MC68HC05P9A Datasheet, PDF (89/158 Pages) Freescale Semiconductor, Inc – HCMOS Microcontroller Unit

Operation
Pin Functions
PD7/TCAP
TCMP
Input Capture
Freescale Semiconductor, Inc.
Timer
Operation
The timing reference for the input capture and output compare functions
is a 16-bit free-running counter. The counter is preceded by a divide-by-
four prescaler and rolls over every 2¹⁸ cycles. Timer resolution with a 4-
MHz crystal is 2 µs. Software can read the value in the counter at any
time without affecting the counter sequence.
Because of the 16-bit timer architecture, the I/O registers for the input
capture and output compare functions are pairs of 8-bit registers.
The timer uses two pins.
PD7/TCAP is the input capture pin. When an active edge occurs on
PD7/TCAP, the timer transfers the current counter value to the input
capture registers. PD7/TCAP is also an I/O port pin.
TCMP is the output-only output compare pin. When the counter value
matches the value written in the output compare registers, the timer
transfers the output level bit, OLVL, to the TCMP pin.
The input capture function is a means to record the time at which an
external event occurs. When the input capture circuitry detects an active
edge on the PD7/TCAP pin, it latches the contents of the timer registers
into the input capture registers. The polarity of the active edge is
programmable.
Latching values into the input capture registers at successive edges of
the same polarity measures the period of the input signal on the
PD7/TCAP pin. Latching the counter values at successive edges of
opposite polarity measures the pulse width of the signal. Figure 44
shows the logic of the input capture function.
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MOTOROLA
Timer
89
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