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Timer System
General Release Specification
MC68HC(7)05L5 — Rev. 2.0
120
Timer System
MOTOROLA
9.3.3 Input Capture Register
Two 8-bit registers, which make up the 16-bit input capture register, are
read-only and are used to latch the value of the free-running counter
after the corresponding input capture edge detector senses a defined
transition. The level transition which triggers the counter transfer is
defined by the corresponding input edge bit (IEDG). Reset does not
affect the contents of the input capture register.
The result obtained by an input capture will be one more than the value
of the free-running counter on the rising edge of the internal bus clock
preceding the external transition. This delay is required for internal
synchronization. Resolution is one count of the free-running counter,
which is four internal bus clock cycles.
The free-running counter contents are transferred to the input capture
register on each proper signal transition regardless of whether the input
capture flag (ICF) is set or clear. The input capture register always
contains the free-running counter value that corresponds to the most
recent input capture.
After a read of the input capture register ($14) MSB, the counter transfer
is inhibited until the LSB ($15) is also read. This characteristic causes
the timer used in the input capture software routine and its interaction
with the main program to determine the minimum pulse period.
A read of the input capture register LSB ($15) does not inhibit the free-
running counter transfer since they occur on opposite edges of the
internal bus clock.
NOTE:
Since the TCAP pin is shared with the PC3 I/O pin, changing the state
of the PC3 DDR or data register can cause an unwanted TCAP interrupt.
This can be handled by clearing the ICIE bit before changing the
configuration of PC3 and clearing any pending interrupts before
enabling ICIE.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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