HT45F23 datasheet(68/152 Pages) HOLTEK

68 / 152 page

I/O Interfacing

The Timer/Event Counter, when configured to run in the event counter or pulse width measurement

mode, require the use of the external pin for correct operation. 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 implemented by ensuring that the mode select bits in the Timer/Event Counter control

Control Register must be set high to ensure that the pin is setup as an input. Any pull-high resistor on

this pin will remain valid even if the pin is used as a Timer/Event Counter input.

Timer/Event Counter Pins Internal Filter

The external Timer/Event Counter pins are connected to an internal filter to reduce the possibility of

unwanted event counting events or inaccurate pulse width measurements due to adverse noise or

spikes on the external Timer/Event Counter input signal. As this internal filter circuit will consume a

limited amount of power, a configuration option is provided to switch off the filter function, an option

which may be beneficial in power sensitive applications, but in which the integrity of the input signal is

high. Care must be taken when using the filter on/off configuration option as it will be applied not only

to both external Timer/Event Counter pins but also to the external interrupt input pins. Individual

Timer/Event Counter or external interrupt pins cannot be selected to have a filter on/off function.

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 respective 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

synchronized 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. 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.

Rev. 1.20

September 15, 2011

HT45F23/HT45F43

8-Bit Flash MCU with Op Amps & Comparators

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