AN597

DS00597A-page 2

© 1995 Microchip Technology Inc.

FIGURE 2:

TIMING DIAGRAM OF RANGING MODULE CONTROL LINES

Parameter

Number

Symbol

Characteristic

Min

Typ

Max

Units

1

Tpu

Ranging Module Stabilization Time

5.0

–

–

ms

Blank Inhibit Time

0.9

2.38

–

ms

Echo Time

––––

High Time for INIT

100

–

–

ms

Low Time for INIT

100

–

–

ms

3

2

4

5

1

INIT

BINH

ECHO

3

2

The PIC16C74 is configured to use one of its internal

timers, Timer1, in capture mode to measure the time

between signal transmission and echo detection. The

resolution

of

the

timer

is

determined

by

the

microcontroller clock frequency. For this application, a

4 MHz external oscillator was used, giving a resolution

of 1 ms per bit. The PIC16C74 initiates a ranging cycle

by first clearing Timer1. Timer1 is then enabled and

INIT is immediately asserted on the ranging module.

When INIT is asserted, the ranging module transmits a

series of 16 pulses on the transducer at 49.4 kHz. The

transmitted pulses reflect off the object and are

received back at the transducer.

The transducer is used for both transmitting and

receiving sound waves. A blanking interval is needed to

ensure that the transmitted signal has decayed on the

transducer, in order not to receive false echoes. In

normal operation, the ranging module has a blanking

interval of 2.38 milliseconds, which corresponds to a

minimum

detection

distance

of

approximately

17 inches. However, the BINH (blank inhibit) signal can

be manipulated to reduce the blanking time on the

transducer to allow for object ranging as close as

6 inches.

In this implementation, the PIC16C74 asserts the BINH

signal approximately 0.9 milliseconds after signal

transmission. This enables the transducer to receive

reflections off objects at a distance of 6 inches. The

ranging module asserts the ECHO signal when a valid

reflection has been detected. The PIC16C74 uses the

ECHO signal to trigger a capture of the Timer1 value.

The

capture

register

contains

the

16-bit

value

representing

the

elapsed

time

between

signal

transmission and echo detection. The PIC16C74 then

calculates object distance based on the Timer1 value,

microcontroller clock speed, and the velocity of sound

in the atmosphere. The basic equation for calculating

distance is given below:

DESIGN CONSIDERATIONS

There are several design considerations which must be

taken into account and are listed below.

The absolute measuring distance supported by the

ranging module is 6 inches to 35 feet with an accuracy

of +/- 1%.

The distance output from the ranging module can be

averaged over time to filter distance calculations.

In some applications, the gain of the receiver amplifier

may be too low or too high and may need to be

adjusted. For example, if the transducer is mounted in

a cylinder, the gain may need to be lowered to reduce

false echoes within the cylinder. In this case, R1 (refer

to the Polariod Ultrasonic Ranging System manual)

may be replaced with a 20 k

Ω potentiometer to tweak

the gain of the receiver amplifier to reduce false

echoes.

In order for the Polaroid 6500 ranging module to

operate properly, the power supply must be capable of

handling high current transients (2.5 A) during the

Note:

The minimum high and low time for INIT is

100 milliseconds, as seen in Figure 2.