JM16 - Can I use the USBDN or USBPDP line to ALSO connect to BKGD??

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JM16 - Can I use the USBDN or USBPDP line to ALSO connect to BKGD??

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LiveMike
Contributor II

When my board is complete, I pot it in epoxy, with only the USB male connector exposed (and sometimes LEDs).

 

I have ALREADY run a small lead from the USBDN pin on the USB male connector to the BKGD pin on the JM16.  (The other lead runs through a 101 resistor to the USBDN pin on the JM16.)  I would eventually put a resistor on the small BKGD lead as well.

 

THIS WORKS.  I think.  I have a lot of testing to do.

 

THIS IS WHAT I THINK:

The BKGD pin ignores everything happening on the BKGD line.

The BKGD does not send high or alter the USBDN line, therefor not affecting the USB communication of my device.

The only time BKGD does anything is when the MCU gets a reset, and IF this were to happen, the USB communications would fail/interrupt/stop because the MCU is resetting REGARDLESS if there was a lead from the BKGD to USBDN or not.

When it is time for me to flash new firmware, I connect my MultiLink to the male USB Vdd, Ground, and USBDN (background interface) and my JM16 re-flashes perfectly fine.

 

IS THERE ANYTHING GOING ON HERE I NEED TO BE AWARE OF?

Potential errors, flaws, failures???

 

Thanks in advance,

-Mike

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eduardo_viramon
NXP Employee
NXP Employee

I'm not entirely getting your description of the connections, could you upload a simple diagram? From what you described it looks like you have connected USBDN and BKGD to the same cable. BKGD has an internal pull-up, and this might cause the USB host to think it's connecting to a low speed instead of a full speed device. Additionally, the change in impedance may affect your device capability to be USB certified.

A diagram may definitely help understand better.

Eduardo V.

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