HT95R35 datasheet(24/73 Pages) HOLTEK

24 / 73 page

HT95R35

Rev. 1.00

October 7, 2009

Timer/Event Counter will stop counting. It is important to

note that in the Pulse Width Measurement Mode, the

enable bit is automatically reset to zero when the exter-

nal control signal on the external timer pin returns to its

original level, whereas in the other two modes the en-

able bit can only be reset to zero under program control.

The residual value in the Timer/Event Counter, which

can now be read by the program, therefore represents

the length of the pulse received on the external timer

pin. As the enable bit has now been reset, any further

transitions on the external timer pin will be ignored. Not

until the enable bit is again set high by the program can

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.

To ensure that the timer pin is configured to operate as a

pulse width measurement pin the Timer Control Regis-

ter must place the Timer/Event Counter in the Pulse

Width Measurement Mode.

Programming Considerations

When configured to run in the timer mode, the internal

system clock 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 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 syn-

chronised 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 registers, the clock is inhibited to

avoid errors, however as this may result in a counting er-

ror, this should be taken into account by the program-

mer. 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

terrupt associated with the timer will remain inactive.

The edge select, timer mode control bits in timer control

properly configured for the required application. It is also

important to ensure that an initial value is first loaded

into the timer register before the timer is switched on;

this is because after power-on the initial value of the

timer register is unknown. After the timer has been in-

itialised the timer can be turned on and off by controlling

the enable bit in the timer control register. Note that set-

ting 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.

+ 1

+ 2

+ 3

+ 4

T i m e r

E x t e r n a l T M R

P i n I n p u t

T O N ( w i t h T E = 0 )

P r e s c a l e r O u t p u t

I n c r e m e n t

T i m e r C o u n t e r

P r e s c a l e r O u t p u t i s s a m p l e d a t e v e r y f a l l i n g e d g e o f T 1 .

Pulse Width Measure Mode Timing Diagram

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