[time-nuts] Ublox 8T - breakout board

[Bob kb8tq]

Hi

If you are looking at the “raw” output of the uBlox, it is not designed to drive
a low impedance. Even if there is a buffer, it may or may not like a low impedance
load. 

Bob

> On Dec 31, 2020, at 7:39 PM, Matthias Welwarsky <time-nuts at welwarsky.de> wrote:
> 
> 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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> 
> 
> 
> 
> 
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