i2c2 device in sleep - how to disconnect SDA and SCL gpio?

tc531

Hi
We need to turn off (float or set to ground) the SDA and SCL lines on i2c2 when we go into a low power state. We can power down nicely, with around 5uA current, but these two GPIO stay high (3.3V) so when a chip is connected, they try to power it rather than let it sleep.

Our devicetree overlay includes the lines

&i2c2_sleep { group1 { psels = <NRF_PSEL(TWIM_SDA, 0, 2)>, <NRF_PSEL(TWIM_SCL, 0, 1)>; low-power-enable; bias-disable; }; };

but despite this the pins still remain set high when the system sleeps.

We’ve tried setting the GPIO lines in code, and this has also had no effect.

Do you know the correct approach to do this?

Thanks, Tim

jaredwolff

Hey Tim

There are 20k pull-ups on the board which keeps it high (I2C requires external pull-ups). It shouldn’t affect sleep modes though.

Are you worried about the pull-ups powering external sensors, etc?

tc531

Hi Jared.. we have our own pull-up resistors which we power from a separate IO line and disable during sleep. We are after a 10 year life from an AA lithium battery so your 20K pullups will kill us. Can we disable yours? Alternatively, where are they so I can hot-air them off? :-)

tc531

Found it - RN5 is the tiny resistor network by pad labelled D5. Hello hot air, bye-bye warranty 😉 Thanks for your help. We’ve only got 10 boards to do, just a first run of some prototype hardware

jaredwolff

tc531 😆 good luck!

tc531

Didn’t have good luck, unfortunately. Having removed the RN5 pull-ups on one board as a test, I discovered that the NRF9151 needs to chat with the PMIC over I2C before main() gets called in order to set up the supply voltage correctly. I have no opportunity to turn on our external pull-ups to allow PMIC comms. We have several exteral I2C chips which need to be powered off during sleep, and leaving SDA and SCL at 3.2V. I will probably need to disconnect the I2C lines with an analog switch in future, unless you have any other ideas?

jaredwolff

Yea if you’re disconnecting an external sensor you’ll need to both control the power and the I2C lines. I’ve done it before.

Make sure if the voltage levels are different, that leakage won’t happen at all the possible voltage levels on each side. Not speak g form experience or anything 😅