# Master included as device on I2C bus?

Hi When calculating the pull up resistor value using the formula R = 50 / d, where d is the number of devices on the bus, is the master considered a bus device?

for e.g. If I have a micro controller master and two slave devices on the bus should this count as a total of 3?

For extra credit: Am I using the right formula? if I use 3 as number of devices this tells me I should use 16k6 pull-ups, which seems high... Ta

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At what (bit) frequency do you want to run that I2C bus? –  Telaclavo Apr 17 '12 at 12:12
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## 1 Answer

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):

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"function of bus capacitance" - Which would be a related to the number of devices surely? Ta –  Toby Apr 17 '12 at 12:15
"in function of bus capacitance" and speed. –  Telaclavo Apr 17 '12 at 12:16
@Telaclavo - Yes, shown in the graphs, but not in the answer. I'll edit it. –  stevenvh Apr 17 '12 at 12:23
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