MC68HC711PH8 Motorola, MC68HC711PH8 Datasheet - Page 142

• Manufacturer Part Number: MC68HC711PH8
• Description: High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit
• Manufacturer: Motorola

8.1.3
Input capture
The input capture function records the time an external event occurs by latching the value of the
free-running counter when a selected edge is detected at the associated timer input pin. Software
can store latched values and use them to compute the periodicity and duration of events. For
example, by storing the times of successive edges of an incoming signal, software can determine
the period and pulse width of a signal. To measure period, two successive edges of the same
polarity are captured. To measure pulse width, two alternate polarity edges are captured.
In most cases, input capture edges are asynchronous with respect to the internal timer counter,
which is clocked relative to an internal clock (PH2). These asynchronous capture requests are
synchronized with PH2 so that latching occurs on the opposite half cycle of PH2 from when the
timer counter is being incremented. This synchronization process introduces a delay from when
the edge occurs to when the counter value is detected. Because these delays cancel out when
the time between two edges is being measured, the delay can be ignored. When an input capture
is being used with an output compare, there is a similar delay between the actual compare point
and when the output pin changes state.
The control and status bits that implement the input capture functions are contained in the PACTL,
TCTL2, TMSK1, and TFLG1 registers.
To configure port A bit 3 as an input capture, clear the DDA3 bit of the DDRA register. Note that
this bit is cleared out of reset. To enable PA3 as the fourth input capture, set the I4/O5 bit in the
PACTL register. Otherwise, PA3 is configured as a fifth output compare out of reset, with bit I4/O5
8
being cleared. If the DDA3 bit is set (configuring PA3 as an output), and IC4 is enabled, then writes
to PA3 cause edges on the pin to result in input captures. Writing to TI4/O5 has no effect when the
TI4/O5 register is acting as IC4.
TPG
MOTOROLA
TIMING SYSTEM
MC68HC11PH8
8-8