mc68hc05pl4b Freescale Semiconductor, Inc, mc68hc05pl4b Datasheet - Page 55

• Manufacturer Part Number: mc68hc05pl4b
• Description: Low-cost Single-chip Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

9.3
MC68HC05PL4
REV 2.0
RESET
TCAP
To prevent interrupts from occurring between readings of the ACRH and ACRL,
set the I bit in the condition code register (CCR) before reading ACRH and clear
the I bit after reading ACRL.
INPUT CAPTURE REGISTERS
The input capture function is a technique whereby an external signal (connected
to PA2/TCAP pin) is used to trigger the 16-bit timer counter. In this way it is possi-
ble to relate the timing of an external signal to the internal counter value, and
hence to elapsed time.
When the input capture circuitry detects an active edge on the selected source, it
latches the contents of the free-running timer counter registers into the input cap-
ture registers as shown in Figure 9-6.
Latching values into the input capture registers at successive edges of the same
polarity measures the period of the selected input signal. Latching the counter val-
ues at successive edges of opposite polarity measures the pulse width of the sig-
nal.
The input capture registers are made up of two 8-bit read-only registers (ICRH,
ICRL) as shown in Figure 9-7. The input capture edge detector contains a Schmitt
trigger to improve noise immunity. The edge that triggers the counter transfer is
de ned b y the input edge bit (IEDG) in the TCR. Reset does not affect the con-
tents of the input capture registers.
READ
ICRH
Figure 9-6. Timer Input Capture Block Diagram
Freescale Semiconductor, Inc.
& DETECT
SELECT
For More Information On This Product,
LOGIC
EDGE
$0012
TIMER CONTROL REG.
16-BIT PROGRAMMABLE TIMER
Go to: www.freescale.com
LATCH
April 30, 1998
ICRH ($0014)
16-BIT COUNTER
INPUT CAPTURE (ICF)
NOTE
ICRL ($0015)
TIMER STATUS REG.
GENERAL RELEASE SPECIFICATION
$0013
4
INTERNAL
INTERNAL
DATA
DATA
BUS
BUS
INTERNAL
(XTAL 2)
READ
INTERRUPT
ICRL
CLOCK
REQUEST
TIMER
9-5