Post by thread:Port B, pullups

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_OUTPICLISTTakeThisOuTMITVMA.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

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.