
Timing System
Input Capture
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Timing System
145
9.3  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 to the internal timer counter, 
which is clocked relative to an internal clock (PH2). These asynchronous capture 
requests are synchronized to PH2 so that the 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 offset each other 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:
Pulse accumulator control register (PACTL)
Timer control 2 register (TCTL2)
Timer interrupt mask 1 register (TMSK1)
Timer interrupt flag 2 register (TFLG1)
To configure port A bit 3 as an input capture, clear the DDRA3 bit of the PACTL 
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 being cleared. If the DDRA3 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. 
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