MC68HC901P MOTOROLA [Motorola, Inc], MC68HC901P Datasheet - Page 39

• Manufacturer Part Number: MC68HC901P
• Description: Multi-Function Peripheral
• Manufacturer: MOTOROLA [Motorola, Inc]

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.
The pulse width measurement mode functions similarly to the delay mode, with the auxiliary
control signal acting as an enable to the timer. When the control signal is active, the
prescaler and main counter are allowed to operate. When 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
PULSE- WIDTH
PULSE- WIDTH
Figure 6-1. Conceptual Circuit of Interrupt Source Selection
TIMER A
TIMER B
MODE
MODE
FIGURE - CONCEPTUAL CIRCUIT OF INTERRUPT SOURCE SELECTION
14
13
MC68HC901 USER’S MANUAL
TAI
TBI
MC68HC901UM / AD
INTERRUPT
INTERRUPT
CHANNEL
CHANNEL
(0011)
(0110)
MOTOROLA