Hi Roberto,

    You MUST use a resistor between RA4 and the base of your transistor...
why? just make a little equation:

    V(RA4) = Ib * Rb + Vbe

    you have Rb = 0, so you are forcing V(RA4) = Vbe (usually 0.7 V)

    Better now?
    I Hope this helps...

    Regards,

    Gonzalo

----- Mensaje original -----
De: "RoDelta" <rodelta@WNET.COM.BR>
Para: <PICLIST@MITVMA.MIT.EDU>
Enviado: Jueves 4 de Enero de 2001 12:52 PM
Asunto: [PIC]: strange output port.


| Hi.
| I tryed  to use  PIC16C621A, RA4 as output, open drain. I put a 10K
resistor
| to Vcc and the base of a npn transistor in RA4 to drive a relay (relay
| conected on colector with diode self induction protection, and emitter to
| zero).
| When the port come hight imediately drop to zero and sit there.
| I saw that, when the output (in  RA4)drop to any value
| <Vdd/2, by external circuit,  this output become to zero and stay in zero
| even the value of the
| external circuit apply in RA4 return to any value > Vdd/2.
| I made a test.
| The output is high, and I force pin 3 ( RA4 ) with a
| jump to zero. The current is limited by 10K resistor to Vdd. When I remove
| this jump the output stays in zero ! Note, by the program this output is
| high all the time .
| The same test in a PIC16F84 this do not happen (no comparator sets!). The
| output come to zero with a jump to zero and return to high when remove
this
| jump.
| Anyone knows why this happen ?
| I didn't find nothing in data sheet (16C621A) about it.
| My sets:
|  banc1                                       ;macro to bank 1
|  MOVLW  b'00000010'           ;Place option reg setup into W
|  MOVWF  OPTION_REG       ; Set up OPTION register
|  MOVLW   b'00000011'           ; RA0 and RA1 as intput RA2,3 and 4 as
output
|  MOVWF  TRISA                      ;
|  banc0                                        ;macro to bank 0
|  MOVLW  b'00000111'            ;07h comparator off
|  MOVWF  CMCON
|  MOVLW  B'10100000'            ; int only to tmr0
|  MOVWF  INTCON
| VRCON, I left by default. (b'00000000')
|
| Did I forgot any set?
| TIA
| Roberto Locks Guimaraes
|
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|
|

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