HT45R22E Holtek Semiconductor, HT45R22E Datasheet - Page 40

• Manufacturer Part Number: HT45R22E
• Description: Remote Type 8-bit OTP MCU
• Manufacturer: Holtek Semiconductor

Using this method of frequency generation, and if a
crystal oscillator is used for the system clock, very pre-
cise values of frequency can be generated.
I/O Interfacing
The Timer/Event Counter, when configured to run in the
event counter or pulse width capture mode, requires the
use of an external timer pin for its operation. As this pin
is a shared pin it must be configured correctly to ensure
that it is setup for use as a Timer/Event Counter input
pin. This is achieved by ensuring that the mode select
bits in the Timer/Event Counter control register, select
either the event counter or pulse width capture mode.
Additionally the corresponding Port Control Register bit
must be set high to ensure that the pin is setup as an in-
put. Any pull-high resistor connected to this pin will re-
main valid even if the pin is used as a Timer/Event
Counter input.
Programming Considerations
When configured to run in the timer mode, the internal
system clock is used as the timer clock source and is
therefore synchronised with the overall operation of the
microcontroller. In this mode when the appropriate timer
register is full, the microcontroller will generate an internal
interrupt signal directing the program flow to the respec-
tive internal interrupt vector. For the pulse width capture
mode, the internal system clock is also used as the timer
clock source but the timer will only run when the correct
logic condition appears on the external timer input pin. As
this is an external event and not synchronised with the in-
ternal timer clock, the microcontroller will only see this ex-
ternal event when the next timer clock pulse arrives. As a
result, there may be small differences in measured val-
ues requiring programmers to take this into account dur-
ing programming. The same applies if the timer is
configured to be in the event counting mode, which again
is an external event and not synchronised with the inter-
nal system or timer clock.
When the Timer/Event Counter is read, or if data is writ-
ten to the preload register, the clock is inhibited to avoid
errors, however as this may result in a counting error, this
should be taken into account by the programmer. Care
Rev. 1.00
40
must be taken to ensure that the timers are properly in-
itialised before using them for the first time. The associ-
ated timer enable bits in the interrupt control register must
be properly set otherwise the internal interrupt associated
with the timer will remain inactive. The edge select, timer
mode and clock source control bits in timer control regis-
ter must also be correctly set to ensure the timer is prop-
erly configured for the required application. It is also
important to ensure that an initial value is first loaded into
the timer registers before the timer is switched on; this is
because after power-on the initial values of the timer reg-
isters are unknown. After the timer has been initialised
the timer can be turned on and off by controlling the en-
able bit in the timer control register.
When the Timer/Event Counter overflows, its corre-
sponding interrupt request flag in the interrupt control
register will be set. If the Timer/Event Counter interrupt
is enabled this will in turn generate an interrupt signal.
However irrespective of whether the interrupts are en-
abled or not, a Timer/Event Counter overflow will also
generate a wake-up signal if the device is in a
Power-down condition. This situation may occur if the
Timer/Event Counter is in the Event Counting Mode and
if the external signal continues to change state. In such
a case, the Timer/Event Counter will continue to count
these external events and if an overflow occurs the de-
vice will be woken up from its Power-down condition. To
prevent such a wake-up from occurring, the timer inter-
rupt request flag should first be set high before issuing
the HALT instruction to enter the Idle/Sleep Mode.
Timer Program Example
The program shows how the Timer/Event Counter regis-
ters are setup along with how the interrupts are enabled
and managed. Note how the Timer/Event Counter is
turned on, by setting bit 4 of the Timer Control Register.
The Timer/Event Counter can be turned off in a similar
way by clearing the same bit. This example program
sets the Timer/Event Counters to be in the timer mode,
which uses the internal system clock as their clock
source.
HT45R22E
March 17, 2010
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