MC68HC05C8A Datasheet, PDF (44/116 Pages) Motorola, Inc – Microcontrollers

Timer
8.4 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 except when exiting
stop mode.
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. Figure 8-3 shows the logic of the input capture function.
TCMP
15
$0018
87
$0019
0
TIMER REGISTER HIGH
TIMER REGISTER LOW
EDGE
SELECT/DETECT
LOGIC
LATCH ¹⁵
87
0
INPUT CAPTURE REGISTER HIGH INPUT CAPTURE REGISTER LOW
$0014
$0015
TIMER
INTERRUPT
REQUEST
TIMER CONTROL REGISTER
$0012
TIMER STATUS REGISTER
$0013
Figure 8-3. Input Capture Operation
MC68HC05C8A • MC68HCL05C8A • MC68HSC05C8A Data Sheet, Rev. 5.1
44
Freescale Semiconductor