Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
204 Timer Interface A (TIMA) MOTOROLA
Timer Interface A (TIMA)
11.4 Functional Description
Figure 11-1 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 com pare functions. Th e 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.
11.4.1 TIMA Counter Prescaler
The TIMA clock source can be one of the seven prescaler outputs or the
TIMA clock pin, PTE3/TCLKA. 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.
11.4.2 Input Capture
An input capture function has three basic parts:
1. Edge select logic
2. Input capture latch
3. 16-bit counter
Two 8-bit registers, which make up the 16-bit input cap ture re gister, ar e
used to latch the value of the free-runn ing 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 correspo nding input
edge bits (ELSxB and ELSxA in TASC0–TASC3 control regi sters with x
referring to the active channel number). When a n active ed ge occurs on
the pin of an input capture channel, the TIMA latch es 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.