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'[PIC]: 16f877 not connecting properly'
2001\03\20@211919 by Roger Duffus

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Hello All,

I have a question about the Pic 16f877. First let me explain the circuit. I
am using the Romzap programmer to program my 877. I have port d pins 4-7
connected to some ir transmitting circuitry(4 identical circuits). The
transmitting circuitry consists of a 1K resistor connected to the base of a
2n4401 transistor and the other end to the port pin. The emitter is grounded
and the collector is connected to 3 ir transmitter leds in series and a 10
Ohm resistor. The problem is that when I try to connect to the Romzap, the
program that I originally burnt on to the chip starts running. Romzap also
says that it is "Connected to Boot loader" but it is not actually connected.
I think that the circuit is draining too much current and is causing a
communication problem with the Romzap(PC). I think this requires some
carefully placed capacitors. Does anyone have any suggestions?

Regards

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2001\03\20@213323 by Tony Nixon

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Roger Duffus wrote:
{Quote hidden}

Hi Roger,

Perhaps take out the 10R resistors leaving the LED open circuited, and
see what happens.

Alternately, recalculate the resistors and connect the LEDs to the
supply rail before the 5V reg.

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Tony

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2001\03\21@082158 by Olin Lathrop

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> Perhaps take out the 10R resistors leaving the LED open circuited, and
> see oABw happens.
>
> Alternately, recalculate the resistors and connect the LEDs to the
> supply rail before the 5V reg.

Another alternative would be to put a resistor between emitter and ground,
and lose the base resistor.  This makes the transistor function as a current
drain instead of a saturated switch.  The drawback of this is that it eats
up about 4.5V.  This is only OK if your unregulated supply is high enough to
drive the LEDs minus 4.5V.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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