MC68HC901 Datasheet, PDF (39/73 Pages) Motorola, Inc – Multi-Function Peripheral

Timers
If the main counter is loaded with 01 (hexadecimal), a time out pulse will occur every time
the prescaler presents a count pulse to the main counter. If the main counter is loaded with
00, a time out pulse will occur every 256 count pulses.
6.1.2 Pulse Width Measurement Operation
Besides the delay mode, timers A and B may be programmed to operate in the pulse width
measurement mode. In this mode, an auxiliary control input is required; timers A and B
auxiliary input lines are TAI and TBI. Also, in the pulse width measurement mode, interrupt
channels normally associated with I4 and I3 will instead respond to transitions on TAI and
TBI, respectively. General purpose lines I3 and I4 may still be used for I/O, but may not be
used as interrupt generating inputs. A conceptual circuit of the selection of the interrupt
source is shown in Figure 6-1.
TAI
TIMER A
PULSE- WIDTH
MODE
14
13
TIMER B
PULSE- WIDTH
MODE
INTERRUPT
CHANNEL
(0110)
INTERRUPT
CHANNEL
(0011)
TBI
Figure 6-1. Conceptual Circuit of Interrupt Source Selection
The pulse width measurement mode functions similarly to the delay mode, with the auxiliary
control signal acting as a^Fn^IGUe^Rn^E a^{- C}b^Ol^Ne^CEt^Po^TUt^Ah^Le^CIRti^Cm^UITe^Or^F. ^INW^TEh^RRe^Un^PTt^Sh^Oe^URc^Co^{E S}n^Et^Lr^Eo^Cl^TIs^Oi^Ngnal is active, the
prescaler and main counter are allowed^Mto^C68o^Hp^C9e⁰¹r^Ua^Mte^{/ A}.^DWhen the control signal is negated, the
timer is stopped. So, the width of the active pulse on TAI or TBI is measured by the number
of timer counts which occur while the timer is allowed to operate.
The active state of the auxiliary input line is defined by the associated interrupt channel's
edge bit in the active edge register (AER). GPIP4 of the AER is the edge bit associated with
TAI, and GPIP3 is associated with TBI. When the edge bit is a one, the auxiliary input will
be active high, enabling the timer while the input signal is at a high level. If the edge bit is
zero, the auxiliary input will be active low and the timer will operate while the input signal is
at a low level.
6-2
MC68HC901 USER’S MANUAL
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