68HC05P9A datasheet(48/90 Pages) FREESCALE

48 / 90 page

GENERAL RELEASE SPECIFICATION

TIMER

MC68HC05P9A

8-4

Rev. 2.0

8.3

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 time 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.

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68HC05P9A Datasheet(PDF) 48 Page - Freescale Semiconductor, Inc