Read Modify Write problem?

[markedwards]

I am writing to a 7segment LED display with a 16F627A and believe I might be experiencing a Read Modify Write problem, some segments will not turn off.
If I use a pullup resistor on the output the segments will turn off. It seems like the output pin is being read as an input.

I turned off analog comparators and set TRISA = %00001100 and
TRISB = %00000010.

Any suggestions how should I modify controlling the outputs?
X0 is my variable that has the segment pattern and I write
each output pin. Hardware serial port uses portB.1 & portB.2
and prevents writing a word to the entire portB.


PORTB.0 = X0.0 ' Send segments to LED
PORTA.4 = X0.1
PORTA.6 = X0.2
PORTB.3 = X0.3
PORTB.4 = X0.4
PORTB.5 = X0.5
PORTB.6 = X0.6
PORTB.7 = X0.7

Thanks,

Mark

[Darrel Taylor]

Hi Mark,

Instead of trying to set the PORT's either high or low, set the PORT to the active ON state, then set the TRIS bit to INPUT(1) or OUTPUT(0) to control the LED's. No R-M-W that way.
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[markedwards]

Thanks Darrel,

Is this what you meant?
Variable X0 is the segment pattern and is sent to the output ports.
TRISB. I use OR to maintain portB.1 as input
TRISA. I use OR to maintain PortA.2 and PortA.3 as inputs
Is there an better approach?



'initialize ports
TRISA = %00001100 'configure port A outputs = 0
TRISB = %00000010 'configure port B outputs = 0

Thanks,

Mark

'write segments to ports A and B
PORTB.0 = X0.0 ' Send segments to LED 0 = ON
PORTA.4 = X0.1
PORTA.6 = X0.2
PORTB.3 = X0.3
PORTB.4 = X0.4
PORTB.5 = X0.5
PORTB.6 = X0.6
PORTB.7 = X0.7

TRISB = X0 OR %00000010 ' OFF segments = 1 (set to inputs)
X1=X0 AND %01010000 ' isolate X1= PortA.4 and PortA.6 bits
TRISA = X1 OR %00001100 'maintain PortA.2 and PortA.3 inputs

[Darrel Taylor]

uh, nope.

I was thinking something more like this.

PORTA = 0
PORTB = 0

'write segments to ports A and B
TRISB.0 = X0.0 ' Send segments to LED 0 = ON
TRISA.4 = X0.1
TRISA.6 = X0.2
TRISB.3 = X0.3
TRISB.4 = X0.4
TRISB.5 = X0.5
TRISB.6 = X0.6
TRISB.7 = X0.7
<br>

[markedwards]

Thanks Darrel,

That is much cleaner! I will give it a try.

Mark