I2C SDA Ack half voltage

[Axel Astrom]

Hello,

Im using a NRF52840 custom made board and im having some issue with the I2C communication. There are 3 units on the I2C line, the MCU and two sensors, the diagram illustrates the setup:



My problem is that during startup when communication starts with AS3935 the SDA line goes bad. Every Ack bit from the sensor becomes half the voltage for some unknown reason. A screen capture from a oscilloscope shows the problem, see below:

Every ninth bit results in 1.5V when VDD voltage actually is 3.0V. 

When searching for this specific problem i have found that this problem can be related to the configuration of the I2C pins. More specific that they are not configured as Open-drain.

I am using the Zephyr stack and have dived deep down and found that the configuration of the SDA pin is : 

#define TWIM_PIN_INIT(_pin) nrf_gpio_cfg((_pin),                     

                                         NRF_GPIO_PIN_DIR_INPUT,     

                                         NRF_GPIO_PIN_INPUT_CONNECT, 

                                         NRF_GPIO_PIN_PULLUP,        

                                         NRF_GPIO_PIN_S0D1,          

                                         NRF_GPIO_PIN_NOSENSE)

Does this configuration of the SDA pin results in open-drain?

Have anyone else experienced this problem and if so, what was causing the problem?

The addresses for the sensors are correct and I have tried different pull ups with same result. The internal pull-up (13k) are used and accounted for.

[awneil]

You seem to have a rather slow rise on SCL - suggest the pullups are too big.

More important, it looks like something is trying to actively drive the SDA line high - compare it to the SCL trace, where all the rising edges are slow.

The AS3935 does both I2C and SPI - are you sure you have it correctly configured?

[Axel Astrom]

Hello,

Thank you for your input! Yes im sure the AS3935 is configured correctly, I have the SI (Select Interface) pin connected to VDD which according to the datasheet should enable the I2C interface.

[awneil]

awneil said:

it looks like something is trying to actively drive the SDA line high

re-reading the OP:

Axel Astrom said:

found that this problem can be related to the configuration of the I2C pins. More specific that they are not configured as Open-drain.

Yes, it does appear that something on the bus is not open-drain; ie, something is actively driving the SDA line high.

You could try disconnecting things to find which one it is ...

Your SCL pin looks OK - so try comparing the SDA config with the SCL config ...

[hmolesworth]

You are violating the guidelines on SDA pull-up for this part; given there is an internal 10k pull-up on SDA the guidelines show no pull-up for stand-alone operation and a 10k external pull-up for up to 3 other I2C devices. The drive output on the AS3935 is poor and has a 100kHz limit with multiple I2C devices, so try just removing the 10k external on SDA given the internal 13k is already enabled.

13k || 10k || 4k7 = 2.6k

Also CS and MISO should be connected to Gnd in I2C mode, worth a check.

[Axel Astrom]

Thanks for the reply,

I have checked the design and CS and MISO are indeed connected to GND.

Seems I have been looking at an old version of the datasheet, I was not aware of the limitation regarding the number of devices allowed on the I2C to be able to run at 400 kHz.

I have tried removing the external pull-ups and measured the pull-up resistance on SDA to 12 k Ohm and the pull-up on SCL to be 13k Ohm. The problem still persist.