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I wanted to debug my chromebook so I was looking for SuzyQable which enables to activate debug features on chromebook. Unfortunately this cable is hard to get in my location so after noticing this https://chromium.googlesource.com/chromiumos/third_party/hdctools/+/master/docs/ccd.md#making-your-own-suzyq official google guide to make own diagnostic cable I decided to give it a try. At first I was a bit sceptical about it as the schematic diagram of SuzyQable (https://cdn.sparkfun.com/assets/9/e/f/8/2/951-00273-01_20180607_suzyqable_SCH_1.pdf) is far more advanced than what google is saying. However the hub seems not be necessary there. So I bought usb type A and C breadboards and connected pins exactly as described on chromium page linked above although with one caveat - I currently don't have 56K Ohm resistors so I connected in series: 56k Ohm = 22 + 22 + 10 + 1 + 1 [k ohm]

Once the whole setup is connected I measured voltage on CC1 & CC2 pins:

Voltage on CC1: Position 1 -> 4,81V; Position 2 -> 4,78 V

Voltage on CC2: Position 1 -> 0,44V; Position 2 -> 0,46V

Unfortunately after plugging it in the device is not recognizable as tty serial USB one, which should be visible under /dev/ttyUSBx

Obvious problems which I already eliminated:

  1. They say chromebooks discover this debug interface in only on usb type c orientation so I already tried both, same effect.

  2. I have usb to serial module (FT232R) I connected it to both devices (chromebook and Ubuntu host) and it was discovered as ttyUSB0 immediately. So this means both devices are serial capable.

Questions:

  1. I wonder if the series connection, of 56 k ohm resistor, introducing voltage drop can be responsible for the issue?

  2. Is the hub which is on sparkfun schematic really superfluous and can be ommited?

  3. What voltage levels are needed for USB to be discoverable as serial device?

UPDATE: Schematic diagram of my connection (according to chromium page)

enter image description here

Picture of real connections on a breadboard enter image description here

My Chromebook model is: Acer Chromebook 311 bobba

PS: Chromebook sometimes log weird dmesg messages like:

"PDLOG (date here) P1 SINK (not charging) charger ?? 5000mV max 5000 mV / 500 mA"

Ubuntu on the other hand logs:

"usb usb1-port3: over current condition"
"usb usb1-port4: over current condition"

Yes I have no idea why it mentions two ports if I only tinker with one!

Thanks for any advice!

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  • \$\begingroup\$ Have you checked your device actually supports "Closed Case Debugging"? (See list here) \$\endgroup\$ Mar 21, 2020 at 11:42
  • \$\begingroup\$ Also, draw a schematic and take a photo of how exactly you wired up your breakout boards. \$\endgroup\$ Mar 21, 2020 at 11:46
  • \$\begingroup\$ Thanks for the suggestions. I have updated the post to include My chromebook model and add schematic diagram + real time picture of connections. Yes this chromebook does have close case debugging support. \$\endgroup\$
    – DannyS
    Mar 21, 2020 at 14:04
  • \$\begingroup\$ Also, I have noticed sometimes the host ubuntu complains about overcurrent condition od USB port 3 and 4. Yes this is weird why two ports? oO. As for chromebook it also log some weird message in the dmesg like "PDLOG (date here) P1 SINK (not charging) charger ?? 5000mV max 5000 mV / 500 mA" \$\endgroup\$
    – DannyS
    Mar 21, 2020 at 14:08
  • \$\begingroup\$ This question isn't likely to be answerable, and has the additional issues of fundamentally depending on external links, when site rules are that questions must stand on their own. On the one hand, either a wiring mistake or incompatible software are quite possible explanations. On the other, hooking up USB connections via a breadboard and loose jumper wires is not supposed to work - sure, sometimes you luck out and it does, but it's a gross violation of all applicable specifications. \$\endgroup\$ Apr 27, 2020 at 21:14

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I have managed to root cause this - the problem was the USB male to male cable I was using, basically it doesn't pass both CCx signals but just one and connect the other to this.

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