Rev. 1.40
72
March 29, 2019
Rev. 1.40
73
March 29, 2019
HT45F23A/HT45F24A
TinyPower
TM
Flash MCU with OPA & Comparators
HT45F23A/HT45F24A
TinyPower
TM
Flash MCU with OPA & Comparators
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 register, select either the event counter or pulse width measurement mode. Additionally
the Port 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.