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weak/strong pull-up.

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New Member
Hey guys,

I oftenly hear the terms weak pull-up and strong pull-up resistors.
Could you please give me an example when a strong pull-up is needed, and when a weak pull-up is enough?

Thanks for any help.


Weak pullups are often fine for digital inputs, such as a switch a user might push. A strong pull-up may be needed on something like an I2C bus. If there is a fair amount of capacitance on the line, the pullups may take a long time to pull the line back to a logic HIGH which would limit the maximum bus speed. By increasing the strength of the pullup, the bus is pulled up quicker, and the speed can be increased.

As a very fat thumbed rule of thumb, I start with a 10k pullup and work from there. In battery powered applications I'll try to go up to 100k or more if I can get away with it, and in bus communications I've gone to a couple hundred ohms. Cell phones use pullups in the mega-ohm range.


Well-Known Member
Lets take the PIC18F1320 for example. in the datasheet on page 252 you will see all the info you need.

if using a 10k resistor on 5v thats a pull up of 0.0005mA aka 500uA

For the 1320:
PORTB weak pull-up current : 50-400uA

so if you need to make a week pull up for that you have to :

5v / 400uA = 5 / .0004 = 12500 = 12.5k Ohm resistor.

so a 12.5kohm resistor will give you the 400uA to make a Week Pullup.


Well-Known Member
Most Helpful Member
When you use an EEPROM or a temperature sensor on a 1-wire bus, you need a weak pull-up when the processor is talking to the sensor, but a strong pull-up when the 1-wire device is drawing power.
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