weak/strong pull-up. - Electronic Circuits Projects Diagrams Free

alphacat · 27th May 2009, 08:23 PM

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.

Wp100 · 27th May 2009, 08:42 PM

Think this post will help -

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Noggin · 29th May 2009, 12:15 PM

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.

AtomSoft · 29th May 2009, 03:17 PM

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.

Diver300 · 29th May 2009, 05:19 PM

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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27th May 2009, 08:23 PM	#1
alphacat	weak/strong pull-up. 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.

27th May 2009, 08:42 PM	#2
Wp100	Think this post will help - Jargon Buster [Archive] - Electronic Circuits Projects Diagrams Free

29th May 2009, 12:15 PM	#3
Noggin	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. __________________ A rectangular bear is just a polar bear after a coordinate transform. -- I dunno who. A recent study shows that research causes cancer in rats. -- I dunno who said that one either.

29th May 2009, 03:17 PM	#4
AtomSoft	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. __________________ AtomSofts eBay Store AtomSoftTech: C18 TIPS & TRICKS v9 PDF Nokia 6100 Driver/Software My Name: Jason Lopez http://atomsofttech.info/ \| My YouTube Videos! My Favorite Store: dipmicro Electronics

29th May 2009, 05:19 PM	#5
Diver300	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. __________________ http://nottheboss.wordpress.com/