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This question already has an answer here:

I'm connecting a PIC18F46K22 (master) to a PIC18F4620 (slave) using I2C. The wire between the devices is ~30cm. I'm using 400kHz without slew rate control. Later on, I'll add another 18F4620 on ~5cm from the master (~35cm from the other slave).

How do I calculate the pull-up resistance needed on both the SCL as the SDA line?

Does it matter where I place the pull-up (near the master, near the slave, both, in the middle, ...)?

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marked as duplicate by nidhin, Daniel Grillo, Dwayne Reid, Passerby, Scott Seidman Mar 21 '15 at 15:42

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The pull-ups are generally located near the master device.

This app note goes into some of the calculations. Essentially, the size of the pull-up will be defined by the total bus capacitance and the pull-down capabilities of the devices on the bus:

\$ R_p \ge \dfrac{V_{dd}-V_{OL}}{I_{OL}}\$

and

\$ R_p \le \dfrac{300ns}{0.847298 \times C_{bus}}\$

where \$V_{OL}\$ is the logic low voltage needed, \$I_{OL}\$ is the sinking current, and \$C_{bus}\$ is the bus capacitance.

Of course, once your sample is built, the best thing to do is scope the waveforms and make whatever adjustments are needed.

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When the line is pulled low, the driver pin (SCL on master, or SDA on master or slave) supplies current V/R to do that; so if it's a 10k resistor from 5V, the driver will need to sink 0.5 mA. A larger resistor means less current is needed, but the line will pull up more slowly (especially on a long line) and this may limit your rate. A smaller resistor pulls up faster, but at some point you'll burn out the driver pin (check the data sheet). 3.3K to 10K is probably fine; if it's 3.3V maybe 2.2K-4.7K

It doesn't matter much where you put them electrically, but near the master is better if the cable might be disconnected; or you could put e.g. a 10k pullup at each end, instead of a single 4k7, which will keep both ends happy when the cable is disconnected.

Generally, for cable lengths to +-20cm, SCL at 100kHz will be happy with 10k. With increasing frequency and/or length, you may go as low as 1k.

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