After a few private emails I have changed the MPU to the PIC32MX2xx and have added a USB port, the reasons being that
1. We can now support a USB bootloader and possibly some of the Arduino like pic32 environments like the chipkit
2. I can add a small NiMH battery charger circuit to recharge the AA cells via the usb.
The original prototype way back used a bigger chip and supported USB but all were removed in the simplification that followed. I've decided to risk adding it to the new PCB for completeness.
The new circuit and board have been updated to Rev5 for anyone interested so you can check out the changes which are:
1. Changed from PIC32MX1xx to PIC32MX2xx and added USB. 4 pins are lost due to USB support on this chip so I have had a bit of a shuffle around of the pins and removed the LCD_RESET which was not really necessary IMHO. K5, K6 and K7 have also been displaced and the scan/power on is only used for powering on the boost converter now (though I was toying with connecting it to the reset MCLR pin for a soft reset)
2. Added a charger circuit. note the charger circuit is pretty base as I will need to experiment with resistor and/or pulse settings to ensure safe charging, by default charging is off by setting the gate of the FET high, the idea is when I detect USB power (VUSB) then I will turn on the gate by setting it low for 5 seconds, turn it back off then check the voltage, this will repeat until I see the correct voltages on VBat (I put a resistor divider here but then realised the mav battery volts will always be less than 3.3V so really this is not required.
3. I have added a FET in the WiFi power line, I should have left this in from the last version but was deceived by the ESP8266 chip enable. There are a couple of big caps here, I will experiment with the values and/or if they are required (the ESP8266 has a bit of a surge voltage on transmit and power on) by default on power up the wifi is off, I will power it on -> send -> receive -> power off hopefully in a few seconds so each time it will boot from cold and hopefully get around some of the issues others are having with the firmware/quality of these devices.
As a note to self I will not solder in a wifi module until I test it connects and sends. I'm hoping the quality will improve as more manufacturers make these if not I will add the ESP8266 chip directly onto the circuit. This is a bit too much of a step at the moment as I still do not have all the time I wish on this project due to other commitments (ce'st la vie!)
4. I have removed the MOSFET for the backlight on the GLCD it is only using 10mA and can be easily sourced direct from the pic.
The new boards (and a stencil) are being manufactured now, fingers crossed I've got it right!
On the good side I'm getting quite familiar now with the PIC32 and MPLABX and I think I've discovered most of the problems I had in my original firmware and I"ve also met a mechanical engineer called Eddie who has a laser cutter and 3D printer so I can plan the mark 2 version without worrying about the case.