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someone help me
PostPosted: Sun Feb 22, 2004 11:29 am     Reply with quote
should pins on port C have pull down resistors when interfacing with say a 74hc00. if no pullup or downs are used could it affect the speed of the switching of the pins?

i am asking because the pic at 20mhz wont sample right,

please give a guy some advice
dyeatman



Joined: 06 Sep 2003
Posts: 1934
Location: Norman, OK

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Pullups
PostPosted: Sun Feb 22, 2004 11:50 am     Reply with quote
Offhand, no, Port C does not need pullups or pulldowns.

If you try to change read/write at the port too quick you run into the read-modify-write issue on the PIC. This is not a CCS C problem but the way the port hardware on the PIC works.

Post the section of code where you set up Port C then 10 or so lines around where you try to sample it so we can see what you are trying to do.

Also include the PIC model and CCS C version and whether you are using a debugger, if so which one etc.
jds-pic2
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Re: pullup resistors on high speed circuits
PostPosted: Sun Feb 22, 2004 9:42 pm     Reply with quote
newbie wrote:
should pins on port C have pull down resistors when interfacing with say a 74hc00. if no pullup or downs are used could it affect the speed of the switching of the pins?

i am asking because the pic at 20mhz wont sample right,

please give a guy some advice


at 20MHz you should definitely be using pull-up/downs and not depending upon the PIC's internal on-die resistors. in general, the faster the circuit is, the lower the pullup/pulldown value should be. those 100K-ish "weak" pullups on some PIC ports are fine for low speed stuff, saving you a few cents in your circuit. but at your rates you should be using *maximum* 2K for pullups and pulldowns, and perhaps even lower values. otherwise you could end up with all kinds of problems due to slow rise times and ringing on your I/O. if you want clean 20MHz signals, tie some 470ohm resistors on your I/O. power up, and take a look at the waveforms with a 'scope. then, to reduce power consumption, replace the 470's with 1K's and look again at the waveforms. if everything still looks good, continue on. ps, make sure your 'scope probe is not contributing to Mr Heisenberg's fund.

and, please, as a convenience to everyone else and simply for common courtesy -- use a descriptive subject line. this way, when others are trying to solve similar problems, they can find your post in the archives. your subject line is simply a plea for help and provides no information whatsoever regarding the content.

jds-pic
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