HT82B40A HOLTEK [Holtek Semiconductor Inc], HT82B40A Datasheet - Page 20

• Manufacturer Part Number: HT82B40A
• Description: I/O MCU with USB Interface
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

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 TMR0 and TMR1 pins for
correct operation. As these pins are shared pins they
must be configured correctly to ensure they are setup
for use as Timer/Event Counter inputs and not as a nor-
mal I/O pins. This is implemented by ensuring that the
mode select bits in the Timer/Event Counter control reg-
ister, select either the event counter or pulse width mea-
surement mode. Additionally the Port Control Register
bits for these pins must be set high to ensure that the pin
is setup as an input. Any pull-high resistor configuration
option on these pins will remain 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 mea-
surement 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 internal timer clock, the 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 programmers 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
Rev. 1.10
20
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. However irre-
spective of whether the interrupts are enabled or not, a
Timer/Event counter overflow will also generate a
wake-up signal if the device is in a Power-down condi-
tion. 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 exter-
nal events and if an overflow occurs the device will be
woken up from its Power-down condition. To prevent
such a wake-up from occurring, the timer interrupt re-
quest flag should first be set high before issuing the
Timer Program Example
This program example shows how the Timer/Event
Counter registers are setup, along with how the inter-
rupts 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 Counter to
be in the timer mode, which uses the internal system
clock as the clock source.
HALT instruction to enter the Power Down Mode.
HT82B40R/HT82B40A
September 4, 2009