68HC11 Timing System

Clearing the TOF bit
Many of the flag bits in the 68HC11 control registers, including the
TOF, are cleared by writing a 1 into the bit position of the flag.
Writing a 0 into the bit position has no effect on the flag.
A routine to clear TOF is:
SUB1: LDAA #$80 ; Set bit 7 to 1
STAA $1025 ; Write it to TFLG2
RTS
; Return
A flag can also be cleared using a BCLR instruction, with a 0 in
the bit position to be cleared. This is because the 68HC11 uses
the inverse mask to clear the bit. To clear TOF, for example, the
following instructions would work:
LDX #$1000
BCLR $25 , X $7F
10