Various documentation and example code for Turning Technologies ResponseCard XR/XR2 model RCXR-01

inspired by Travis Goodspeed's most excellent reverse engineering of older versions of these clickers. The RCXR-01 seems to be quite handy as a portable ATmega + nRF24 platform (i.e. handheld control/monitoring). I hope that you will find lots of nice hacks

This is not about reverse engineering the existing software, but rather reusing the hardware completely. You get a nice lcd, matrix keypad, NRF24 radio, usb-uart, and mcu with plenty of flash that can run Arduino (or pure AVR C if that's your thing ;).

(Note that the NXT [RCXR-02/03] is completely different insides - based on MSP430F5528)

• ATmega 644PV mcu
  • 64k flash, 4k ram, 2k eeprom
  • internal RC 1MHz clock (CKDIV8 fuse set)
  • 32khz crystal on TOSC1/2
    • timer2 can be setup as rtc
    • may be missing some loading caps? In some experiments, I'm having a tough time getting the crystal to start. I used 6pF caps, and I think that helped. But also doing some experiments where just grounding the crystal can it seems to help it start up (without needing extra load caps).
• NRF24L01 radio
  • on hardware spi bus (SCK/MISO/MOSI)
  • CE -> PB0, CSN -> PC1
• CAT25128 16KB SPI EEPROM
  • CS -> SS (PB4)
  • doesn't look like TT used the external eeprom for anything.
• FTDI FT232R usb to uart
  • TXD/RXD seem to be connected through some isolator? to mcu
  • has custom PID: 0xdb14
    • flashing to default ftdi PID: sudo ./ft232r_prog --old-pid 0xdb14 --new-pid 0x6001 so serial port is setup on connect
    • https://github.com/eswierk/ft232r_prog
  • DTR/RTS not connected. May want to connect either these to mcu RST (top pad of D5) w/ in-line 1uF cap for auto-reset, i.e. for arduino bootloader (I found 4.7uF cap works better for me.)
• when usb power is present, PB2 is pulled low (via Q3)
• testpoint pads under battery compartment (from left to right)
  • MOSI, RST, SCK, MISO, GND, VDD3v3, SS, TCK, TMS, TDO, TDI
• LCD panel
  • 96x32 matrix plus several custom symbols, possibly ST7565 chipset.
  • U8x8 lib customized: U8X8_ST7565_RCXR01_4W_HW_SPI
  • mcu connectivity: SPI for SCLK & SI. CS -> PD5, RST -> PD6, RS(A0) -> PB3

• (E:FE, H:9D, L:62)
• http://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega644p&LOW=62&HIGH=9D&EXTENDED=FE&LOCKBIT=FF

• use something like usbasp, ftdi, etc. Cheap usbasp adapter worked fine for me. Something similar to this adapter from Banggood
• https://github.com/MCUdude/MightyCore/tree/master/avr/bootloaders/optiboot_flash/atmega644p
• choose a bootloader option:

• slower option (recommend instead 8MHz@38400)
• use optiboot_flash_atmega644p_9600_1000000L.hex
• set fuses: avrdude -v -p atmega644p -c usbasp -P usb -e -U lock:w:0x3f:m -U efuse:w:0xfe:m -U hfuse:w:0xd6:m -U lfuse:w:0x62:m
• flash bootloader: avrdude -v -p atmega644p -c usbasp -P usb -U flash:w:optiboot_flash_atmega644p_9600_1000000L.hex:i -U lock:w:0x0f:m
• now should be able to use arduino bootloader at 9600 for uploading (see note above about connecting RTS/DTR to RST for auto-reset.)

• use optiboot_flash_atmega644p_38400_8000000L.hex
• set fuses: avrdude -v -p atmega644p -c usbasp -P usb -e -U lock:w:0x3f:m -U efuse:w:0xfe:m -U hfuse:w:0xd6:m -U lfuse:w:0xe2:m (unsets CKDIV8)
• flash bootloader: avrdude -v -p atmega644p -c usbasp -P usb -U flash:w:optiboot_flash_atmega644p_38400_8000000L.hex:i -U lock:w:0x0f:m
• should allow you to use 38400 upload speed. Note: adjust upload_speed in platformio.ini
• bootloader will run at 8MHz. This should be safe because with usb, vcc should always be above 2.7V. Application sketch should run at <=4MHz in order to run off of battery down to BOD voltage of 1.8V. Demo app here sets clock prescaler down to 1MHz.

• using MightyCore board support for Arduino framework
• using platformio for build/upload
• pinouts and pin number mappings: https://github.com/MCUdude/MightyCore#pinout