[time-nuts] Ublox 8T - breakout board

[Matthias Welwarsky]

Quick heads-up for everyone who might have ordered the breakout board based on 
design files I uploaded to github before Dec. 28th.

As Peter Bell found out, the voltage divider that provides VDD_USB to the LEA 
module is not adequate, the impedance is way too high and since the current 
drawn by the VDD_USB pin is quite variable, there is little chance to 
dimension it correctly. However, it might still be possible to salvage the 
boards. I suggested, and Peter verified it, that replacing R7 with a red LED 
(0603 footprint), oriented such that the cathode connects to VDD_USB, will 
provide an adequate supply to the pin and might even double as an USB 
indicator light.

I've updated the files on github on Dec. 28th and added a 3.3V LDO regulator 
instead of the resistor divider. This is the solution recommended in the 
hardware integration manual for the LEA-M8.

My own boards still haven't arrived.

BR,
Matthias


On Samstag, 26. Dezember 2020 10:06:11 CET Peter Bell wrote:
> Just a quick note for anyone building these - I got my boards back and
> assembled them
> and everything works except for the USB port.  This is being caused by the
> VDD_USB
> pin being fairly low-impedance and dragging down the voltage of the R7/R9
> potential
> divider to about 700mV, at which point the module doesn't recognize it's
> connected.
> 
> I guess the "proper" fix would be to add another 3.3V LDO to the board that
> runs from
> VBUS and generates VDD_USB - but for testing purposes I removed R7 and
> connected
> the VDD_USB pin to the 3.3V on pin 2 of U4 - after doing this, the board
> connected
> and enumerated as a virtual COM port.
> 
> Regards,
> 
> Pete
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