MC68HC08AZ60A Datasheet, PDF (408/480 Pages) Motorola, Inc – Microcontrollers

Timer Interface Module A (TIMA)
23.4.1 TIMA Counter Prescaler
The TIMA clock source can be one of the seven prescaler outputs or the
TIMA clock pin, PTD6/ATD14/TACLK. The prescaler generates seven
clock rates from the internal bus clock. The prescaler select bits, PS[2:0],
in the TIMA status and control register select the TIMA clock source.
23.4.2 Input Capture
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 free-running counter contents are transferred to the TIMA channel
register (TACHxH–TACHxL see TIMA Channel Registers on page 429)
on each proper signal transition regardless of 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 2 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
Advance Information
408
Timer Interface Module A (TIMA)
MC68HC08AZ60A — Rev 0.0
MOTOROLA