MC68HC908AT32 Datasheet, PDF (291/378 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
25.3 Functional Description
Figure 25-1 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 six TIMA channels are programmable independently as input capture or output compare channels.
25.3.1 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.
25.3.2 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 result obtained by an input capture will be two more than the value of the free-running counter on the
rising edge of the internal bus clock preceding the external transition. This delay is required for internal
synchronization.
The free-running counter contents are transferred to the TIMA channel status and control register
(TACHxH–TACHxL, see 25.8.5 TIMA Channel Registers) 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 two 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
captured. To measure a pulse width, two alternate polarity edges are captured. Software should track the
overflows at the 16-bit module counter to extend its range.
Another use for the input capture function is to establish a time reference. In this case, an input capture
function is used in conjunction with an output compare function. For example, to activate an output signal
a specified number of clock cycles after detecting an input event (edge), use the input capture function to
record the time at which the edge occurred. A number corresponding to the desired delay is added to this
captured value and stored to an output compare register (see 25.8.5 TIMA Channel Registers). Because
MC68HC908AT32 Data Sheet, Rev. 3.1
Freescale Semiconductor
291