The main controller U1 writes a file (hex) to the microSD (fat32 SDHC) and issues a signal to the controller U2 to start programming.
The U2 controller receives a signal to start programming from U1, reads a file (char * fname) from the SD root directory, updates the sketch to U1 and transfers control to U1.
To access the SD card using SPI interface.
For communication between U1 and U2, the serial port is used (pin 0, 1)
Provide connection of TTL Terminal sSerial to U1 where control information will be displayed.
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Checking the performance of the circuit and the program is performed by reading test programs.