HT46R92 datasheet(22/61 Pages) HOLTEK

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HT46R92

Rev. 1.10

November 5, 2008

the timer begin further pulse width measurements. In

this way, single shot pulse measurements can be easily

made.

It should be noted that in this mode the Timer/Event

Counter is controlled by logical transitions on the exter-

nal timer pin and not by the logic level. When the

Timer/Event Counter is full and overflows, an interrupt

signal is generated and the Timer/Event Counter will re-

load the value already loaded into the preload register

and continue counting. The interrupt can be disabled by

ensuring that the Timer/Event Counter Interrupt Enable

bit in the Interrupt Control Register is reset to zero.

As the external timer pin is shared with an I/O pin, to en-

sure that the pin is configured to operate as pulse width

measurement pins, two things have to happen. The first

is to ensure that the Operating Mode Select bits in the

Timer Control Register place the related Timer/Event

Counter in the Pulse Width Measurement Mode. The

second is to ensure that the port control register

configures the timer pin as an input.

Prescaler

Bits PSC0~PSC2 of the TMRC register can be used to

define the pre-scaling stages of the internal clock source

for the Timer/Event Counters. In the Event Counter

Mode the prescaler has no effect.

I/O Interfacing

The Timer/Event Counter, when configured to run in the

Event Counter or Pulse Width Measurement Mode, re-

quires the use of the external PA2/TMR pin for correct

operation. As this pin is a shared pin it must be config-

ured 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 PAC bit 2

must be set high to ensure that the pin is setup as an in-

put. Any pull-high resistor configuration option on this

pin will remain valid even if the pin is used as a

Timer/Event Counter input.

Programming Considerations

When configured to run in the timer mode, one of the in-

ternal clocks 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

nal interrupt signal directing the program flow to the re-

spective internal interrupt vector. For the pulse width

measurement mode, one of the internal system clocks 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 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 properly in-

itialised before using them for the first time. The associ-

ated 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 regis-

ter must also be correctly set to ensure the timer is prop-

erly 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 reg-

isters are unknown. After the timer has been initialised

the timer can be turned on and off by controlling the en-

able 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 oper-

ation 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

will in turn generate an interrupt signal. However irre-

spective 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 condi-

tion. 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 exter-

nal 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 re-

quest flag should first be set high before issuing the

HALT instruction to enter the Power Down Mode.

HT46R92 Datasheet(PDF) 22 Page - Holtek Semiconductor Inc

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