MC908AS60ACFU Datasheet, PDF (310/414 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Timer Interface Module A (TIMA)
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 free-running counter contents are transferred to the TIMA channel 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 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
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
both input captures and output compares are referenced to the same 16-bit modulo counter, the delay
can be controlled to the resolution of the counter independent of software latencies.
Reset does not affect the contents of the TIMA channel register (TACHxH–TACHxL).
25.3.3 Output Compare
With the output compare function, the TIMA can generate a periodic pulse with a programmable polarity,
duration and frequency. When the counter reaches the value in the registers of an output compare
channel, the TIMA can set, clear or toggle the channel pin. Output compares can generate TIMA CPU
interrupt requests.
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
310
Freescale Semiconductor