mk68901 STMicroelectronics, mk68901 Datasheet - Page 12

• Manufacturer Part Number: mk68901
• Description: Multi.function Peripheral
• Manufacturer: STMicroelectronics

MK68901
Figure 15 : A Conceptual Circuit of the MFP Timers in the Pulse Width Measurement Mode.
Changing the prescale value with the timer running
can cause the first Time Out pulse to occur at an in-
determinate time, (no less than one nor more than
200 timer clock cycles times the number in the time
constant register), but subsequent Time Out pulses
will then occur at the correct interval.
In addition to the delay mode described above, Ti-
mers A and B can also function in the Pulse Width
Measurement mode or in the Event Count mode. In
either of these two modes, an auxiliary control signal
is required. The auxiliary control input for Timer A is
TAI, and for Timer B,TBI is used. The interrupt chan-
nels associated with 14 and 13 are used for TAI and
TBI, respectively, in Pulse Width mode. See Figure
15.
The pulse width measurement mode functions
much like the delay mode. However, in this mode,
the auxiliary control signal on TAI or TBI acts as an
enable to the timer. When the control signal on TAI
or TBI is inactive, the timer will be stopped. When it
is active, the prescaler andmain counter are allowed
to run. Thus the width of the active pulse on TAI or
TBI is determined by the number of timer counts
which occur while the pulse allows the timer to run.
The active state of the signal on TAI or TBI is de-
pendent upon the associated Interrupt Channel ’ s
edge bit (GPIP 4 for TAI and GPIP 3 for TBI : see
Active Edge Register in figure 5). If the edge bit as-
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sociated with the TAI or TBI input is a one, it will be
active high ; thus the timer will be allowed to run
when the input is at a high level. If the edge bit is a
zero, the TAI or TBI input will be active low. As pre-
viously stated, the interrupt channel (13 or 14) as-
sociated with the input still functions when the timer
is used in the pulse width measurement mode. How-
ever, if the timer is programmed for the pulse width
measurement mode, the interrupt caused by tran-si-
tions on the associated TAI or TBI input will occur on
the opposite transition.
For example, if the edge bit associated with the TAI
input (AER-GPIP 4) is as one, an interrupt would
normally be generated on the 0-1 transition of the 14
input signal. If the timer associated with this input
(Timer A) is placed in the pulse width measurement
mode, the interrupt will occur on the 1-0 tran-sition
of the TAI signal instead. Because the edge bit
(AER-GPIP 4) is a one, Timer A will be allowed to
count while the input is high. When the TAI input
makes the high to low transition, Timer A will stop,
and it is at this point that the interrupt will occur (as-
suming that the channel is enabled). This allows the
interrupt to signal the CPU that the pulse being mea-
sured has terminated ; thus Timer A may now be
read to determine the pulse width. (Again note that
13 and 14 may still be used for I/O when the timer
is in the pulse width measurement mode). If Timer
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