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Topic: I2C transfer rates using Wire library. (Read 13404 times) previous topic - next topic

WanaGo

Thanks Nick

So 1K may not be the best choice then...

My boards are 80mm x 80mm, each with a Adum1250 I2C isolator on them (http://www.analog.com/static/imported-files/data_sheets/ADUM1250_1251.pdf), and the I2C signals are just going board to board - which are all plugged on top of each other. So the longest I2C wire would be say 100mm long before it goes to an Adum1250 isolator. The backbone between the Adum1250's however may be 400mm in length, but there are no devices attached directly to this, only via the Adum1250's.

Whether or not that makes sense, hopefully it does.

So in summary, should I be putting something like 2.2K on both sides of the Adum1250, to achieve the 'best' speed and stability?

Thanks

nickgammon

That sounds reasonable, but bear in mind what I am doing is largely theory, backed up with a few measurements. If possible, try a test rig, and be prepared to tweak the operating frequency. I note this from page 10 of their datasheet:

Quote

However, the input logic low threshold at Side 2 is designed to be at 0.3 VDD2, consistent with I2C requirements. The Side 1 and Side 2 pins have open-collector outputs whose high levels are set via pull-up resistors to their respective supply voltages.


That appears to suggest that pull-up resistors that pulled the low level higher than 300mV would be too much (ie. too low ohms). So that would rule out the 470 ohm idea.

Also I note from what you appear to be describing that you will have multiple I2C circuits, so you would need to consider them separately, to an extent. My tests are done at 5V, I assume you will be using 5V and not 3.3V? Page 1 of the datasheet says:

Quote
30 mA current sink capability


... so it seems we are within spec there.


It sounds to me you are in the ball park, probably with the 2.2K. And as you can see from my pictures, a fairly wide range was tolerated, and indeed I often run I2C (eg. to LCD screens) only using the internal pull-ups, so you probably have a bit of room to move.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

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