ht82m73e Holtek Semiconductor Inc., ht82m73e Datasheet - Page 16

• Manufacturer Part Number: ht82m73e
• Description: 2.4ghz Mouse Tx 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

As in the case of the other two modes, when the counter
is full and overflows, the timer will be reset to the value
already loaded into the preload register. If the timer in-
terrupt is enabled, an interrupt signal will also be gener-
ated. To ensure that the external pin PA2/TMR is
configured to operate as a pulse width measuring input
pin, two things have to happen. The first is to ensure that
the TM0 and TM1 bits place the timer/event counter in
the pulse width measuring mode, the second is to en-
sure that the port control register configures the pin as
an input.
I/O Interfacing
The Timer/Event Counter, when configured to run in the
event counter or pulse width measurement mode, re-
quire the use of the external PA2 pin for correct opera-
tion. As this pin is a shared pin it must be configured
correctly to ensure it is setup for use as a Timer/Event
Counter input and not as a normal I/O pin. This is imple-
mented by ensuring that the mode select bits in the
Timer/Event Counter control register, select either the
event counter or pulse width measurement mode. Addi-
tionally the Port Control Register PAC bit 2 must be set
high to ensure that the pin is setup as an input. Any
pull-high resistor configuration option on 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 inter-
nal interrupt signal directing the program flow to the re-
spective internal interrupt vector. For the pulse width
measurement 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 syn-
Rev. 1.00
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c h r o n i s e d w i t h t h e i n t e r n a l t i m e r c l o c k , t h e
microcontroller will only see this external event when the
next timer clock pulse arrives. As a result, there may be
small differences in measured values requiring pro-
grammers to take this into account during 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 internal 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 must be taken to ensure that the timers are prop-
erly initialised before using them for the first time. The
associated timer enable bits in the interrupt control reg-
ister 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 register must also be correctly set to ensure the
timer is properly 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 registers are unknown. After the
timer has been initialised the timer can be turned on and
off by controlling the enable bit in the timer control regis-
ter. Note that setting the timer enable bit high to turn the
timer on, should only be executed after the timer mode
bits have been properly setup. Setting the timer enable
bit high together with a mode bit modification, may lead
to improper timer operation if executed as a single timer
control register byte write instruction.
When the Timer/Event counter overflows, its corre-
sponding interrupt request flag in the interrupt control
register will be set. If the timer interrupt is enabled this
will in turn generate an interrupt signal. But the timer
overflow can t wake-up if MCU is in a Power down con-
dition.
HT82M73E
May 14, 2008