MC68HC705CT4 Datasheet, PDF (62/152 Pages) Freescale Semiconductor, Inc – General Release Specification

16-Bit Timer
Freescale Semiconductor, Inc.
8.4 Output Compare Register
The 16-bit output compare register is made up of two 8-bit registers at
locations $16 (MSB) and $17 (LSB). The output compare register is
used for several purposes, such as indicating when a period of time has
elapsed. All bits are readable and writable and are not altered by the
timer hardware or reset. If the compare function is not needed, the two
bytes of the output compare register can be used as storage locations.
The output compare register contents are continually compared with the
contents of the free-running counter. If a match is found, the
corresponding output compare flag (OCF) bit is set and the
corresponding output level (OLVL) bit is clocked to an output level
register. The output compare register values and the output level bit
should be changed after each successful comparison to establish a new
elapsed time-out. An interrupt can also accompany a successful output
compare provided the corresponding interrupt enable bit (OCIE) is set.
After a processor write cycle to the output compare register containing
the MSB ($16), the output compare function is inhibited until the LSB
($17) is also written. The user must write both bytes (locations) if the
MSB is written first. A write made only to the LSB ($17) will not inhibit the
compare function. The free-running counter is updated every four
internal bus clock cycles. The minimum time required to update the
output compare register is a function of the program rather than the
internal hardware.
The processor can write to either byte of the output compare register
without affecting the other byte. The output level (OLVL) bit is clocked to
the output level register regardless of whether the output compare flag
(OCF) is set or clear.
General Release Specification
MC68HC705CT4 — Rev. 2.0
62
16-Bit Timer
MOTOROLA
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