Why some USB C cables work, or not, J-Link

Summary

September 28, 2025 - 5 mins readDo you have a J-Link Compact and have noticed it doesn’t work with some USB C to C cables? I do, and I did, so I decided to investigate…BackgroundThe J-Link Compact is a really nice debugger for various microcontrollers. The non compact version is rather large and uses USB-B(or Ethernet). It comes with a branded ~1m C to C USB cable, which works great. I plug it into my laptop, the green LED turns on and I’m ready to debug!One day, I needed to reach a device that was more than 1m away, so I grabbed a 2m macbook charging cable and plugged it in. It didn’t work. The light didn’t turn on and the device didn’t enumerate. I immediately tried another cable and it didn’t work either. The original cable worked, but none of the long ones did. I tried a long USB A to C cable, and it worked just fine… That gave me a hint to what the problem could be…InvestigationType-C DetourThe fact that some type-c cables work but not others strongly points to issues with the CC pull down resistors. To understand what that means, we need to look at the USB-C specification. In short, all USB-C cables connect one of the two CC lines between host and device. The sink/device must have a 5.1kΩ pull down resistors on each of the CC lines. The host has a pull up whose value depends on how much power it can source. The second CC line remains disconnected and both devices can use that information to determine cable orientation, among other things.The figure below(from the USB spec) shows a schematic of the connection. For most “basic” USB 2.0 cables, Ra is not there and the pin is effectively left floating. That basic resistor configuration is enough for a device to advertise up to 5V@3A of power. Devices can further negotiate for more power using the PD protocol, but to get the maximum power available(over 3A and up to 48V) requires the cables to be “electronically marked” or eMarked. This means there’s a small microcontroller inside the cable with info...