NEO-M8T More than 100 frame errors, UART RX was disabled - RX line idles high

0 votes
Hello All,

I am trying to get a NEO-M8T u-blox receiver to accept configuration commands through UART from a microprocessor however, after a few seconds, it starts to send out a GNTXT NMEA message stating "More than 100 frame errors, UART RX was disabled". I've read through the other posts on this issue and it seems the main two reasons for this would be a low idle on the RX line or a bad ground plane.

I did notice that when I wasn't sending any messages from the micro, the u-blox RX line was in-fact low. I used a 10K resistor to pull up the RX and TX lines (to 3.3V), however the frame errors message remains.

I've also connected the u-blox to my PC through an RS-232 level-shifter to try and get communication working with u-center and I am still seeing the same issue. TX from the u-blox works fine in both applications. I am able to see and parse the default NMEA messages.

Is there something I am misunderstanding in the way these ICs should be integrated? Thanks in advace!
by avlasov1 asked Jul 11
Thank you both for the responses. We took another look at our design, and corrected a few items found in the hardware integration manual. Now I can send messages from u-center and receive acknowledgement. Now to get the same functionality working with the microprocessor.
+1 vote
It is suggestive of some cross-connectivity on the RX pin. You should probably review signal integrity with a scope or logic analyzer.

Check also supply voltages.

Check with the PC when you connect to the RX pin (RX NEO to RX USB-to-CMOS Serial) and see if you see data/traffic/noise at 9600 baud.

If you follow the directions in the Hardware Integration Manual you should end up with a viable design.
by clive1 answered Jul 11
by avlasov1 selected Jul 12
Best answer
+1 vote
Another possible cause is that the RX input on NEO was damaged by ESD or by connecting a drive voltage higher than 0.7 V above NEO VCC or in excess of 3.6V
by grampy answered Jul 12
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