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?

up vote 5 down vote accepted

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.

  • How about my side question ? – kellogs Nov 9 at 14:46
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    @kellogs 32KHz is really slow for I2C. Depending on the capacity of the bus you can probably use 10k or more. Use a scope to see the rising edge of SCL and SDA to determine the value of the pull-up. SDA must be able to rise from low to high in the low period of SCL. – Peter Karlsen Nov 9 at 17:06

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.

  • It also takes energy to charge and discharge the parasitic capacitance. I think another factor that affects power consumption is active time percentage of the bus. – Long Pham Nov 9 at 14:34
  • How about my side question ? – kellogs Nov 9 at 14:46
  • The power drawn charging and discharging the capacitance would (assuming you do not change the circuit and that your clock is slow enough for this happen to effective completion) be determined by the number of clock cycles but not by the clock rate. – Chris Stratton Nov 9 at 15:05
  • I'm curious about the 75% of the time value. I would have said about 50% (or better 56, including the ack), since all the values are equally probable. 75% means that the average number of zeros per byte is between 5 or 6; was yours a pessimistic estimation or 75% is the actual statistical value? – frarugi87 Nov 9 at 16:40
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    @frarugi87 pessimistic estimation. Based on the habit of using low numbers for addresses of various control registers and the values they take. So I guestimated that the top 3 bits are very often just 0. – ratchet freak Nov 9 at 17:02

Your thinking is correct, as long as you can achieve a higher speed with the same pull up resistors.

  • How about my side question ? – kellogs Nov 9 at 14:46
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    @kellogs cannot answer that as I have no idea what your lines capacitance is. So I can only tell you that we are running 100 kHz with 100 kOhm resistors in one of our products with no problems. I would guess that you are fine. – Arsenal Nov 9 at 14:55
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    any way to guesstimate it ? – kellogs Nov 9 at 15:02
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    @kellogs well, 10 pF for any pin connected to the bus, 50 pF per meter for the length of the line would be a conservative guess I think. If your I²C bus is on a single PCB I have a hard time to imagine why it wouldn't work with 3k3 pull up resistors. – Arsenal Nov 9 at 15:07

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.

  • How about my side question ? – kellogs Nov 9 at 14:46

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