MC68HC05B6_13 Datasheet, PDF (61/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

5.1.1 Counter register and alternate counter register
Timer counter high
Timer counter low
Address bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
$0018
1111 1111
$0019
1111 1100
Address bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
State
on reset
Alternate counter high
$001A
1111 1111
Alternate counter low
$001B
1111 1100
5
The double-byte, free-running counter can be read from either of two locations, $18-$19 (counter
register) or $1A-$1B (alternate counter register). A read from only the less significant byte (LSB)
of the free-running counter ($19 or $1B) receives the count value at the time of the read. If a read
of the free-running counter or alternate counter register first addresses the more significant byte
(MSB) ($18 or $1A), the LSB is transferred to a buffer. This buffer value remains fixed after the first
MSB read, even if the user reads the MSB several times. This buffer is accessed when reading
the free-running counter or alternate counter register LSB and thus completes a read sequence
of the total counter value. In reading either the free-running counter or alternate counter register,
if the MSB is read, the LSB must also be read to complete the sequence. If the timer overflow flag
(TOF) is set when the counter register LSB is read then a read of the timer status register (TSR)
will clear the flag.
The alternate counter register differs from the counter register only in that a read of the LSB does
not clear TOF. Therefore, where it is critical to avoid the possibility of missing timer overflow
interrupts due to clearing of TOF, the alternate counter register should be used.
The free-running counter is set to $FFFC during power-on and external reset and is always a
read-only register. During a power-on reset, the counter begins running after the oscillator start-up
delay. Because the free-running counter is 16 bits preceded by a fixed divide-by-4 prescaler, the
value in the free-running counter repeats every 262,144 internal bus clock cycles. TOF is set when
the counter overflows (from $FFFF to $0000); this will cause an interrupt if TOIE is set.
In some particular timing control applications it may be desirable to reset the 16-bit free running
counter under software control. When the low byte of the counter ($19 or $1B) is written to, the
counter is configured to its reset value ($FFFC).
The divide-by-4 prescaler is also reset and the counter resumes normal counting operation. All of
the flags and enable bits remain unaltered by this operation. If access has previously been made
to the high byte of the free-running counter ($18 or $1A), then the reset counter operation
terminates the access sequence.
Warning: This operation may affect the function of the watchdog system (see Section 9.1.4). The
PLM results will also be affected while resetting the counter.
MC68HC05B6
Rev. 4.1
PROGRAMMABLE TIMER
Freescale
5-3