GENERAL RELEASE SPECIFICATION
August 27, 1998
10.2 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 10-4. 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 flag 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
OSC
RESET
÷ 4
16-BIT COUNTER
(f
÷ 2)
Figure 10-4. Alternate Counter Block Diagram
The alternate counter registers (ACRH, ACRL) shown in Figure 10-5 are read-
only locations which contain the current high and low bytes of the 16-bit free-run-
ning 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
ACRL7
1
1
ACRL6
1
1
ACRL5
1
1
ACRL4
1
1
ACRL3
1
1
ACRL2
1
1
ACRL1
0
1
ACRL0
0
ACRL
$001B
R
W
reset:
Figure 10-5. 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 fixed 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 fixed 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 flag bit.
MOTOROLA
10-4
16-BIT TIMER
MC68HC05SB7
REV 2.1
FREESCALE [ Freescale ]
