5V nRF52833 I2C connection to 1.8V device

[Kaveh.M]

Hi, 

I have connected my nRF52833 chip to a sensor. 

nRF52833 is supplied at 5V. The sensor supply voltage is 1.8V.

My sensor communicates with nRF52833 on I2C bus. I2C bus of sensor is connected to 1.8V by pull up resistors. Does this system work fine or I still need a level shifter for I2C proper functionality? 

See below diagram.

[Vidar Berg]

Hi,

Even if you can get reliable communication, 5v may damage the inputs on the sensor. For comparison, the nRF52833 does not support more than VDD + 0.3v on its GPIO pads (Absolute maximum ratings). But have you measured 5 V on the i2c lines? As you may know, the IO voltage on the nRF is not controlled by the VDDH input but by the output of the REG0 regulator.

The output voltage of REG0 can be adjusted through the non-volatile REGOUT0 register in UICR. It defaults to 1.8v after a full chip erase.

Best regards,

Vidar

[Kaveh.M]

I have not measured anything yet. I am in the design stage. 

I want to know if the architecture that I shared work, and if not, what needs to be added to my design. 

So you mean when I get a bare erased nRF52833 chip, its default I2C bus voltage is 1.8V and my design can work without any issues? 

[Vidar Berg]

I don't see any problems with this design. Just note that you need to select a configuration that supports "VDDH/high voltage mode" for your board design if you want to supply the chip directly from a 5v source.

Reference circuitry

[Kaveh.M]

My design is not working as expected meaning that the voltage on my I2C bus is not 1.8V but it is leaning toward 3V. How can I set I2C bus voltage to 1.8V (or 3V) using  REGOUT0 register in UICR? 

Is there a sample code for it? 

Thanks

[Kaveh.M]

I have used this below line in main()  to set my I2C bus level to 1.8V: 

NRF_UICR->REGOUT0 = UICR_REGOUT0_VOUT_1V8;

But still seems my I2C line is at 3V. Please note that my processor supply voltage is 3.3V.

[Kaveh.M]

This is a scope capture of my I2C SDA and SCL voltage profiles. As you see my sensor initially operate at 1.8V but then I2C voltage line slowly increases to 3V and then return to 1.8V. The reason for return is that shortly after communication I disable and uninit my I2C line in the code. 

It seems my sensor I2C operate at its 1.8V voltage level, but nRF52833 pulls high to 3V and then release it back. 

How can I prevent this behaviour and keep I2C voltage at 1.8V?

[Vidar Berg]

It's easiest to change the REGOUT0 register with nrfjprog. E.g.,

$ nrfjprog --recover //Erase chip and disable APPROTECT
$ nrfjprog --memwr 0x10001304 --val 0 // Configure output to 1v8

You can also check the current setting with nrfjprog:

$ nrfjprog --memrd 0x10001304

But I'm not sure why you are measuring more than 1.8v on the bus lines. Can you please post your schematic here? 

[Kaveh.M]

I applied :$ nrfjprog --memwr 0x10001304 --val 0 // Configure output to 1v8

and confirmed: $ nrfjprog --memrd 0x10001304

The issue is still there. It seems like I should have used a level shifter between my nrf52833 and sensor chip. 

With my debugger, when I put a breakpoint for the code to stop right after I2C communication the I2C line bus voltages go exactly to 3V. It seems the reason that the voltage is returning back to 1.8V is because I disable and uninit I2C after my communication. 

[Vidar Berg]

I'm not sure how to explain the voltage fluctuations. VDD and the IO voltage should be stable at 1.8V with this setting. Can you please post the schematics for your board?

[Kaveh.M]

I m using BM833 that has nRF52833 chip. I have connected 3.3V DC to its VDD pins 14 and 28. 

I have left VDDH pin open. 

BM833.pdf

[Vidar Berg]

This means your board is configured to run in normal voltage mode with the REG0 regulator disabled (see Main voltage regulators). I assumed you had configured the board to run in high voltage where VDD is supplied from the REG0 output.

To make the module run in high voltage mode, you need to connect your supply to VDDH, not VDD. Same as shown for Config. 2 in the Reference circuitry chapter of the PS.