Sorry members but this is entirely hardware info. I bought a couple of these cheap NEO-8MU modules from Ali. rather than just copy and upload a sketch I thought I would see how it worked first (and didn’t). My modules had a remote active antenna on a short pigtail co-ax, some have the antenna fixed to the board, others have an SMA connector for a remote active antenna.
I have a CH340 USB UART module which has a 5V/3.3V switch. This is important because the NEO module RX/TX data lines are direct to the NEO chip which operates at 3.3V. This confused me at first, because I can connect the NEO module VCC to 3.3V or 5V. On inspection the module has a 3.3V regulator, therefore its VCC is 5V but serial lines are 3.3V. This is kind of explained when hooking up to U-Blox software, but their UART interface takes care of input voltages.
So I connect the CH340 board, USB power comes from PC and I get power on leds but not much else. Then I reversed the TX/RX lines and the TX led starts flashing but I’m not seeing anything on the U-Blox monitor app. I then ran Windows Terminal for 9600 Baud (PuTTY) or others work and I see scrolling screens of junk polling when the RX led lights.
After messing about I discover the NEO module has a red ‘lock’ led. Well it isn’t that, but the 1PPS sat. clock output when locked to 3 or more satellites. That does mean you may not yet have a stable location until more satellites are locked.
I realized having the antenna integrated was pain for testing, because indoor reception can be very poor. I soldered a 1.6mm pcb SMA connector on the board to allow a remote GPS antenna to be hung out of a window. It took about 4 minutes to get a flashing PPS led. during this time U-Blox app wasn’t doing anything. It can hang and once the NEO module gets time lock, it’s best to restart the app.
So far non of this needs the CH340 UART to test the NEO module, you just need +5V on the VCC line and see if the PPS led flashes. I’m now concerned at the long acquisition times, particularly when power is removed and restored. The first test was to disconnect and reconnect the antenna with the module powered, Time lock came back within 2 seconds which I’d expect from a Hot start. But removing power, the lock time was 2-3 minutes??
These modules have EEPROM memory backup (24C32). There are 2 specs. I assume they used the low voltage version with a minimum voltage of 1.8V. I checked it and with power on I got 3.3V, and with power removed it was 0.2V. Next step look at the backup battery which I think is a super capacitor? It was charging at 3.3V but off charge was only 0.2V. Conclusion: On the slow boat from China and time spent in their warehouse, the memory capacitor was fully discharged and either failed or would not accept charge?
I tried removing it with a hot air gun - Disaster because their cheap pcb track started lifting. Eventually I got it off using a soldering iron. I had a larger memory capacitor 0.22F, 3 X the original capacity. Not the best solution but I glued it to the NEO module case.
I can disconnect the antenna or power source for an hour and the module will hot start within 2 seconds. I haven’t used it with a sketch yet, but I’m surprised that before it locks, it sends out blocks of corrupt data? I’ll attach some photos.
