capacitancei2c
What is a reasonable value to use as the capacitance a I2C device adds to a bus if so is not stated in the datasheet?
With this value I intend on calculating the pull-up resistors to eventually use in a PCB.
I need to append to the bus the following devices:
Three. Electrically the master controls the bus in exactly the same way as the slaves.
I'm not sure where your formula comes from. The spec states that pull-up resistors are to be chosen in function of bus capacitance and speed(page 55):
I2C will allow you to add as many as there are unique addresses available for the device dependent on line conditions. If you need more addresses than available then you will need to use a I2C hub or multiple I2C buses.
SPI allows as many devices as you can provide nSS lines for, again based on line conditions. A multiplexer such as the '138 can help here.
Regardless of which you pick, the more devices you have the slower you may have to run the clock in order to allow enough slew on the clock and data lines.
Best Answer
Since none of the three of those datasheets give any details on data line capacitive loading, design around a total bus capacitance of 400pF (the maximum allowed within the I2C spec). Obviously, once you have a prototype PCB up, use a scope to check the signal waveforms and adjust the pullups appropriately.