Dear Chico,

Have you checked value of R0.

What I believe is you have posted only serial ISR, isnt it?

If my understanding is correct, you are initialising R0 in say Nth occurance of ISR & using it in (N+1)th occurance of ISR. but in between the 2 occurances, your code is getting executed at some other location. Now this execution may disturb the value of R0.

To overcome this problem, use different BANK for the ISR. make sure this bank or direct address of register is not used in any part of your code

To do this modify your code little bit like this

SERI:
push acc

push psw
mov psw, #08 ;selecting bank 1

jb RI,LER

(remaining part of your code as it is till before MidiEnd)

MidiEnd:

pop psw

pop acc
reti


Try this out and reply.


Note: make sure you are not using R0 of BANK1 anywhere else in code. Also make sure you are not manipulating content of memory address 0x08 directly or indirectly


Regards,
Mahesh