This is part of a series of posts on the design and technical steps of creating Himblick, a digital signage box based on the Raspberry Pi 4.
Updates
This post seem to have become quite popular, so to be clear, the intention is to document the issue that we spent a whole day chasing so that others won't have to.
It's been fantastic to see how this has made the rounds and resulted in actual RF measurements being made, and better pinpointing of the issue than we could ever have achieved.
I greatly admire Raspberry Pi's work. Thanks to them we get to have an affordable standard platform with a wide use base, which over time gets extremely well known, and as such can be a very well mapped ground to build on.
- Hacker News thread
- mentioned the test with a different distance from the AP madengr's comment reminded me we also did that
- Mike Walters took and posted some measurements
- Android Authority had a story, where Raspberry Pi's Eben Upton chimed in
- Mentioned that one of the cables we tried was the one that came with the Pi 4; added photo.
- Mentioned that we did not try to go to higher resolutions than 2560x1440.
- If you arrived here from a site called "The Inquirer": we only see the problem at 2560x1440, and we see no reason to avoid Full HD: 1920x1080 is the resolution we are going to use in the signage screens, and it leaves WiFi alone. Although we do not have a 4K monitor at hand, many have reported that 4K resolutions also work for them, together with WiFi. I would prefer that the people who wrote about this made an effort to be accurate and useful.
Original post
One full day of crazy debugging, and the result is that if the Raspberry Pi 4 outputs HDMI at a resolution of 2560x1440, the WiFi stops working.
Any lower resolution we tried, from 2048x1080 down, does not show this problem.
We did not try any resolution above 2560x1440, for lack of a monitor that would do it.
We reproduced this:
- on both microHDMI outputs
- with two different cables: one with a microHDMI to HDMI dongle adapter; one direct microHDMI to HDMI. The latter is the one I bought in the Raspberry Pi Store in Cambridge together with one of the Pi 4 units.
- with three different RaspberryPi units
- with 4 different power supplies: one rated at 2A, one rated at 3A, one rated at 3A bought in the Raspberry Pi shop in Cambridge, and a laptop USB-C charger
- with stock Raspbian Buster Lite
- with stock Raspbian Buster
- killing every process in the system, starting the network manually with
wpa_supplicantanddhclient, and starting X manually withsudo X - with two different SD cards
- connected to an AP some meters away, and connected to a phone hotspot next to the Raspberry PI
At the bottom of this forum thread
(guestxyz dated Aug 07) someone mentioned screen resolution, which is what
finally prompted us to try that. Thanks, guestxyz!
After confirming what the trigger was that caused the problem and chatting about it on IRC, olasd found this forum thread where more people are experiencing similar issues.
Further things left to try after chatting about it on IRC:
- whether disconnecting the HDMI cable from the Pi end (with X still started at high resolution and everything) make the WiFi work again
- switching the monitor to another input while the Pi is at 2560x1440
- letting the monitor go into power saving mode while the Pi is at 2560x1440
- cable chokes