HT46R01A datasheet(21/58 Pages) HOLTEK

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HT46R01A

Rev. 1.10

August 13, 2008

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 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.

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

The program shows how the Timer/Event Counter regis-

ters are setup along with how the interrupts 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 Counters to be in the timer mode,

which uses the internal system clock as their clock

source.

org 04h

; external interrupt vector

org 08h

; Timer Counter 0 interrupt vector

jmp tmr0int

; jump here when Timer 0 overflows

org 20h

; main program

;internal Timer 0 interrupt routine

tmr0int:

; Timer 0 main program placed here

begin:

;setup Timer 0 registers

mov a,09bh

; setup Timer 0 preload value

tmr0,a

mov a,081h

; setup Timer 0 control register

mov tmr0c,a

; timer mode and prescaler set to /2

;setup interrupt register

mov a,00dh

; enable master interrupt and both timer

; interrupts

intc0,a

set tmr0c.4

; start Timer 0

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