NRF52832 TWI at 50 KHz (SMBus Compatibility)

[Greg Nowak]

I am running the NRF52832 (BMD-300 module) and I'm trying to talk to battery over SMBus. SMBus is compatible with devices up to 100 KHz.

I was wondering if there was a way to talk to an I2C device on the NRF52832 with a lower clock rate than the 100 KHz rate given in the TWI library.

I see here that the frequency is not configurable beyond 3 predefined clock rates, 100K, 250K, and 400K. infocenter.nordicsemi.com/index.jsp

What I'm experiencing is inconsistent readings between my battery and board. I have about a 60% success rate when reading. The battery manufacturer stressed that I need to be SMBus compliant, not just I2C. They suggested that I use a slower clock rate than 100 KHz.

I was wondering if anyone out there knew if it was possible to use a slower clock rate for I2C communication using the NRF52832?

Thanks,

Greg

[RK]

you could just try halving the value in the frequency register and seeing if it works. 0xCC0000 would be the value to try. No guarantees of course but worth the 10 minutes to test.

[MartinBL]

Hi,

The formula for calculating the frequency vs the register values is more complicated than one might think. This is partly due to how the algorithm handles rounding errors, and partly because it relies on how you configure the TWI. In other words, it is not as simple as just dividing a fixed master clock frequency with a register value. Furthermore, if you read the TWIM's electrical specifications you will see that it is specced between 100k and 400kHz. However, this doesn't mean that it wont work at all outside these boundaries, but Nordic has not tested it and therefore gives no guarantees. That being said, halving the register values as RK suggest will probably get you close to 50kHz (I haven't tried), but if you do this you are doing so at your own discretion.

[studzinski]

You can always use software TWI coding using GPIO levels with all frequencies you want... In cuch case you define your clock. It seems that in earlier SDK there is such library...

[RK]

do you mean bitbang it? With the softdevice running that's not going to work, this is discussed very often, bitbanging things with any kind of timing requirement is often tried and almost always fails.

If you didn't mean that, what did you mean?

[studzinski]

I do not know the meaning of bitbang term. Since years I use bit coding for I2C of LIS accelerometer (I generate the clock with one GPIO levels and read value on other data GPIO). However, it is true, I did not tested it on Nrf52832.