Notecarrier-B I2C Differences between J22 and QWIIC connectors

I have two scenarios for using the Notecard I2C connections using the Notecarrier-B and noticed that the signals going to the the J22 Edge connector have series resistors while the signals going to the QWIIC connector are connected directly to the Notecard connector. Is there a reason that it was implemented this way? Is there guidance as to how to use the two interfaces?

The two I2C scenarios:

  1. Connect an RPi Zero 2W which has 1.8K pullups to 3.3V
  2. Connect an Arduino Nicla Vision which has no pullups

I am not using both simultaneously.

Thanks,
Ralph

Hey @ralphjy

The series resistors are there for protection in case the non-notecarrier side of the pins are configured incorrectly or shorted. Since I2C is pulled to 3.3V you’d have 3.3V/200Ohms = 0.00165A which is a very low draw if you shorted the I2C pins.

  1. RPi Zero 2W: I would recommend removing the 1.8k pullups (R24 & R23) from the Pi Zero 2 W. The notecard already has 10k pull ups. I2C is “open drain” meaning the drain pin on the mosfet is open circuit but pulled to 3.3 with a 10k. If you leave the 1.8k resistors on the the bus it may fail since 10k +1.8k in parallel gets you ~1.5k and thats a very strong pullup. Therefore the SDA & SCL line will not be able to pull low and the I2C bus won’t operate as expected.
    image

  2. Arduino Nicala Vision: as long as you have a cable or a connection to match the pinouts between the qwiic connector and the nicala vision line up go for it. The Nicala Vision most likely has the internal pullups configured inside the STM32H7 which is typically 40k internally so 10k + 40k in parallel is an 8k pullup which is strong but should function properly.

Keep an eye on your I2C pullups across your entire bus, keeping it within 4.7k-10k is really the sweet spot nominally for devices to play nice together.

Thanks for the reply. I’ll be sure to be careful if I use a QWIIC cable that doesn’t have a connector on the other end (as I might have done in both these cases). I’ll just use the J22 connections for now.

Interesting that all the RPi’s have that 1.8K pullup (except the Pico). I guess I’ve been lucky or maybe because the RPi is usually the controller.

I am trying to reproduce Carlton Henderson’s portal demo. Maybe I’ll give it a try before removing the pullups from the Rpi.