MC68HC05B6_13 Datasheet, PDF (64/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

5.2.2 Timer status register (TSR)
The timer status register ($13) is a read only register and contains the status bits corresponding
to the four timer interrupt conditions – ICF1,OCF1, TOF, ICF2 and OCF2.
Accessing the timer status register satisfies the first condition required to clear the status bits. The
remaining step is to access the register corresponding to the status bit.
Timer status (TSR)
Address bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
$0013 ICF1 OCF1 TOF ICF2 OCF2
Undefined
5
ICF1 — Input capture flag 1
This bit is set when the selected polarity of edge is detected by the input capture edge detector 1
at TCAP1; an input capture interrupt will be generated, if ICIE is set. ICF1 is cleared by reading
the TSR and then the input capture low register 1 ($15).
1 (set) – A valid input capture has occurred.
0 (clear) – No input capture has occurred.
OCF1 — Output compare flag 1
This bit is set when the output compare 1 register contents match those of the free-running
counter; an output compare interrupt will be generated if OCIE is set. OCF1 is cleared by reading
the TSR and then reading or writing the output compare 1 low register ($17).
1 (set) – A valid output compare has occurred.
0 (clear) – No output compare has occurred.
TOF — Timer overflow status flag
This bit is set when the free-running counter overflows from $FFFF to $0000; a timer overflow interrupt
will occur if TOIE is set. TOF is cleared by reading the TSR and the counter low register ($19).
1 (set) – Timer overflow has occurred.
0 (clear) – No timer overflow has occurred.
When using the timer overflow function and reading the free-running counter at random times to
measure an elapsed time, a problem may occur whereby the timer overflow flag is unintentionally
cleared if:
1 The timer status register is read or written when TOF is set, and
1) The LSB of the free-running counter is read, but not for the purpose of
servicing the flag.
Reading the alternate counter register instead of the counter register will avoid this potential
problem.
Freescale
5-6
PROGRAMMABLE TIMER
MC68HC05B6
Rev. 4.1