The PC interface will be how I change firmware. I can also use the USB connector for charging of the battery. I can use a dedicated USB to UART bridge or I can use a microcontroller for this purpose. The dedicated USB to UART bridge is very simple to set up. The microntroller will need it’s own firmware which can be updated via the USB connection.
The least expensive FTDI chip to go from USB to UART(FT230XS-R) is $1.438 on DigiKey. Microchip has a microcontroller that can be used as a USB client without a crystal. The Microchip PIC16F1455 is $1.18 in quantities of 100. Having a seperate microcontroller on the board can be used to create more pin availability if it becomes necessary.
I decided on the microcontroller option. I think I am going to want two ADC channels. I can use the microcontroller to read these analogue values and report them over serial. I might also use the PWM to operate a switching voltage regulator for the VPP voltages.
I have created the PIC16F1455 in the Programmer library. I then added it to the schematic, I added a programming connector, and USB connnector. I also added a Capacitor to the Vusb pin of the chip. This is required for proper regulation of the USB interface voltage. Microchip recommends a 0.47 µF.
I Received the ESP-12E Modules this week. I will start basic testing.