BS2824 Holtek Semiconductor Inc., BS2824 Datasheet - Page 58

• Manufacturer Part Number: BS2824
• Description: Touch Key Mcu
• Manufacturer: Holtek Semiconductor Inc.

BS2824/BS2854/BS2868
Touch Key MCU
Rev. 1.00
I/O Interfacing
Programming Considerations
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 input. Any pull-high resistor connected to this
pin will remain valid even if the pin is used as a Timer/Event Counter input.
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 respective 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 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 written 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 properly initialised before using them for
the first time. The associated 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 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 register.
When the Timer/Event Counter overflows, its corresponding 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 enabled 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 device will be woken up from its
Power-down condition. To prevent such a wake-up from occurring, the timer interrupt request flag
should first be set high before issuing the HALT instruction to enter the Idle/Sleep Mode.
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January 27, 2010