HT82B40R datasheet(20/57 Pages) HOLTEK

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HT82B40R/HT82B40A

Rev. 1.10

September 4, 2009

I/O Interfacing

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

event counter or pulse width measurement mode, re-

quire the use of the external TMR0 and TMR1 pins for

correct operation. As these pins are shared pins they

must be configured correctly to ensure they are setup

for use as Timer/Event Counter inputs and not as a nor-

mal I/O pins. This is implemented by ensuring that the

mode select bits in the Timer/Event Counter control reg-

ister, select either the event counter or pulse width mea-

surement mode. Additionally the Port Control Register

bits for these pins must be set high to ensure that the pin

is setup as an input. Any pull-high resistor configuration

option on these pins 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, 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

interrupt signal directing the program flow to the respec-

tive internal interrupt vector. For the pulse width mea-

surement 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 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 prop-

erly initialised before using them for the first time. The

associated timer enable bits in the interrupt control reg-

ister 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 regis-

ter. 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.

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

²HALT² instruction to enter the Power Down Mode.

Timer Program Example

This program example shows how the Timer/Event

Counter registers are setup, along with how the inter-

rupts are enabled and managed. Note how the

Timer/Event Counter is turned on, by setting bit 4 of the

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 Counter to

be in the timer mode, which uses the internal system

clock as the clock source.

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