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Prescaler

The single timer in the HT48R10A-1/HT48C10-1 and HT48R30A-1/HT48C30-1, Timer/Event

Counter 0 in the HT48R50A-1/HT48C50-1 as well as Timer/Event Counter 2 in the HT48RU80/

HT48CU80 all possess a prescaler. Bits 0~2 of their associated timer control register, namely bits

PSC0~PSC2, T0PSC0~T0PSC2 or T2PSC0~T2PSC2, define the pre-scaling stages of the inter

nal clock source of the Timer/Event Counter. The Timer/Event Counter overflow signal can be

used to generate signals for the PFD and as a Timer interrupt.

I/O Interfacing

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

ment mode, require the use of the external timer pin for correct operation. This external timer pin

may be pin-shared with other I/O pins, depending upon which device is selected. For pin-shared

timer pins, pull-high resistors can be selected for connection to the timer input pins. The timers can

also be setup to drive the pin-shared buzzer pins. When the buzzer pins are selected by selecting

the correct configuration option, the output of the chosen timer can be made to drive this at a fre

quency determined by the contents of the timer register and where appropriate the timer.

Programming Considerations

When configured to run in the timer mode, the internal system clock or RTC is used as the timer

clock source and is therefore synchronized with the overall operation of the microcontroller. In this

mode, when the appropriate timer register is full, the microcontroller will generate an internal inter

rupt signal directing the program flow to the respective internal interrupt vector. For the pulse width

measurement mode, the internal system clock or RTC 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 program-

ming. The same applies if the timer is configured to be in the event counting mode which again is

an external event and not synchronized with the internal system or timer clock.

Timer Program Example

The following example program section is based on the HT48R50A-1/HT48C50-1 devices, which

contain one internal 8-bit Timer/Event Counter and one internal 16-bit Timer/Event Counter. Pro

gramming the Timer/Event Counters for other devices is conducted in a very similar way. The pro

gram shows how the Timer/Event Counter registers are setup along with how the interrupts are

enabled and managed. Points to note in the example are how, for the 16-bit Timer/Event Coun

ters, the low byte must be written first, this is because the 16-bit data will only be written into the ac

tual timer register when the high byte is loaded. Also note how the Timer/Event Counter is turned

on, by setting bit 4 of the respective 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 Counters

to be in the timer mode, which uses the internal system clock as their clock source. The

Timer/Event Counter clock sources are setup via configuration options.

I/O Type MCU

Holtek HT48R30A-1 User Manual

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