HT37A70 datasheet(33/85 Pages) HOLTEK

33 / 85 page

HT37A70/60/50/40/30/20

Rev. 1.00

February 17, 2009

Configuring the Pulse Width Measurement Mode

In this mode, the width of external pulses applied to the

pin-shared external pin PA6/TMR0 or PA7/TMR1 can be

measured. In the Pulse Width Measurement Mode, the

timer clock source is supplied by the internal clock. For

the timer to operate in this mode, bits TM0 and TM1

must both be set high. If the TE bit is low, once a high to

low transition has been received on the PA6/TMR0 or

PA7/TMR1 pin, the timer will start counting until the

PA6/TMR0 or PA7/TMR1 pin returns to its original high

level. At this point the TON bit will be automatically reset

to zero and the timer will stop counting. If the TE bit is

high, the timer will begin counting once a low to high

transition has been received on the PA6/TMR0 or

PA7/TMR1 pin and stop counting when the PA6/TMR0

or PA7/TMR1 pin returns to its original low level. As be-

fore, the TON bit will be automatically reset to zero and

the timer will stop counting. It is important to note that in

the Pulse Width Measurement Mode, the TON bit is au-

tomatically reset to zero when the external control signal

on the external timer pin returns to its original level,

whereas in the other two modes the TON bit can only be

reset to zero under program control. The residual value

in the timer, which can now be read by the program,

therefore represents the length of the pulse received on

pin PA6/TMR0 or PA7/TMR1. As the TON bit has now

been reset any further transitions on the PA6/TMR0 or

PA7/TMR1 pin will be ignored. Not until the TON bit is

again set high by the program can the timer begin fur-

ther pulse width measurements. In this way single shot

pulse measurements can be easily made. It should be

noted that in this mode the counter is controlled by logi-

cal transitions on the PA6/TMR0 or PA7/TMR1 pin and

not by the logic level.

Prescaler

Bits PSC0~PSC2 of the TMRC1~ TMRC2 registers can

be used to define the pre-scaling stages of the internal

clock sources of the Timer/Event Counter.

Note: Because the two timers are used by MIDI the ex-

ternal timer pin functions are disabled.

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 PA6/TMR0 or PA7/TMR1

pin for correct operation. As this pin is a shared pin it

must be configured 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 se-

lect bits in the Timer/Event Counter control register, se-

lect either the event counter or pulse width

measurement mode. Additionally the PA share pin op-

tion must be selected to ensure that the pin is setup as

an TMR0 and TMR1 input.

Programming Considerations

When configured to run in the timer mode, the internal

system clock fOSC/8 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-

chronized 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

+ 1

+ 2

+ 3

+ 4

T i m e r

E x t e r n a l T i m e 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 f O S C / 8 o r R C 1 2 K .

Pulse Width Measure Mode Timing Diagram

T i m e r + 2

E x t e r n a l T i m e r

P i n I n p u t

T 0 E o r T 1 E = 1

I n c r e m e n t

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

T i m e r + 3

T i m e r + 1

Event Counter Mode Timing Diagram

HT37A70 Datasheet(PDF) 33 Page - Holtek Semiconductor Inc

HT37A70 Datasheet(HTML) 33 Page - Holtek Semiconductor Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85

Datasheet Download

Link URL