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In my current project, I need a microcontroller to retrieve the temperature of 60 temperature sensors which are located on the same PCB with distances not exceeding 50 cm. The temperature sensor's reference is MAX30205. It communicates using I2C and offers 32 different I2C addresses.

I was thinking of using three I2C buses (which is the maximum with my microcontroller), and putting 20 sensors in each bus.

I do not have big contrains regarding the bus clock, as I will only need to get the temperature of sensors from time to time, so we can suppose I will use I2C in its standard mode (speed = 100 kHz).

The MAX30205 will be supplied at 3 V, meaning that the minimum possible pull-up resistors value is 1 kohm (because I2C spec dictates a maximum current of 3 mA on SCL and on SDA). Given the number of sensors, I will choose R as small as possible, so maybe around 1.1 kohm.

The MAX30205 has an input capacitance of 3 pF, and the microcontroller has an input capacitance of 5 pF, meaning each bus will have at least a bus capacitance of 103 pF. The maximum capacitance for standard mode I2C is 400 pF. There is a remaining bus capacitance due to PCB and traces.

Is it possible to estimate it ? Can it be close to 300 pF ? Do you think I need an I2C buffer ?

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TI's application report Effect of Parasitic Capacitance in Op Amp Circuits says:

Circuit traces on a PCB with a ground and power plane will be about 1−3 pF/in.

100 in ≈ 2.5 m


If you still have doubts about the rise times, consider using an I²C accelerator like the LTC1694, or a buffer like the TCA9803.

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