Mario.
I made a 8051 loader for in application flash programming that has a complete packet protocol including CRC check bytes on assembler language. I would let you look at the code. Right now it is coded as a slave responder. You would have to configure a master level above the send/receive routines. Also you would have to add the addressing byte to the packet layout. I coded specifically with interrupts and for
a Cygnal processor but the code can be adapted to UART and timer on any 8051 part. Send email if you want to see it.
Michael Karas