View Full Version : pullup resistors value recommendation?

07-22-2009, 01:18 PM
I want to pull up resistors for the following scenarios:
1) to interface to CMOS open collector outputs (10khz range) pull up to 3.3V
2) to interface to TTL open collector logic (pull up high value to +5V)

Any suggestions on a value? I've seen 1k or 4.7k as common.


Nick Mueller
07-22-2009, 02:58 PM
> Any suggestions on a value? I've seen 1k or 4.7k as common.

You can have anything between 1k to 10k or even 1M.
It's just a question of speed and noise immunity.
You'll always be on the save side if you look at how much current the device can sink/source and stay at about 30..50% of that.
The rest is simple math: U = R * I


Never use force, just go for a bigger hammer!

The DIY Digital-Readout for mills, lathes etc.:
YADRO (http://www.yadro.de)

07-23-2009, 08:49 AM
It is always a good idea to keep resistor values below 20K for noise reasons. I use
10K for pull ups and 4.7K series resistors for 5V to Propeller input interface. The I2C
signals are an exception, I use 2.2K pull ups for fast signal transitions.

Phil Pilgrim (PhiPi)
07-23-2009, 08:52 AM
Any pullup from a Prop pin to +5V will have to be 2.2K or greater. This is to accommodate the 500uA maximum forward current through the Prop's body diodes.