Unexpected power consumption on imx6 VBus pin!

The VBus pin (SODIMM 137 / “USBC_CableDet”) on the imx6 has been plaguing us through 6 pcb prototypes. There must be some undocumented behavior!!

  • We are assuming it’s 3.3V, not 5V like all other VBus_detect pins we’ve ever encountered. We’ve tried driving it to 3.3V from the actual VBus pin on the usb connector through resistive dividers, FET level-translators, and 74AHC-series 24mA(!) buffers. It never works. The pin always reads <2.5V when driven from a 3.3V logic source.

  • If we connect VBus directly to the 3.3V power supply, the USB client will begin working. This leads us to wonder if it is even necessary to connect? (Of course, when it has a spurious level of 2.5V from inadequate drive levels, the USB client never connects.)

  • We have also connected the pin directly to the Usb connector so it gets 5V, and it does work at that point, but we worry that we are running out-of-spec and will eventually damage the board!

  • We notice on the Colibri Eval Schematic v3.2A, that it is connected directly to the usb connector through a 560-ohm + 1k-ohm voltage divider, so we assume that it is, in fact, not 5V compliant…also, a 1K divider suggests that it uses a lot more current than typical for a digital input.

  • We need to know the current consumption and correct voltage level for this pin. The documentation is clearly inadequate, and we even paid the $1k for a schematic review. The only time it works is when we incorrectly (we assume) connect it directly to 5V_USB! Extremely disappointing…

The USBC_CableDet pin is only a 3.3V tolerant as it clearly stated in Colibri iMX6 datasheet (https://docs.toradex.com/102075-colibri-imx6-datasheet.pdf) and Colibri Carrier board design guide (https://docs.toradex.com/102491-colibri-arm-carrier-board-design-guide.pdf).

This pin is used for detect cable connection to host when Colibri acts as a client. If you have a HW permanently connected to USBC you can connect Pin 137 directly to 3.3V.

Please refer to section 4.1 and 5.4 of Colibri iMX6 datasheet and section 2.3 of Colibri Carrier board design guide for more details.