68HC705JB4 datasheet(60/134 Pages) FREESCALE

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GENERAL RELEASE SPECIFICATION

February 24, 1999

MOTOROLA

16-BIT TIMER

MC68HC05JB4

9-4

REV 2

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 ﬂag bit and Timer interrupts. The alternate counter

registers include a transparent buffer latch on the LSB of the 16-bit timer counter.

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.

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.

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

ACRH

ACRH7

ACRH6

ACRH5

ACRH4

ACRH3

ACRH2

ACRH1

ACRH0

$001A

reset:

11111111

ACRL

ACRL7

ACRL6

ACRL5

ACRL4

ACRL3

ACRL2

ACRL1

ACRL0

$001B

reset:

11111100

Figure 9-5. Alternate Counter Registers (ACRH, ACRL)

ACRH ($001A)

TMR LSB

16-BIT COUNTER

÷ 4

INTERNAL

(fOSC ÷ 2)

RESET

CLOCK

ACRL ($001B)

INTERNAL

($FFFC)

DATA

READ

ACRH

READ

ACRL

READ

LATCH

BUS

Freescale Semiconductor, Inc.

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