HT95R34 datasheet(22/59 Pages) HOLTEK

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HT95R34

Rev. 1.10

February 18, 2009

To ensure that the timer pin is configured to operate as

an event counter input pin the Timer Control Register

must place the Timer/Event Counter in the Event

Counting Mode. It should be noted that in the event

counting mode, even if the microcontroller is in the

Power Down Mode, the Timer/Event Counter will con-

tinue to record externally changing logic events on the

timer input pin. As a result when the timer overflows it

will generate a timer interrupt and corresponding

wake-up source.

Configuring the Pulse Width Measurement Mode

In this mode, the Timer/Event Counter can be utilised to

measure the width of external pulses applied to the ex-

ternal timer pin. To operate in this mode, the Operating

Mode Select bit pair in the Timer Control Register must

be set to the correct value as shown.

Control Register Operating Mode

Select Bits for the Pulse Width

Measurement Mode

Bit7 Bit6

In this mode the internal clock is used as the

Timer/Event Counter clock. After the other bits in the

Timer Control Register have been setup, the enable bit,

which is bit 4 of the Timer Control Register, can be set

high to enable the Timer/Event Counter, however it will

not actually start counting until an active edge is re-

ceived on the external timer pin.

If the Active Edge Select bit, which is bit 3 of the Timer

Control Register, is low, once a high to low transition has

been received on the external timer pin, the Timer/Event

Counter will start counting until the external timer pin re-

turns to its original high level. At this point the enable bit

will be automatically reset to zero and the Timer/Event

Counter will stop counting. If the Active Edge Select bit

is high, the Timer/Event Counter will begin counting

once a low to high transition has been received on the

external timer pin and stop counting when the external

timer pin returns to its original low level. As before, the

enable bit will be automatically reset to zero and the

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.

+ 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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