Due to my special interests in a customized RC transmitter and receiver system I started with reverse engineering of a Corona RP8D1 35MHz receiver.
The RP8D1 is my favourite candidate cause it´s a double superheterodyne receiver, uses a PLL synthesizer and is controller by an AVR ATmega88V.
Interesting fact about the micro-controller:
Corona obviously has been changed the controller type on actual receiver boards. The first receiver I bought in mid 2010 uses the mentioned ATmega88V.
Two newer receiver, bought in January 2011, are using a controller marked “225K01” with manufacturer “QRG”.
Fact is the PCB layout is 100% identical on both receivers so must be the pin-out of the different controllers!
So the 225K01 is either a clone or some official AVR derivative, already contacted Atmel for details.
Used components, followed by pictures and the controller pin-out:
- Controller AVR ATmega88V on mid 2010 board
- Controller QRG 225K01 on 2011 boards
- Rohm BU2630 Dual PLL
- Rohm BH4126 Wideband IF Detector
- Philips SA616DK High Performance Mixer FM IF System
Board top layer:
Board top layer, filter and crystal removed:
A word about the second LED (PLL lock detect) on the RP8D1: The LED will signal the status of the PLL control loop. If the LED lights the PLL is out of sync, that means the synthesizer can not set the receiving frequency correctly. For normal operation the LED shall be always off.
Board bottom layer:
There is a typo in the picture, the controller type is 225K01.
The RSSI signal can be picked up for external use easily on one of the resistors instead of removing the filter and wiring to the SA616 pin 5.
Receiver board with AVR ATmega88V controller:
Receiver board with QRG 225K01 controller:
||Bind button input
||Bind LED output
||PPM sum signal input
||Channel 1 PPM output
||Channel 2 PPM output
||Channel 3 PPM output
||Channel 4 PPM output
||Channel 5 PPM output
||Channel 6 PPM output
||RSSI signal input
||PPM signal threshold for analog comparator, level set by ext. voltage divider
||PLL CE signal output
||PLL DATA signal output
||PLL CLK signal output
||Channel 9 PPM output
||Channel 8 PPM output
||Channel 7 PPM output
Unfortunately pin PC6, the RESET input, is used as IO pin, this means the ISP serial interface is disabled and one can´t dump the flash and EEPROM content. (Already tried 😉 ) Therefore only parallel programming is the way to go which is quite complicated due to the MLF package and required number of wiring connections.
Or maybe there is a hidden boot loader, the USART pins are available on the channel sockets, just need to find out how to activate boot mode if exists.
I made a HV programming adapter and tried to dump the flash of the controller. Unfortunately the memory protection fuse is set, so that´s not an option.
The boot reset fuse isn´t set so there is no bootloader installed.
A fact: The QRG 225K01 is definately an ATmega88 since the signature bytes match in AVR-Studio.
Some more work to do. In any case a firmware for the AVR can must be programmed from scratch.