MK68901 datasheet(12/33 Pages) STMICROELECTRONICS

12 / 33 page

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 and main 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-

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

V000332

MK68901

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