Timer Interface Module B (TIMB)
Functional Description
MC68HC908AZ60A — Rev 2.0 Technical Data
MOTOROLA Timer Interface Module B (TIMB) 321
20.4 Functional Description
Figure 20-1 shows the TIMB structure. The central component of the
TIMB is the 16-bit TIMB counter that can operate as a free-running
counter or a modulo up-counter. The TIMB counter provides the timing
reference for the input capture and output compare functions. The TIMB
counter modulo registers, TBMODH–TBMODL, control the modulo
value of the TIMB counter. Software can read the TIMB counter value at
any time without affecting the counting sequence.
The two TIMB channels are programmable independently as input
capture or output compare channels.
20.4.1 TIMB Counter Prescaler
The TIMB clock source can be one of the seven prescaler outputs or the
TIMB clock pin, PTD4/ATD12/TBCLK. The prescaler generates seven
clock rates from the internal bus clock. The prescaler select bits, PS[2:0],
in the TIMB status and control register select the TIMB clock source.
20.4.2 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
TBSC0 through TBSC1 control registers with x referring to the active
channel number). When an active edge occurs on the pin of an input
capture channel, the TIMB latches the contents of the TIMB counter into
the TIMB channel registers, TBCHxH–TBCHxL. Input captures can
generate TIMB 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.
