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COUNTER/TIMERS BASIC SECTION 8

Page 8-2

Figure 8-2 Pulse Width Measurement

USING INTERRUPT 0

Interrupt 0 (INT 0) may be used for COM 1 serial

communications. You cannot simultaneously use INT 0

for the counter and COM 1.

Interrupt 0 is level sensitive. This means that as long as

the line at INT 0 (available at J7-3 or J8-6) is low, code

starting at the line set in ONITR will execute if an RETI

instruction was given. If a RETURN is given, then

interrupts are turned off.

Normally this is not going to be a problem using the

counter if your counting rate is greater than 100 Hz. If

the count rate is very slow, then you can monitor the

interrupt line through one of the lines at J3 and use the

ON LINE multi-tasking command to generate an

interrupt.

If you are using the counter in quadrature mode, then the

only requirement is to reset the interrupt latch:

LINEB,0A700h,0

resets the latch. See COU NT1. BAS for a program

example.

Review EXT ERNA L INTE RRUPTS SECT ION 10 for

more information.

PULSE WIDTH & FREQUENCY

MEASUREMENT

You can measur e a pulse wid th or a frequency, with

limitations.

Pulse width measurement

COM 1 UART can be put into a mode where the CTS

line outputs a cr ystal contro lled frequ ency. This

frequency is programmable from 9600 H z to 3.6864

MHZ . U sing this output in conjunction with the input

pulse allows period m easurement.

See COUNT4. BAS program for a list of frequencies and

a demonstration program of how to measure a pulse

width. Note that is program best measures pulse widths

with long dwell periods. Pulse widths may be very short

(> 1 micro-sec ond) but the repetition rate should be less

than 100 Hz.

Pulse width to measure is applied to J11-5. Period

measurement begins when the signal is low. If you w ant

to measure a high going pulse, you must put an inverter

in front of the signal.

A reference clock signal is applied to J11-2.

CLOCK4.BAS uses a clock available from COM 1

UART. If the UART is needed, then an external clock

must be supplied. A simple 1 MHZ or 10 MHZ

oscillator applied to J22-2 will work.

A problem measuring pulse widths using this demo is the

repetition r ate is limited. The pulse w idth (low signa l)

may be under 1 micro-second. H owever, the repetition

rate should be slower than 100 Hz to ensure reliable

operation. This is because the program has to read and

reset the counter then perform what ever analysis on the

incoming data.

Frequency measurement

Frequencies to 20 MH Z may be m easured. T he

program uses the 2 Hz clock output to gate the

frequency. Effectively, gate time is 0.25 seconds.

Usually this is adequate for high frequency

measurements. Note that the gate time is dependent

upon the RTC accuracy. For very high accuracy you

will need to add an external clock.

A gating signal is applied to J11-5 w hile the frequency to

be measured is ap plied to J11-2. When the gate signal is

low, the counter is enabled and counts pulses applied to

J11-2.

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 59 60

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