MOTOROLA
5-6
MC68HC05B6
Rev. 4
PROGRAMMABLE TIMER
5
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 satises the rst condition required to clear the status bits. The
remaining step is to access the register corresponding to the status bit.
ICF1 — Input capture ag 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 ag 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 overow status ag
This bit is set when the free-running counter overows from $FFFF to $0000; a timer overow interrupt
will occur if TOIE is set. TOF is cleared by reading the TSR and the counter low register ($19).
1 (set)
–
Timer overow has occurred.
0 (clear) –
No timer overow has occurred.
When using the timer overow function and reading the free-running counter at random times to
measure an elapsed time, a problem may occur whereby the timer overow ag 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 ag.
Reading the alternate counter register instead of the counter register will avoid this potential
problem.
Address
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
Timer status (TSR)
$0013
ICF1
OCF1
TOF
ICF2
OCF2
Undened
TPG
62
05B6Book Page 6 Tuesday, April 6, 1999 8:24 am