Freescale Semiconductor, Inc.
GENERAL RELEASE SPECIFICATION
April 30, 1998
9.2
ALTERNATE COUNTER REGISTERS (ACRH, ACRL)
The functional block diagram of the 16-bit free-running timer counter and alternate
counter registers is shown in Figure 9-4. The alternate counter registers behave
the same as the timer registers, except that any reads of the alternate counter will
not have any effect on the TOF ag bit and Timer interrupts. The alternate counter
registers include a transparent buffer latch on the LSB of the 16-bit timer counter.
INTERNAL
DATA
BUS
READ
ACRL
LATCH
ACRL ($001B)
TMR LSB
READ
ACRH
READ
ACRH ($001A)
($FFFC)
INTERNAL
CLOCK
RESET
÷
4
16-BIT COUNTER
(XTAL
÷ 2)
Figure 9-4. Alternate Counter Block Diagram
The alternate counter registers (ACRH, ACRL) shown in Figure 9-5 are read-only
locations which contain the current high and low bytes of the 16-bit free-running
counter. Writing to the alternate counter registers has no effect. Reset of the
device presets the timer counter to $FFFC.
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
ACRH
$001A
R ACRH7 ACRH6 ACRH5 ACRH4 ACRH3 ACRH2 ACRH1 ACRH0
W
reset:
1
1
1
1
1
1
1
1
ACRL
$001B
R ACRL7 ACRL6 ACRL5 ACRL4 ACRL3 ACRL2 ACRL1 ACRL0
W
reset:
1
1
1
1
1
1
0
0
Figure 9-5. Alternate Counter Registers (ACRH, ACRL)
The ACRL latch is a transparent read of the LSB until the a read of the ACRH
takes place. A read of the ACRH latches the LSB into the ACRL location until the
ACRL is again read. The latched value remains xed even if multiple reads of the
ACRH take place before the next read of the ACRL. Therefore, when reading the
MSB of the timer at ACRH the LSB of the timer at ACRL must also be read to
complete the read sequence.
During power-on-reset (POR), the counter is initialized to $FFFC and begins
counting after the oscillator start-up delay. Because the counter is sixteen bits and
preceded by a xed divide-by-four prescaler, the value in the counter repeats
every 262,144 internal bus clock cycles (524,288 oscillator cycles).
Reading the ACRH and ACRL in any order or any number of times does not have
any effect on the 16-bit free-running counter or the TOF ag bit.
16-BIT PROGRAMMABLE TIMER
MC68HC05PL4
REV 2.0
9-4
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