RJTE Bootloader Protocol

Serial port configuration is 115200 baud 8N1.
One page is 64 bytes.
PROG is active high. It is actually the DTR line on the serial port.

Host	Target
Drive PROG active
	Reset: Bootloader begins execution
	PROG is active so bootloader continues (if PROG is inactive then user code is executed)
	Send 54 31 to host
Read 54 31 from target
Send 48 01 to target
	Read 48 01 from host

Send page of data to target		Loop until all data is transferred
	Read page of data from host
	Write data to flash
	Send checksum to host
Read checksum from target
Verify checksum

Drive PROG inactive
	Detect PROG is inactive and execute user code (this is actually the error-detecting case for waiting for another page of data from the host).

Checksum calculation

The checksum is calculated as the sum of each 16-bit little-endian word sent from the host. It is sent to the host as a 16-bit little-endian value.

Error detection and handling

An error has occurred if:

Either handshake byte from the host (48 01) takes more than 30ms to arrive.
Invalid data is received (incorrect handshake bytes or bad checksum)
The PROG line goes inactive at any time

If the host detects an error, it informs the user, drives PROG inactive, and performs no further communication with the target.

If the target detects an error, it executes the user code. The host causes an error condition intentionally to stop downloading data and cause the user code to be executed.