to hold them high.

So when using them as inputs you write a one to the bit position. Then external electronics can override the pull-up and force the pin low. So while a write of 0cFF makes all pins usable as inputs, the reading of the port will indicate the actual pin state after external electronics have had a chance to override.

But that also mean that when you write a zero to the port pin, it gets locked as low and will read back as low.

Do you now see an issue with having a loop that multiple times reads and writes the port and tries to shift the result?
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0 Eduardo Reis over 11 years ago in reply to ImPer Westermark

Is this pull-up resistor "configuration" important if I'm only running the code on the Keil debugger?

"Do you now see an issue with having a loop that multiple times reads and writes the port and tries to shift the result?"

Not exactly. This is my code/issue:


                ORG             0000h
                JMP             0100h
                ORG             0100h


        MOV     R0,#7

LOOP1:  CLR     C
                MOV             A,P1
                RRC             A
                MOV             P1,A ;This one works perfectly
                DJNZ    R0,LOOP1

                MOV     R0,#7

LOOP2:  CLR     C
                MOV             A,P1
                RLC             A
                MOV             P1,A ;But, for some reason, this one just send the 
;value to the latch
                DJNZ    R0,LOOP2

                END