# Sensor on I2C bus

We are using the Sensor. The sensor communicates with the MCU via I2C. Now, I2C specifications provision the max bus capacitance of 400pF. So, this dictates the no of slaves on my I2C bus. The loading capacitance of the said sensor is as shown below -

It states 400pF. So I can only have a max of just 1 sensor ? Is it so ?

It appears they mean this parameter $C_b$ to be the capacitance due to other loads on the SDI and SCK lines. You'll see that just below the table you showed, they define

$C_b$ = bus capacitance on SDA line

which isn't very helpful, but then on the next page in Table 33, they give different timing parameters for different values of $C_b$, implying that it's something coming from outside the device rather than a property of the device itself.

There is a picture of the actual I/O structure (very simplified) in section 8.2

For typical drivers / receivers of this type, pin and ESD diode capacitances normally dominate.

I would estimate about 15pF per pin (to be safe, it is likely to be lower). Track capacitance is about 1.1pF / inch on FR-4 with a track width of 4 thou and 4 thou to plane. Do not forget to add track and via capacitance to the calculation.

So you could have quite a few of these and I2C should work fine. As ThePhoton noted, the capacitance in table 32 is the maximum permissible on the total bus.

[Update to add note on interfacing with wires]

You can calculate the effective capacitance of twisted pair at this question.

• Thank you so much. And if I am using wires, say 24 gauge. How can I calculate the capacitive loading please ? – Board-Man Jul 21 '16 at 10:17