mc68hc908as60 Freescale Semiconductor, Inc, mc68hc908as60 Datasheet - Page 381

• Manufacturer Part Number: mc68hc908as60
• Description: Mc68hc908as60 Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

22.4.2 Input Capture
MC68HC908AS60 — Rev. 1.0
An input capture function has three basic parts: edge select logic, an
input capture latch, and a 16-bit counter. Two 8-bit registers, which make
up the 16-bit input capture register, are used to latch the value of the
free-running counter after the corresponding input capture edge detector
senses a defined transition. The polarity of the active edge is
programmable. The level transition which triggers the counter transfer is
defined by the corresponding input edge bits (ELSxB and ELSxA in
TASC0 through TASC5 control registers with x referring to the active
channel number). When an active edge occurs on the pin of an input
capture channel, the TIMA latches the contents of the TIMA counter into
the TIMA channel registers, TACHxH–TACHxL. Input captures can
generate TIMA CPU interrupt requests. Software can determine that an
input capture event has occurred by enabling input capture interrupts or
by polling the status flag bit.
The result obtained by an input capture will be two 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.
The free-running counter contents are transferred to the TIMA channel
status and control register (TACHxH–TACHxL) (see
Channel
whether the TIMA channel flag (CH0F–CH5F in TASC0–TASC5
registers) is set or clear. When the status flag is set, a CPU interrupt is
generated if enabled. The value of the count latched or “captured” is the
time of the event. Because this value is stored in the input capture
register two bus cycles after the actual event occurs, user software can
respond to this event at a later time and determine the actual time of the
event. However, this must be done prior to another input capture on the
same pin; otherwise, the previous time value will be lost.
By recording the times for successive edges on an incoming signal,
software can determine the period and/or pulse width of the signal. To
measure a period, two successive edges of the same polarity are
captured. To measure a pulse width, two alternate polarity edges are
captured. Software should track the overflows at the 16-bit module
counter to extend its range.
Freescale Semiconductor, Inc.
For More Information On This Product,
Timer Interface Module A (TIMA-6)
Registers) on each proper signal transition regardless of
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Timer Interface Module A (TIMA-6)
22.9.5 TIMA
Functional Description
Technical Data