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'[PIC]: Pull ups/ pull downs'
2002\01\02@191454 by Benjamin Bromilow

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Hi all,

Can I just ask about pull-ups/pull downs? I've searched the datasheets and I
can't find anything which makes sense to me!
I've got a 16F877 connected via one of the I/O pins to a 16F84. Both the '84
and the '877 use these pins to receive and transmit data but not at the same
time- the '877 sends serial data to the '84 which (when it has received 12
bits of data) makes it's input into an output and puts it high for 30
cycles. I know the '84 is getting the data because it's acting upon it (I'm
using it as a LED driver). However, when I turn on the error correction /
feedback system it (ie the feedback) doesn't work.
My hardware is as follows: In case both tried to become outputs at the same
time, I've put a 500 ohm resistor between the two pins (limits short circuit
current to 20mA). I've also used a 2000 ohm pull-down to avoid a floating
output.
They're both running at 4Mhz from crystal oscillators. The feedback pulse
stays high for 20 cycles and I'm testing for it in the middle of this cycle
period so if it is there I should be able to sense it.....
The hardware looks something like this (but with solder splashes- kidding :)
!)

[877 Port B0] =====  [ 500 ohm ] ===== [84 Port B0]
                                                           I
                                                       2000 ohm
                                                           I
                                                           I
                                                       GROUND

The only difference I see is that the 877 has effectively a 2500 ohm
pull-down while the 84 has a 2000 pull-down resistance. However, I've used
2000 ohm pull-downs before and they've worked fine so I can't imagine it's
that.... and I know the 2500 pull-down is not causing problems...
Any ideas??

Ben
ps Sadly I haven't got a scope!!
pps Would pull-ups make a difference??

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2002\01\02@193544 by Barry Gershenfeld

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>Ben
>ps Sadly I haven't got a scope!!
>pps Would pull-ups make a difference??

That bit with the resistors is probably good enough to start with,
though I am sure you will get many suggestions.

Happily, you are the programmer, and if you have to, you can
create very large delays and make your program run very slowly
and then you can use a voltmeter instead of a scope.  This
will also let you see if you guesses with the resistors
give you good enough results at each pin.

Barry

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2002\01\02@195413 by Benjamin Bromilow

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> Happily, you are the programmer, and if you have to, you can
> create very large delays and make your program run very slowly
> and then you can use a voltmeter instead of a scope.

That's a very good point :)

Ben

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