Servo - June 2008

Building a Sonar System

overrode the 20 MHz crystal you see in Schematic 2 with the PIC18F2620’s internal oscillator.

When the rising edge of the PW ranging signal triggers an interrupt, we immediately read the value of the CCP1 holding registers and preserve that value in the pulsestart variable. The next step we perform is to configure the CCP1 input to trigger on a falling edge. Meanwhile, the TIMER1 clock is still counting. When the falling edge occurs, another interrupt is generated and we preserve the contents of the CCP1 holding registers in the pulseend variable.

The pulse width is the difference in the value of the pulseend variable and the pulsestart variable. Since the pulsewidth value is in microseconds, we can easily determine the target distance in inches by dividing the pulsewidth value by 147. Here’s a look at the PW pulse capture interrupt handler code:

void interrupt MEASURE(void)
{

if(CCP1IE && CCP1IF)
{

//interrupt caused by rising edge of PW
if(flags.rising_edge)
{

//save start count
pulsestart = make16(CCPR1H,CCPR1L);

CCP1CON = 0b00000100;
//capture falling edge
flags.rising_edge = 0;
//setup for falling edge

}
else
//interrupt caused by falling edge of PW
{

//save end count
pulseend = make16(CCPR1H,CCPR1L);

+6VDC to +9VDC

LM2940

VR1

5.0V LM3940

VR2

5.0V 3.3V

J1

C1 + .1uF C2

220uF

IN OUT

GND

C3 220uF

+ C4

.1uF

IN OUT

GND

PIC VOLTAGE SELECT JUMPER

R1 + 3.3V

390

C5 220uF

+ C6

.1uF

JP2

VCC

LED1

+ 5.0V

VCC

ICSP CONNECTOR

VCC

+ 5.0V

63 ₅₆32 ₄₅21 41

R3 10K

C8 .1uF

OPTIONAL 390

LV-Max Sonar-EZ0

R1 100

C7 .1uF

OPTIONAL ACTIVITY LED

BW PW AN RX TX VCC GND

R2 1K

U2

7

6

5

4

3

2

1

MCLR

2 AN RA0 ₃

RA1 ₄

RA2 ₅

RA3 ₆RA4/T0CKI ₇

RA5 ₁₀OSC2/RA6

C11 20pF

Y1 20 MHz

28

RB7/PGD

27 9

RB6/PGD OSC1/RA7 ₂₁_RX

RB0 ₂₂_BWRB1 ₂₃

RB2 ₂₄

RB3 ₂₅

RB4 ₂₆

RB5 ₁₁

VCC ^RC012

RC1/CCP2 ₁₃_PW₂₀RC2/CCP1 ₁₄

_C9VDD RC3 ₁₅

RC4 ₁₆

.1uF 8 ^RC517

₁₉GND RC6/TX ₁₈_RX

GND RC7/RX

C12 20pF

VCC

R5 10K

3

1 BC846B

PIC18F2620/PIC18LF2620

SCHEMATIC 2. As you have come to expect, there’s no rocket science in my PIC design. All of the advanced hardware and firmware resides within the confines of the LV-MaxSonar-EZ0 ultrasonic rangefinder.