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V2.2 Stuck at white LCD but no USB device registers

George Joseph 2020/09/03 08:21

I've been using my V2.2 quite happily for a 2 weeks now but did something stupid the other day...  I knocked the unit off the workbench and of course, it landed on the USB cable connector and snapped the PCB connector off.  Luckily I had some spare connectors so I fired up my rework station, removed the bits that were still soldered to the board, and resoldered a new connector on.   I've tested the connector for shorts and opens and it looks good but when I connect the unit to my computer (linux) I get power to the unit but only get the LCD backlight and no USB device registers on the computer.  I've double checked that there's nothing accidentally shorting the "left" button that might keep the MCU from starting and I've tried shorting S301 to force DFU mode but that didn't work.  Is there anything else I should be looking at?

A few other questions...
Can the unit run without the LCD touch panel connected if it's not needed?  I use the unit exclusively when connected to a computer.
What do the non-IP5305 LEDs indicate?  LED301 LED302 LED401 LED601?


OwO 2020/09/03 23:30

Does it still show up as a USB device in device manager?

If the LCD is not connected USB should still work.

George Joseph 2020/09/03 08:33

On Thu, Sep 3, 2020 at 08:30 AM, OwO wrote:

> Does it still show up as a USB device in device manager?

This is Linux but no, the USB device never registers and there are no enumeration messages shown in dmesg.

> If the LCD is not connected USB should still work.

Cool.  Thanks!

OwO 2020/09/03 23:38

I would try going over the MCU (U303) pins with a soldering iron after
applying some flux, and if you have a ST-LINK dongle you can try
attaching to the SWD header on the right side of the board to see if the
MCU can be detected. If the MCU is detected but it shows white screen
and doesn't enumerate over USB, it's either a clocking issue or bad
contact on the MCU pins.

George Joseph 2020/09/03 09:27

I'll try going overt the pins and I can grab a ST-link dongle tomorrow if it's still an issue.  Thanks!

George Joseph 2020/09/03 13:27

It looks like in the process, I blew U4.  Is there a reason it's a separate 3V LDO instead of 3.3V (or fed off U111).   The 3V version is is a 10 week backorder in the US.   The 3.3V version I can get from Amazon on Sunday.

OwO 2020/09/04 12:20

If the 3V rail isn't powered it's also possible there's a short to
ground. 3V rail powers the MXD8641 RF switches which have "recommended"
maximum voltage 3.0V, but absolute max voltage 3.3V, that's why a 3.0V
regulator is used but I think 3.3V is fine for a one-off repair.

George Joseph 2020/09/04 07:44

I did find some solder paste across some of the MCU pins (which speaks to my rework skills) so it's possible there was or still is a short.  At this point I'm just going to buy a new one though.  It's cheaper than spending more time trying to fix the current one.

I really appreciate the help and quick responses!  Thanks!

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