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 > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow > > the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
