> T0_INTR: DJNZ TCOUNT,T0_EXIT
> MOV TCOUNT,#20

This means that timer SFRs will NOT
be reloaded for the next 19 overflows.
First overflow will occur after 65281
machine cycle, and other -- after
65536.
These two lines should be put just
before CPL P1.5.
By the way, what clock frequency one
should have to get an overflow after
50 ms, while T0 is set up for -00FFh =
0FF01h (65281) cycles? My calculations show 15.667 MHz. I never saw such
crystals, but nevertheless...