Device working with USB A-C cable, but not C-C...sometimes - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 25, 2025, 08:34:15 pm
content: I'm building an RP2350-based device which communicates over a USB-C port. It's powered solely by an external supply, and VBUS does not connect to anything except the TVS diode array. It works fine when connected with a USB A-C cable, but some computers have trouble with C-C cables.Some Apple and Dell laptops do not enumerate at all, while other desktop computers work fine. I've tried official Apple cables, including a fancy thunderbolt cable. I'm using two 5.1k pulldowns on the CC lines as per the spec. Curiously, using an A-C cable with an A-C adapter works just fine.Does anyone know why some computers would act up like this? Is it because I'm not using VBUS? Did I find an edge case in some USB controllers which look for a minimum current draw?

author: thm_w
author_rank: Super Contributor
timestamp: April 25, 2025, 09:40:40 pm
content: Where is the rest of the schematic showing the D+ and D- connections to the MCU?

author: Someone
author_rank: Super Contributor
timestamp: April 25, 2025, 10:40:45 pm
content: Leaving Vbus open should present no problem and I have tested it widely.

author: wraper
author_rank: Unknown
timestamp: April 25, 2025, 11:19:04 pm
content: I'm not familiar with RP2350 but it may need VBUS detection. Also if you don't use VBUS at all, what's the point connecting it to ESD protection and a capacitor? It does no good at all and only adds a potential failure point.

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 25, 2025, 11:26:01 pm
content: Here's the USB connection to the RP2350. The microcontroller does not use VBUS detection.Just for fun, I put a resistor across VBUS and GND, to draw 150mA. This made it behave perfectly on all computers with a USB C-C cable.

author: Someone
author_rank: Super Contributor
timestamp: April 25, 2025, 11:34:18 pm
content: Quote from: Rachie5272 on April 25, 2025, 11:26:01 pmThat is odd. I've tested many combinations including devices negotiating power with the Vbus disconnected and have not seen that behaviour.

author: Someone
author_rank: Super Contributor
timestamp: April 25, 2025, 11:37:23 pm
content: Quote from: wraper on April 25, 2025, 11:19:04 pmThat (common) type of ESD suppressor dumps positive transients to the supply rail. If it were unpowered then the USB data lines would need to charge up the +Ve protection side.

author: thm_w
author_rank: Super Contributor
timestamp: April 26, 2025, 12:43:06 am
content: Quote from: Rachie5272 on April 25, 2025, 11:26:01 pmTry a capacitor in place of the resistor. Minimum is 1uF. But I wonder if resistor would still be needed to discharge.https://microchip.my.site.com/s/article/Minimum-Capacitance-Required-on-VBus"In order to account for variances in actual components - we recommend using a 2.2uF capacitor on the VBUS pin. This will greatly reduce the possibility of failing the VBUS capacitance compliance test."https://ez.analog.com/interface-isolation/f/q-a/112628/the-input-capacitance-on-vbus-of-usb-2-0

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 26, 2025, 01:41:39 am
content: Since you seem to say that drawing some current from VBUS "works", I would suggest looking at your USB device descriptors to see if, by any chance, you advertise it as bus-powered.