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'Port B, pullups'
1996\05\10@135437 by Michael S. Hagberg

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Where in the pic 74 manual does it state what value the "weak pullups"
resisters are on port b.  I though I read they are ~5K, but my brother
thinks they might be ~50K.

thanks

1996\05\10@144242 by Ed VanderPloeg

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    Wow, a genuine PIC question on the PIC mailing list!  This is nice for
    a change.

    All the PIC16C7X databooks I got have the spec in the "DC
    Characteristics" section in the back.  An "older" databook (by
    Microchip standards) which I'm using here at my office #1 is the
    1995/1996 "PIC 16/17 Microcontroller Data Book", see page 2-662.

    The pull up current for Port B is given, not the resistor value, and
    it's listed as 50 uA minimum, 200 uA typical, and 400 uA maximum.

    Note that the lovely standard Microchip phrase #18 applies here:

         "Data in "Typ" column is at 5V, 25 deg C unless otherwise
         stated.  These parameters are for design guidance only and
         are not tested".

    -Ed V.



______________________________ Reply Separator _________________________________
Subject: Port B, pullups
Author:  pic microcontroller discussion list <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> at
InterNet
Date:    5/10/96 11:13 AM


Where in the pic 74 manual does it state what value the "weak pullups"
resisters are on port b.  I though I read they are ~5K, but my brother
thinks they might be ~50K.

thanks

1996\05\15@103408 by Mark Peterson

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At 01:03 PM 5/10/96 GMT, you wrote:
>Where in the pic 74 manual does it state what value the "weak pullups"
>resisters are on port b.  I though I read they are ~5K, but my brother
>thinks they might be ~50K.
>
>thanks
>
>

Your brother was the more correct. As Ed said the 74 pull-ups have a nominal
current of 200uA. In the Embedded Control Handbook AN557 p3-142 under
"Hardware" line 5 states that the pull-ups have a nominal value of 20k.
5v/20k=250uA. MCHIP gives a range of 50uA to 400uA with a disclaimer as Ed
said. The author of AN557 is then safe with a 20k estimate. If I'm doing
this right extrapolating from the 74's datasheet shows corresponding value
range of the weak pull-ups to be 100K (@50uA min spec) to 12.5k(@400uA max
spec) with the typical value of 25k(@200uA typ spec). All at 5v, 25degC,
disclaimer noted. From a design standpoint, both the min and max parameters
need to be accounted for. I havn't used the weak pullups, but I might
someday. I notice that a series resistor of 220ohms is used to limit current
during key depression or LED display (5v/220ohm=23mA). I do know that some
devices don't like the portb pullups, I think I read that LCD's don't. I dunno.

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