ht82k72e Holtek Semiconductor Inc., ht82k72e Datasheet - Page 17

• Manufacturer Part Number: ht82k72e
• Description: Ht82k72e/ht82k72a -- One Channel Keyboard With R-f Type Mcu
• Manufacturer: Holtek Semiconductor Inc.

counting. It is important to note that in the Pulse Width
Measurement Mode, the TON bit is automatically reset
to zero when the external control signal on the external
timer pin returns to its original level, whereas in the other
two modes the TON bit can only be reset to zero under
program control. The residual value in the timer, which
can now be read by the program, therefore represents
the length of the pulse received on pin PA2/TMR. As the
TON bit has now been reset any further transitions on
the PA2/TMR pin will be ignored. Not until the TON bit is
again set high by the program can the timer begin fur-
ther pulse width measurements. In this way single shot
pulse measurements can be easily made. It should be
noted that in this mode the counter is controlled by logi-
cal transitions on the PA2/TMR pin and not by the logic
level.
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 to ensure that the TM0 and TM1 bits place the
timer/event counter in the pulse width measuring mode.
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
Rev. 1.00
17
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-
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.
HT82K72E/HT82K72A
February 20, 2008