I2c pull up resistors - Githubissues

PaxInstruments / labwiz-board

A development board based on the STM32 series processors specifically targeting the STM32F103RxT series.

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I2c pull up resistors #68

Closed charlespax closed 7 years ago

charlespax commented 7 years ago

Redo calculations and find what would be a good value. Can we even have a fixed value without knowing the total capacitance on the lines?

~~We could go nuts and use digital pots.~~

Notes

Drew did not have success with 1.5k resistors, but did with 10k resistors
Refer to T400 i2c issue: https://github.com/PaxInstruments/t400-electronics/issues/10
Stack Exchange topic: http://electronics.stackexchange.com/questions/1849/is-there-a-correct-resistance-value-for-i2c-pull-up-resistors
Equations to use to determine i2c resistor range.

charlespax commented 7 years ago

I made a graph showing the pullup resistor range for 100kHz and 400 kHz. i2c

We should keep the pullup resistors values as low as necessary to allow for a larger enough bus capacitance range. This is because the modules affect the bus capacitance.

charlespax commented 7 years ago

I wonder if the internal pullup resistors were enabled on the i2c lines. That would explain why we needed such a high value on the external resistors; the internal and external resisotrs might be running in parallel. Does the STM32F103 even have internal pullup resistors for the i2c lines?

charlespax commented 7 years ago

I'm keeping the 1.5k in the design for now. When revision 4 comes back I will try different values. See i2c pullup resistor test #135