When to use I2C pull-up jumpers on Pmod HAT?

[Brooklynoutlier]

I am looking to connect a Pmod HYGRO module to a Raspberry Pi 3B through a Pmod HAT. The Pmod HYGRO uses the I2C protocol, and I understand that this requires pull-up resistors on the SCL/SDA lines. However, from what I've read, the RPi has built in resistors to handle this, and I'm wondering when/if I would use the pull-up jumpers on the Pmod HAT?

[JColvin]

Hi @Brooklynoutlier,

I was not able to find evidence of resistors already being implemented on the RPI 3B (based on their schematic here: https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/rpi_SCH_3b_1p2_reduced.pdf), so my understanding of the correct time to use the pull-up jumpers would be when the connected I2C device does not already implement the pull-up resistors.

In the case of the Pmod HYGRO, both it and the Pmod HAT Adapter have jumpers to optionally implement pull-up resistors, so you can choose either set of jumpers.

Let me know if you have any questions about this.

Thanks,
JColvin

[Brooklynoutlier]

Thanks for your reply.

The schematic shows pull-up resistors on GPIO2 and GPIO3, which are the pins used for the "allowed" I2C.1 protocol on the Raspberry Pi 3B. Pins GPIO0 (ID_SD) and GPIO1 (ID_SC) are used for the "forbidden" I2C.0 protocol, reserved for updating EEPROM, I gather (this is way above my pay grade), and don't have pull-up resistors.

[JColvin]

Ah, ok. I would still consider using the extra pull-up resistors since the 1.8 kOhms may not be sufficient, though they could still work on their own(I would presume the resistors would only be implemented directly in the Raspberry Pi 3B if it properly adhered to the I2C standards).

Let me know if you have any more questions.

Thanks,
JColvin