[time-nuts] Ublox 8T - breakout board

[Matthias Welwarsky]

Happy New Year, Peter.

If you get the chance, would you also have a look at the buffered 1PPS signal? 
I received my boards meanwhile and after mounting the LEA module I connected 
the 1PPS output to a scope to measure the signal rise time. I'm getting around 
6ns for the 10%-90% rise and the output is not quite reaching 2.5V. That is 
not bad but not as fast as I'd like. I'm thinking of reducing R16/R17 to 68 
Ohm to get the impedance closer to 50 Ohm.

Best regards,
Matthias

On Mittwoch, 30. Dezember 2020 17:56:37 CET Matthias Welwarsky wrote:
> 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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