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Raspberry Pi admits to faulty USB-C design on the Pi 4
The Raspberry Pi 4 was announced two weeks ago as a major new upgrade to the line of cheap single-board hobbyist computers. The Pi 4 featured a faster CPU, options for up to 4GB of RAM, and a new, modern USB-C port for power delivery. The Pi 4 was the Raspberry Pi Foundation's first ever USB-C device, and, well, they screwed it up.
As detailed by Tyler Ward, the Raspberry Pi 4 has a non-compliant USB-C charging port and doesn't work with as many chargers as it should. Thanks to the open nature of Raspberry Pi (even the schematics are online!), Ward was able to discover that Raspberry Pi just didn't design its USB-C port correctly. Two "CC" pins on a USB-C port are supposed to each get their own 5.1K ohms resistor, but Raspberry Pi came up with its own circuit design that allows them to share a single resistor. This is not a compliant design and breaks compatibility with some of the more powerful USB-C chargers out there.
[...] The Pi 4 is not the first high-profile device to get the USB-C spec wrong. The Nintendo Switch also has a non-compliant USB-C port and has issues with certain USB-C cables as a result.
After reports started popping up on the Internet, Raspberry Pi cofounder Eben Upton admitted to TechRepublic 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 went on to say, "I expect this will be fixed in a future board revision, but for now users will need to apply one of the suggested workarounds. It's surprising this didn't show up in our (quite extensive) field testing program."
Probably not a dealbreaker (the cables that do work are cheaper), but could be annoying.
Previously: Raspberry Pi 4 Model B Launched
(Score: 2, Interesting) by Anonymous Coward on Wednesday July 10 2019, @09:17PM (4 children)
I'm glad you're not designing it then...
Sure we can use 7805 type linear regulators, typically with about a ~2V minimum dropout voltage... Add in the ~1.5V or so drop in the bridge rectifier and we'll need an input voltage of about ~9V, so this power supply, by itself, is consuming almost as much power as the rest of the board combined!
Let's say we need 15W out of this, 7805 type regulators typically have a max output current of 1A so three are required and together with the bridge rectifier, represents (in volume!) about $1 of extra BOM cost on a device which retails for $35, ignoring whatever cooling upgrades are required for this.
Using an LDO type regulator can help with efficiency, but usually have much lower power output (and are also much more finicky to use) so this will probably be an even more expensive option. Likewise efficiency of the rectifier can be improved using more intricate designs (-> $$$)...
(Score: 3, Funny) by fyngyrz on Wednesday July 10 2019, @10:23PM (1 child)
Well, here's the thing. None of your concerns are my concerns. 😊
--
Every once in a while declare peace. It confuses your enemies.
(Score: 2) by RS3 on Thursday July 11 2019, @03:33AM
I know you weren't trying to be funny, but I actually laughed out loud a bit. And it might be because you started with "Well, here's the thing." I generally don't watch and can't stand most sitcoms, but there was this one called "Becker" starring Ted Danson who played a doctor of sorts. His assistant Linda was stunningly ditzy and whenever she did something stupid she'd start her explanation with "here's the thing", and something funny always followed.
(Score: 3, Informative) by Knowledge Troll on Thursday July 11 2019, @01:44AM (1 child)
I'd have gone even further and not tried to cost cut with the linear regulators and go all the way to switching regulators. Switching regulators with 5 volt output, 25 watts power handling capacity, and tolerant of inputs up to 16 volts can be had at retail for less than $4 in the form of r/c aircraft battery eliminator circuits (BEC). That's $4 full retail cost to the end user. To retail at that cost they must have got the entire unit down around $1.
So lets run with $1 added to the $35 retail price of the RPI: that's a 2.8% increase in cost. That single dollar bought the user not having all these common power delivery issues as well as having the full 5 volts available at the USB ports and also not having to use crappy USB power supplies. Another amazing benefit is that the forums wouldn't be clogged up with people having power issues.
The poor power regulation was a really bad place to try to cost cut. That has caused a ton of pain for a lot of people.
(Score: 0) by Anonymous Coward on Friday July 12 2019, @05:22AM
A switching design would make much more sense for this product but the earlier poster made it quite clear that they did not want to use a switching regulator for EMI reasons (justified or not).
Well this is a non sequitur: a $1 increase in BOM cost does not imply a $1 increase in the retail price of the finished product. I'd guestimate adding onboard regulation without changing anything else would turn this in to a $40 product.
I'm not really familiar with this particular product design... However, these issues seem really quite minor, easily worked around and will likely be fixed in the next board revision. So I'd say it remains to be seen whether this was really a bad decision. If you take shortcuts somewhere else instead people will complain about that too: this is just life with products built to a price. If that's a problem one can get products where the designers never make any tradeoffs to save costs...
What does seem strange to me, however, is that TFA suggests they made a deliberate design decision to reduce the number of 5.1k resistors from two to one, contrary to the USB specification. This essentially does not save anything at all, because resistors often literally cost nothing beyond the reel space (which they've already committed because they still have this resistor value in the design). Judging by the photos it does not look like there would be any real problem fitting another one onto the board...