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Referring to just what the I2C lines draw, am I wrong thinking that the higher the clock frequency the shorter the time there will be (the same amount of) current flowing through the pullups and thus lower power consumed?
side qeustion
I don't think I am going to reach 100 kHz, that's way over the limit of my hardware. I am alternating between about 32 and 4 kHz. Will the same resistor value (3.3k @ 3V) be good for both?
Higher clock frequency usually require lower pull-up value, thus increasing the current.
Increasing the clock frequency from 100kHz to 400kHz usually requires the pull-up to be reduced with a factor of 4-5.
Since the power is inverse proportional to the resistance the power consumed will be almost the same.
The I2C data and clock lines draw power when they get pulled low. Because then power is sunk through the pull-up resistors.
While a line is pulled low it will draw 5V/4.7k\$~\Omega \approx\$ 1mA. Assuming 5V VCC and 4.7k pullup resistors.
The clock line will have a 50% duty cycle. The data line is low at least 1 out of every 9 clock cycles (every ack for a successful byte) but you are rarely going to send/receive only 0xff bytes. It's more likely going to be pulled low 75% of the time.
But indeed faster clock means shorter transmission which means less power lost through the pull-ups. However faster transmission may require lower value resistors to overcome the parasitic capacitance between the lines and ground.
Your thinking is correct, as long as you can achieve a higher speed with the same pull up resistors.
As @ratchet-freak stated, In terms of time, you could have 75% of the time the bus pulled-down, hence, if you increase the clock rate, your consumption by the bus will decrease as long as you have the same value for pull-up resistors. But, at higher speeds, resistor values should be reduced.
Having this, the consumption of the bus will be lower, but slaves and masters devices could increase their consumption depending on the clock rate.
Regarding your side question, if 3.3kohms suits both 4khz and 32khz, you have to check the capacitance of your bus. This capacitance depends on the length of the bus, the distance between lines and the number of devices attached to it. It could be difficult to calculate the real capacitance, but you can check the waveform of your data in the bus at both frequencies and see if there is any distortion of the signal at 32khz using 3.3k.
At high data rate power consumption will be more: - MCU need to work faster - Low-value pullups will be required to get proper rise/falling edge of the pulses
