MC68HC05P18 Datasheet, PDF (87/144 Pages) Freescale Semiconductor, Inc – Advance Information

Freescale Semiconductor, Inc.
16-Bit Timer
Timer
Because the timer has a 16-bit architecture, each function is represented
by two registers. Each register pair contains the high and low byte of that
function. Generally, accessing the low byte of a specific timer function
allows full control of that function; however, an access of the high byte
inhibits that specific timer function until the low byte is also accessed.
NOTE:
The I bit in the condition code register (CCR) should be set while
manipulating both the high and low byte registers of a specific timer
function. This prevents interrupts from occurring between the time the
high and low bytes are accessed.
10.3 Timer
The key element of the programmable timer is a 16-bit free-running
counter, or timer registers, preceded by a prescaler which divides the
PH2 clock by four. The prescaler gives the timer a resolution of 2.0
microseconds when a 4-MHz crystal is used. The counter is incremented
to increasing values during the low portion of the PH2 clock cycle.
The double byte free-running counter can be read from either of two
locations: the timer registers (TMRH and TMRL) or the alternate counter
registers (ACRH and ACRL). Both locations will contain identical values.
A read sequence containing only a read of the LSB of the counter
(TMRL/ACRL) will return the count value at the time of the read. If a read
of the counter accesses the MSB first (TMRH/ACRH), it causes the LSB
(TMRL/ACRL) to be transferred to a buffer. This buffer value remains
fixed after the first MSB byte read, even if the MSB is read several times.
The buffer is accessed when reading the counter LSB (TMRL/ACRL)
and thus completes a read sequence of the total counter value. When
reading either the timer or alternate counter registers, if the MSB is read,
the LSB must also be read to complete the read sequence. See
Figure 10-2 and Figure 10-3.
MC68HC(8)05P18 — Rev. 2.0
MOTOROLA
16-Bit Timer
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Advance Information
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