Controlling PWM fans with the Raspberry Pi CM4 IO Board's EMC2301

Noctua 120mm PWM fan connected to Raspberry Pi CM4 IO Board

When I initially reviewed the Compute Module 4 IO Board, I briefly mentioned there's a 4-pin fan connector. It's connected to the Pi's I2C bus using a little PWM chip, the EMC2301.

But wait... what's I2C, what's PWM, and what's so special about a 4-pin fan connector? I'm glad you asked—this post will answer that and show you how you can control a fan connected to the IO Board, like the quiet Noctua NF-P12 pictured above with my IO Board.

If you plug a fan like that into the CM4 IO Board, it will start running full blast, 24x7. If you need that much cooling, that's great, but a lot of times, I don't mind my Pi's CPU getting warmer if it means I can run the fan silent most of the time.

The Raspberry Pi 4 has a fan now - the Case Fan

Last year, I wrote a blog post titled The Raspberry Pi 4 needs a fan.

And in a video to go along with that post, I detailed the process of drilling out a hole in the top of the official Pi 4 case and installing a 5v fan inside.

Raspberry Pi 4 Case with Fan drilled into top of case

But that solution wasn't great. The fan was a little loud and annoying, and would stay on constantly. And who wants to damage the nice-looking Pi Case by putting a hole right in the top?

The Raspberry Pi 4 might not need a fan anymore

tl;dr: After the fall 2019 firmware/bootloader update, the Raspberry Pi 4 can run without throttling inside a case—but only just barely. On the other extreme, the ICE Tower by 52Pi lives up to its name.

Raspberry Pi 4 cooling options including ICE tower cooling fan and a case mod fan
Three options for keeping the Pi 4 cozy: unmodified Pi 4 case, modded case with fan, and the ICE Tower.

A few months ago, I was excited to work on upgrading some of my Raspberry Pi projects to the Raspberry Pi 4; but I found that for the first time, it was necessary to use a fan to actively cool the Pi if used in a case.

Two recent developments prompted me to re-test the Raspberry Pi 4's thermal properties:

The Raspberry Pi 4 needs a fan, here's why and how you can add one

December 2020 Update: Lo and behold, the Pi Foundation tacitly acknowledges the Pi needs a fan in the official case, because now they sell the Case Fan!

The Raspberry Pi Foundation's Pi 4 announcement blog post touted the Pi 4 as providing "PC-like level of performance for most users". The Foundation even offers a Raspberry Pi 4 Desktop Kit.

The desktop kit includes the official Raspberry Pi 4 case, which is an enclosed plastic box with nothing in the way of ventilation.