Jeroen, your code has many problems. Firstly you alter the A reg in the interrupt service routine without saving it, you probably alter the PSW register also. As you expand your code, this will cause weird problems.


clr TR0
mov TH0, #48h ; set for 20 ms
mov TL0, #00 ;
setb TR0
push a
push psw

mov a,tcount ;use ram instead of a register - makes to code easier to expand
jz,xx1
dec tcount
sjmp xx2
xx1
mov tcount,#250
mov a,#'C'
mov SBUF,a
clr TI
xx2
pop psw
pop a
reti


You don't need to clear TF0 - the interrupt hardware does this for you.