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  1. #1
    Join Date
    Jun 2005
    Posts
    52

    Question 16F627A portA4 problem?

    I have recently changed from red LEDs to blue LEDs and encountered
    a strange problem on port A4 on a 16F627A. Port A4 is an open drain output
    connected to the LED cathode, the LED anode is connected to 5 vdc via a 1 K resistor. The red LED works as expected but the Blue LED will not turn off
    unless I use a 10 K pullup around the blue LED. The blue LED has a larger
    forward drop than the red and should have a lower current than the red LED.

    Does anybody have any ideas? Am I missing something?

    I defined my I/O and disabled the comparators

    TRISA = %00001100
    TRISB = %00000010
    CMCON=%00000111 ' Comparators OFF


    IF k = 1 Then
    F = $F8
    low portb.0 'turn on LED 1
    low porta.4 'turn on LED 2
    low porta.6 'turn on LED 3
    Return
    EndIF
    IF k = 2 Then
    F = $FE
    low portb.0 'turn on LED 1
    high porta.4 'turn off LED 2
    high porta.6 'turn off LED 3
    Return

  • #2
    Join Date
    Jul 2003
    Location
    Colorado Springs
    Posts
    4,959

    Default

    Hi Mark,

    This sounds like it might be a "read-modify-write" problem.

    Maybe you could try using INPUT instead of HIGH to turn off PORTA.4.

    <pre><b>INPUT porta.4 'turn off LED 2</b></pre>
    DT

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