Raspberry Pi 4 has a USB-C design flaw, some power cables don't work

Raspberry Pi 4 was released last month, with much hype and promotions. It has a 1.5GHz quad-core 64-bit ARM Cortex-A72 CPU, three memory options of up to 4GB, full-throughput gigabit Ethernet, and a USB-C port as a power connector. The USB-C power connector was the first of its kind addition in the Pi 4 board. However, four days after its release, Tyler Ward, an electronics and product engineer disclosed that the new Pi4 is not charging when used with an electronically marked or e-marked USB-C cables, the type used by Apple MacBooks and other laptops. Two days ago, Pi's co-creator Eben Upton also confirmed the same.

Upton says that, “A smart charger with an e-marked cable will incorrectly identify the Raspberry Pi 4 as an audio adapter accessory, and refuse to provide power.” Upton adds that the technical breakdown of the underlying issue in the Pi 4's circuitry, by Tyler Ward offers a detailed overview of why e-marked USB-C cables won't power the Pi.

According to Ward’s blog, “The root cause of the problem is the shared cc pull down resistor on the USB Type-C connector. By looking at the reduced pi schematics, we can see it as R79 which connects to both the CC lines in the connector.”

“With most chargers this won’t be an issue as basic cables only use one CC line which is connected through the cable and as a result the pi will be detected correctly and receive power. The problem comes in with e-marked cables which use both CC connections”, he adds. 

Ward has suggested some workarounds for this problem, firstly he recommends to use a non e-marked cable, which most USB-C phone charger cables are likely to have, rather than the e-marked cable. Also, the older chargers with A-C cables or micro B to C adaptors will also work if they provide enough power, as these don’t require CC detection to get charged. The complete solution to this problem would be if Pi would, in a future board revision, add a 2nd CC resistor to the board and fix the problem. Another option is to buy the $8/£8 official Raspberry Pi 4 power supply.

In a statement to TechRepublic, Upton adds that “It's surprising this didn't show up in our (quite extensive) field testing program.” 

Benson Leung, a Google Chrome OS engineer has also criticized Raspberry Pi in a medium blogpost which he has sarcastically titled,“How to design a proper USB-C™ power sink (hint, not the way Raspberry Pi 4 did it)”. Leung has identified two critical mistakes on Raspberry Pi’s part.

He says that Raspberry Pi should have copied the figure from the USB-C Spec exactly, instead of designing a new circuit. Leung says that Raspberry Pi “designed this circuit themselves, perhaps trying to do something clever with current level detection, but failing to do it right.”

The second mistake, he says, is that they didn’t actually test their Pi4 design with advanced cables. “The fact that no QA team inside of Raspberry Pi’s organization caught this bug indicates they only tested with one kind (the simplest) of USB-C cables.”, he adds.

Many users agreed with Leung and  expressed their own views on the faulty USB-C design on the Raspberry Pi 4. They think it’s hard to believe that Raspberry Pi shipped these models before trying it with a MacBook charger.

A user on Hacker News comments, “I find it incredible that presumably no one tried using a MacBook charger before this shipped. If they did and didn't document the shortcoming that's arguably just as bad. Surely a not insignificant number of customers have MacBooks? If I was writing some test specs this use case would almost certainly feature, given the MacBook Pro's USB C adapter must be one of the most widespread high power USB C charger designs in existence. Especially when the stock device does not ship with a power supply, not like it was unforeseeable some customers would just use the chargers they already have.”

Some are glad that they have not yet ordered their Raspberry Pi 4 yet.

https://twitter.com/kb2ysi/status/1148631629088342017

However, some users believe it’s not that big a deal.

https://twitter.com/kb2ysi/status/1148635750210183175

A user on Hacker News comments, “Eh, it’s not too bad. I found a cable that works and I’ll stick to it. Even with previous-gen Pis there was always a bit of futzing with cables to find one that has small enough voltage drop to not get power warnings (even some otherwise “good” cables really cheap out on copper). The USB C thing is still an issue, and I’m glad it’ll be fixed, but it’s really not that big of a deal.”

No schedule has been disclosed on the release of the revision by Upton nor Raspberry Pi till now.

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