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Author Topic: Increasing I2C pull-ups via additional resistor?  (Read 546 times)
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Hi, I am working on an old project at the moment that had some issues and I think I have found that the reason is weak pull up resistors on the SCL & SDA lines.  Now, one I2C sensor is not accessible and has 1K pull ups on the board that I cannot get at to change.  Am I right in thinking that I cannot simply add a couple more pull up resistors, even higher value ones, where I can get at the circuit as this will simply reduce the resistance as they would be in parallel?
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the reason is weak pull up resistors on the SCL & SDA lines.  Now, one I2C sensor is not accessible and has 1K pull ups on the board that I cannot get at to change.

A weak pull-up is a pull-up resistor of very high value (50k). 1k pull ups are very strong pull-ups.

Adding a weak pull-up (high value resistor) to a 1k pull-up will not make it weak; it will make it (marginally) stronger.
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Ah - that would be my lack of understanding of what exactly a pull up resistor is doing  smiley-red  Thanks for clarifying, I need to go a read some more reading I think.

I guess I'm expecting the pull up value needs to be closer to 4K7 rather than 1K - heance the pondering on how to make it so.
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It would not make much of a difference having 1K as opposed to 4K7.
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I guess I'm expecting the pull up value needs to be closer to 4K7 rather than 1K

Typically 4.7k, though I have used 220 - 10k.
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It would not make much of a difference having 1K as opposed to 4K7.

The main thing is there are typically limits on how much various I2C devices can sink.
In the olden days of yore, it was commonly 3 mA, so the lowest pullup-R value allowable
was about 1.7K.
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Thanks for your help folks - from what you have said it is most likely not the pullups so the hunt goes on.
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