
Raspberry Pi 4 WiFi stops working at 2560 x 1440 screen resolution - pabs3
http://www.enricozini.org/blog/2019/himblick/raspberry-pi-4-loses-wifi-at-2560x1440-screen-resolution
======
adontz
Answer from forum

    
    
        Had the same problem with Raspi 4B.
        Problem was dependent on screen resolution (!!).
        With 1920x1080, wlan0 became disconnected after it was
        ok at lower screen resolutions. Was in 2.4GHz band.
        After turning on 5GHz in the router and going into
        the network preferences (right click the network icon top
        right on screen) and SSID ... and checking "automatically
        configure options" the connection remains stable (so far :D ).
    

So radio interference it is?

~~~
codetrotter
Reminds me of the MacBook Air that I have (2018 model).

When I connect my USB 3 hub to it, I lose my WiFi :(

I Googled it when it happened a while back, and apparently other people have
this problem with the MacBook Air too.

Some choose to shield the USB 3 cable with tinfoil. Personally I opted to
connect a USB 3 Ethernet interface to the hub and use wired Ethernet when I
use the hub.

~~~
linker3000
In the very distant past (late 1980s, early 90s), Olivetti changed their PC
keyboard design; it went from having the keyboard PCB assembly inside a metal
clamshell to a bare board with a metallised plastic sheet on the back of the
PCB (they 'cheaped out').

We got to know about the design change when a rather large and sprawling local
leisure centre reported that often when someone used their walkie talkie near
a PC, the screen filled with random characters and sometimes the dot matrix
printers would 'go haywire' (CTRL-P = Print what's displayed on the screen).

In effect, the rf signal from the walkie talkies was 'mashing the keys'.

The immediate fix was to swap in some older keyboards, the longer fix was down
to Olivetti using better shielding and some appropriately-placed capacitor
decoupling on the power and signal lines.

~~~
heelix
First time I saw a room full of non-crt computer monitors, found out Merrill
Lynch moved to a new building to save money. What they discovered is the CRTs
would weird out every time the subway train went by. All the cash saved by
moving was lost buying the stupid expensive monitors.

~~~
fragmede
I wouldn't have considered trains to be a source of EM interference but in
retrospect it makes sense. one that's amazed me is that military jets and
warships cause some of the nearby consumer electronics go wonky, but as they
_literally_ run systems that are trying to jam radio waves, it makes a ton of
sense that eg garage doors have trouble keeping up.

------
TwoNineFive
I will really like to hear what the root cause of this is.

This makes me think of the bug that the QCA AR9331 SoC has. The AR9331 is
extremely common in small travel routers, but it has a fun bug where one of
it's clock sources is shared between the 802.11 wifi and the USB port. IF the
USB port is negotiated at USB 1.x speeds AND the 802.11 radio is scanning, the
USB will freak out and die. This generally requires the 802.11 radio to be in
client mode rather than AP mode. You can read some details about this on the
old OpenWRT forum, if it survived the great forum purge of 2018.

~~~
Narishma
The cause seems to be a low-quality HDMI cable that doesn't provide enough
shielding.

------
pvtmert
Also Macbook Air (especially 11 inch model) 2013-2015 versions have similar
problem with (cheap) USB 3.x external drives

Wifi shows as 'connected' but reality is only fraction of packets go through
because of interference problems

some online reference from intel on usb.org:
[https://usb.org/sites/default/files/327216.pdf](https://usb.org/sites/default/files/327216.pdf)

~~~
danieldk
Unfortunately not only with cheap external USB 3.x drives and not only with
old MacBook Airs. Connecting an Anker USB-C dock/hub to my MacBook Pro 2016
and later 2018 would consistently cause interference with the Microsoft Sculpt
Ergonomic and Apple Magic Trackpad (both on 2.4GHz). Then I got an Aukey hub
(since my wife was happy with one) and the problems have vanished.

~~~
xenadu02
This is an issue with poorly made USB 3 cables, connectors, and devices. Intel
has a white paper about it:
[https://www.intel.com/content/www/us/en/products/docs/io/uni...](https://www.intel.com/content/www/us/en/products/docs/io/universal-
serial-bus/usb3-frequency-interference-paper.html)

> the noise from USB 3.0 data spectrum can be high (in the 2.4–2.5 GHz range).
> This noise can radiate from the USB 3.0 connector on a PC platform, the USB
> 3.0 connector on the peripheral device or the USB 3.0 cable. If the antenna
> of a wireless device operating in this band is placed close to any of the
> above USB 3.0 radiation channels, it can pick up the broadband noise. The
> broadband noise emitted from a USB 3.0 device can affect the SNR and limit
> the sensitivity of any wireless receiver whose antenna is physically located
> close to the USB 3.0 device. This may result in a drop in throughput on the
> wireless link.

The money quote:

> With the HDD connected, the noise floor in the 2.4 GHz band is raised by
> nearly 20 dB. This could impact wireless device sensitivity significantly.

Besides having properly shielded devices and cables (which manufacturers often
don't bother doing), they also recommend that the plug in the laptop be fully
shielded or enclosed in a metal chassis (which is fulfilled by having an
entirely metal case).

I don't know of a cheap RF analyzer but I'd like to get one at some point. I'm
curious how many common devices actually adhere to the FCC regulations and/or
standards like USB 3, compared to how many are just cheaply made.

~~~
GrayShade
> I don't know of a cheap RF analyzer but I'd like to get one at some point.

A SDR might work, and they're quite versatile. Unfortunately, the cheap RTL-
SDR ones don't get to 2.4 GHz without a downconverter.

------
VT_Dude
Former communications semiconductor FAE here. We would troubleshoot issued
like this all-day every-day for years, usually under NDA before the product is
ever released to production. The solutions are routinely as weird as some of
the "voodoo" hypothesis tossed around here - wait for it and you'll see. After
a while it seems normal that all unverified combinations are broken and the
moments of delight are when an unverified configuration completely works.

~~~
mceachen
TIL one more TLA.

FAE = Functional Accessibility Evaluator

~~~
ChuckMcM
Typically in the semiconductor business FAE = Field Application Engineer.

------
cjbprime
Aw, I was hoping the explanation would be in the thread. Is it high-frequency
noise from an unshielded clock? Surpassing a current or temperature limit due
to the stress of the high resolution and resulting high memory bandwidth? Time
will tell!

~~~
duskwuff
> Is it high-frequency noise from an unshielded clock?

I bet that's it.

2560x1440 @ 60 Hz with CVT-RB timings has a pixel clock of 241.5 MHz. The TMDS
bit rate is 10x the pixel clock, and 2415 MHz is right in the lower end of the
802.11 band.

If the Pi can be convinced to use CVT blanking, that'll raise the pixel clock
to 312 MHz, which should be fine.

~~~
marcan_42
This is the answer. That mode puts the second harmonic going out of the HDMI
cable smack in the 2.4GHz WiFi band (the fundamental would be at 1/2 the bit
rate).

Now the question is who screwed up. Is it a leaky cable? Is it bad PCB design?
Is it a problem internal to the SoC? Is it a power delivery issue? Knowing the
RPi foundation and Broadcom, I bet one of them screwed this up for all RPis
and it isn't just a bad cable.

~~~
walterman
I'm trying to understand this so I don't know a lot of what's going on here,
but why is the fundamental half the bit rate instead of the bit rate?

~~~
duskwuff
Imagine a signal that alternates every second (e.g. 10101010). This has a
frequency of 1/2 Hz, not 1 Hz, because it repeats every 2 seconds.

------
seminatl
Why do all the proposed avenues of future investigation, and all of the
current comments on this thread, focus on voodoo instead of the far more
likely explanation that the display driver is just stomping on the memory of
the network interface? If there's software anywhere in a system, 99% of the
time that's the problem.

~~~
mjg59
What does "stomping on the memory of the network interface" mean?

~~~
djsumdog
Really, terrible, security vulnerability issues? DRI and networking kennel
modules should absolutely not be able to interact with each other at all.

~~~
fulafel
"kernel module" together with "should absolutely not be able to interact with
each other" are an impossible requirement with Linux.

I think the other operating systems available for the Pi are roughly in the
same boat (Windows & RiscOS). There was a nascent Minix port at some point, I
wonder if it was abandoned.

------
Animats
Is it using main memory as the video controller's memory? It may be out of
memory bandwidth.

~~~
wmf
That was my first thought as well, but scanning out that resolution should use
less than 1 GB/s of memory bandwidth which is nowhere close to the DRAM speed.
And usually in that situation you get horizontal speckles in the video output.

------
forkerenok
Some MacBooks have the same issue (interference). Making sure WiFi is working
in the 5Ghz band is one of the workarounds.

------
Dwolb
I’d want to know if the Bluetooth stops as well since it’s in the same
frequency range.

------
_bxg1
Maybe it doesn't have enough power and prioritizes the GPU over the WiFi
antenna?

~~~
skittleson
First thought as well. Didn't see a way it could determine that tho.

~~~
_bxg1
I know PIs soft-require more power than the supplies most people give them,
and they do things like CPU throttling when undersupplied. I think I've also
read that WiFi and/or Bluetooth may stop working when underpowered. So it may
not be a balancing thing so much as graceful degradation.

------
chx
Ah WiFi radio interference, everyone's favorite! It kills Thunderbolt
sometimes:

[https://www.akitio.com/faq/301-why-does-my-
thunderbolt-3-dev...](https://www.akitio.com/faq/301-why-does-my-
thunderbolt-3-device-not-work-witih-dell-s-xps-laptops)

> Why does my Thunderbolt 3 device not work with Dell's XPS laptops?

> Some users on the Dell forum have found that reducing the power output of
> the WiFi network adapter to 75% fixes the problem.

------
cft
I have been using this resolution (with the console runlevel 3 only) and no
prob with wifi.

------
cyberjunkie
A Xenon flash used to freeze the Pi 2.

~~~
segfaultbuserr
I don't know why it became such a piece of breaking news.

When a current flows through a p&n junction, photons are emitted (and an LED
is just a diode that happens to emit photons at the wavelengths of visible
light). And it works in both ways, if you hit a p&n junction with photons, you
produce a current, not only LEDs - any diode will do that, they're all
potential photodiodes, it's just that some are more sensitive than others.

You can cause a lot of chips to reset if you shine a bright beam of light to
its exposed die, a common way to test chips.

It's also one reason (in addition to cost) that most diodes are sealed in
plastic package, not glass package. Fun experiment: buy some 1N4148 small-
signal diodes in glass package, connect it to a Darlington-pair transistor
amplifier, and shoot the flashlight, you'll see some funny thing on the
oscilloscope.

~~~
nrclark
I think the newsworthy part of all that wasn't the physics involved, but more
the design-choices on the Rpi 2.

Putting a light-sensitive chip (I think it was a wafer-scale package with no
casing) on a board that's intended for use outside of an enclosure was a
really big oversight.

~~~
segfaultbuserr
> _a really big oversight._

I agree.

------
caro_douglos
Sounds like a turkey day mystery!

You were missing a router swap on your list.

While it might seem a bit overkill try restarting your router’s WiFi to see if
it magically works. I had a war with a pi zero W not that long ago...turns out
the 2 band wireless would just die sometimes. Turned out to be an issue with
the router ️

~~~
mike_hock
Was it a bug in the router or was it a bug in the Pi that happened not to get
triggered by a different router?

~~~
caro_douglos
It was ultimately with the router. Specifically With NAT and 2.4 band. 5g kept
going just fine. After looking at the manufacturers website the solution was
to either buy a new router or disable port mapping.

------
WilliamEdward
Monitors creating harmonic interference with wifi is a very common problem,
and most computers don't give any warning of this. I don't see why it wouldn't
be possible to say "the wifi won't connect, it may be because of your refresh
rate".

------
rs23296008n1
Reminds me of an old adage about software:

The plane can't take off because the carpet is the wrong kind of orange.

ObligAnecdote: I once had a keyboard that wouldn't work when the monitor was
outputting at 75hz. Had to be 60hz or else nothing. The joys of wireless
keyboards.

------
smashah
Has anybody else got 4k video working in the pi? Doesn't work for me on
raspbian at all.

~~~
wk0
I have over VNC

------
forinti
I have this issue with a Zero W. Wlan0 would just disappear. I tried jessie,
stretch, and now buster. It had been connected to a ultra-wide monitor (but
not at 2560 width, obviously).

I can't get it to fail now, but it is not connected to a monitor anymore.

------
goombastic
Any other SBCs that can drive a high res display @60hz just for normal
computing purposes? No gaming/video encoding use cases.

~~~
jrockway
I have successfully used the Pi4 at 4K. It is a very slow computer for “normal
computing purposes” though.

~~~
vardump
> It is a very slow computer for “normal computing purposes” though.

Can you elaborate what you're doing that is slow? In my experience, CPU wise
it's plenty fast, matching typical desktop from 2008. Although memory
bandwidth could be better...

~~~
baq
It's slow at 4k. I've got the 4GB variant and in 1080p it's good, I'd even say
surprisingly so.

~~~
vardump
Ah, so you mean graphics performance. Yeah, just not enough memory bandwidth.
32-bit DDR4 memory interface is a bit narrow for 4k use!

------
mic-kul
It works fine with Ar1 Pi case - that extends usb-c ports and puts them 2-3cm
away

------
userbinator
It reminds me of this old demo of electromagnetic interference:
[http://www.erikyyy.de/tempest/](http://www.erikyyy.de/tempest/)

------
paule89
Sounds like an EMC or Clock error to me.

------
AegirLeet
Please don't use code blocks to quote text, they are impossible to read on
mobile.

> _Had the same problem with Raspi 4B. Problem was dependent on screen
> resolution (!!). With 1920x1080, wlan0 became disconnected after it was ok
> at lower screen resolutions. Was in 2.4GHz band. After turning on 5GHz in
> the router and going into the network preferences (right click the network
> icon top right on screen) and SSID ... and checking "automatically configure
> options" the connection remains stable (so far :D )._

------
tosh
Ubuntu Bug 255161: Openoffice can’t print on Tuesdays

[https://news.ycombinator.com/item?id=8171956](https://news.ycombinator.com/item?id=8171956)

------
frereubu
Please don’t post quotes as code - it makes it almost impossible to read on
mobile.

~~~
adontz
Honestly, I have no idea what is the right way to format quotes. And sorry for
inconvenience.

~~~
hayksaakian
You could use "quotes" to format quotes. Just a suggestion.

------
jokoon
The RPi doesn't feel very minimalist.

I wish the RPi was this standalone microserver that had its own flash memory
(SD cards are known to fail) that you could plug into your home network and
act as a personal server.

Maybe one day it will be possible to do some basic GSM data with a RPi.

~~~
lylecheatham
If they didn't use SD cards, the storage would be more reliable, but users
would spend a lot more time fixing bricked boards. By allowing for removeable
storage (in a format that can be plugged into any other computer natively)
they solved that problem because I can just re image the card and get going
again.

The philosophy of the RPi is that they won't really add features to it unless
a bulk of the user base would use the feature. For example they were hesitant
to even build the WiFi into it because users who wanted that could always get
a USB chipset, and building it in adds BOM cost.

Because with GSM you'd also need a plan for it, I don't really predict they'd
add that. Especially since you can get it in hat format already.

~~~
ThrowawayR2
M.2 to USB converters work just fine and a M.2 drive would be far more
reliable than μSD cards, even the high-endurance ones.

~~~
lylecheatham
You're talking about a $40 tool just to flash the storage vs a part (micro SD
adapter) that they give you with the SD card for free because it's so cheap.

Also doing a quick survey for the rated cycle counts on M.2 vs SD card slots:

M.2: I found this one [1] which is $0.768 for only 60 cycles

SD: This one [2] is $0.6256 for 5,000 cycles

I'm not sure why you'd say M.2 is more reliable, considering users often cycle
storage dozens if not hundreds of times.

[1] [https://www.digikey.com/product-detail/en/jae-
electronics/SM...](https://www.digikey.com/product-detail/en/jae-
electronics/SM3ZS067U310AMR1200/670-2690-2-ND/4162221)

[2] [https://www.digikey.com/product-
detail/en/gct/MEM2051-00-195...](https://www.digikey.com/product-
detail/en/gct/MEM2051-00-195-00-A/2073-MEM2051-00-195-00-ATR-ND/10460360)

