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Timer Interface (TIMA-4)
General Release Specification
MC68HC908AT32
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Rev. 2.0
294
Timer Interface (TIMA-4)
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MOTOROLA
18.4 Functional Description
Figure 18-1
shows the TIMA structure. The central component of the
TIMA is the 16-bit TIMA counter that can operate as a free-running
counter or a modulo up-counter. The TIMA counter provides the timing
reference for the input capture and output compare functions. The TIMA
counter modulo registers, TAMODH–TAMODL, control the modulo
value of the TIMA counter. Software can read the TIMA counter value at
any time without affecting the counting sequence.
The four TIMA channels are programmable independently as input
capture or output compare channels.
18.4.1 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.
18.4.2 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 TASC3 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.
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