I took a look inside this old, Futaba-sized, Corona module.
The Corona protocol uses the same CC2500 module that the FrSky protocols use, and look at that chip (click photos for larger views) - an AtMega48V - this is pretty similar to the AtMega328 that our 8-bit Arduinos use, except that it only has 4K of Flash memory. What's more, it appears to have tracks running to PCB holes where we could attach a USBasp programmer.
My V8 ccpm sketch only uses 3274 bytes of flash - so it will fit, and it should be pretty easy to "upgrade" the module to run FrSky V8 protocol using my library - or to the FrSky D8 and D16 protocols, once I release the libraries for those.
The question is, should I bother - or is it better to leave the module working in its classic Corona form?
Edit: I searched one of my boxes of radio gear and found another Corona module - this time in JR module bay format. As expected, it has the same AtMega48V chip and programming port inside. I think at least one of these is ripe for experimentation! I see Corona modules still for sale cheap on BangGood, AliExpress, and Ebay. if they can be easily flashed to use FrSky D, or V, or even X protocol, then they're a cheap alternative to the increasingly difficult to find, or very expensive FrSky modules.
Edit: I've belled out the connector pads - they are indeed an ICSP connector for attaching a (3.3 V) USBasp or similar programmer. It's six pins in a line with 0.1 inch spacing. Starting from pin 1 (the square pad) the connections go: 1:MISO 2:VCC 3:SCK 4:MOSI 5:RST 6:GND
The other set of three connector pads, also on 0.1 inch spacing are GND, TX, and RX. I wonder if it has an Arduino bootloader that works for serial upload on those pins?
Even if it doesn't, those pins will be handy for debugging, setting the fine-tuning value, or transmitter ID.
The two-colour LED is driven by pins PC0 and PC1 (that's A0 and A1 in Arduino language). The bind button links PC2 (A2) to GND when pressed. The PPM pin goes to PB0 (Arduino pin '8'); on the Futaba module it just goes via some passive components (resistors and capacitor) but on the JR module there is also a transistor in the line.
The chip select line for the CC2500, CSn is PB2 (10). The other SPI pins of the CC2500 are connected to the AtMega's SPI hardware pins (and therefore also to the ICSP connector).
PB1 (9), I think, switches the power amplifier of the RF sub-board - when it's transmitting Corona protocol, that line switches high while it sends 3 packets, then low for a while before the next three packets... but in bind mode it stays low always. PD7 (7) also seems to control transmit power - it's always high in normal Corona transmitting mode, and always low when binding.
PD2 (2) switches high about 1.5ms after each SPI operation. It stays high for about 1.5ms and then goes low again. I don't know what that does - maybe some kind of 'idle' signal from the RF sub-board?
PD3, PD5, and PD6 (3, 5, 6) also connect to the sub-board, but I've not seen them change either in normal mode or when binding: 3 seems to always be HIGH; 5, and 6 seem to always be LOW.