AN597
Implementing Ultrasonic Ranging
Author:
THEORY OF OPERATION
Robert Schreiber
Logic Products Division
Ultrasonic ranging entails transmitting a sound wave
and measuring the time that it takes for the sound wave
to reflect off of an object and back to the origin. The
reflection time is proportional to the distance that the
object is from the source. In this implementation, the
sound wave is transmitted and received from the same
transducer. Therefore, a blanking interval is required
between signal transmission and reception to eliminate
false echoes (i.e., a transmitted signal being detected
as its own echo).
INTRODUCTION
Object ranging is essential in many types of systems.
One of the most popular ranging techniques is
ultrasonic ranging. Ultrasonic ranging is used in a wide
variety of applications including:
•
•
•
•
•
Auto focus cameras
Motion detection
Robotics guidance
Proximity sensing
Object ranging
CIRCUIT CONFIGURATION
In this implementation, a PIC16C74 is connected to the
ranging module as shown in Figure 1. The RE0 and
RE1 I/O pins are configured as digital outputs and are
tied to INIT and BINH, respectively. The CCP1 pin is
configured as a digital input and is tied to ECHO
through a pull-up resistor. The pull-up resistor is
needed since the ECHO signal is an open-collector
output. The CCP1 pin is configured for capture mode
(CCP1CON). Figure 2 shows the timing relationship for
VDD and the three signal lines (INIT, BINH, and ECHO).
This application note describes a method of interfacing
PIC16CXX microcontrollers to the Polaroid 6500
Ranging Module.
This implementation uses a
minimum of microcontroller resources, a CCP module
and two I/O pins. The two major components of the
system are:
• Microcontroller
• Polaroid 6500 Ranging Module
Note:
The microcontroller performs the intelligence and
arithmetic functions for ultrasonic ranging, while the
Polaroid 6500 Ranging Module performs the ultrasonic
signal transmissions and echo detection.
FIGURE 1:
The ranging module requires 5.0 milliseconds to stabilize during power-up.
RANGING MODULE INTERFACE
RE1
BINH
RE0
INIT
TRANSMIT
ECHO
POLAROID 6500 RANGING MODULE
CCP1
TRANSDUCER
PIC16C74
1995 Microchip Technology Inc.
DS00597A-page 1
Thi d
t
t d ith F
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404
AN597
FIGURE 2:
TIMING DIAGRAM OF RANGING MODULE CONTROL LINES
VDD
INIT
5
4
1
2
2
BINH
3
3
ECHO
Parameter
Number
Symbol
1
2
3
4
5
Tpu
TBINH
TECHO
TINIT_H
TINIT_L
Characteristic
Ranging Module Stabilization Time
Blank Inhibit Time
Echo Time
High Time for INIT
Low Time for INIT
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
DS00597A-page 2
Min
Typ
Max
Units
5.0
0.9
–
100
100
–
2.38
–
–
–
–
–
–
–
–
ms
ms
–
ms
ms
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:
Distance (inches) = TECHO time / 147.9 microseconds
Note:
The minimum high and low time for INIT is
100 milliseconds, as seen in Figure 2.
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 kΩ 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
1995 Microchip Technology Inc.
AN597
transmit pulse. The instantaneous drain on the power
supply can be mitigated by installing a storage
capacitor across the power lines at the ranging module.
A value of 500 microfarads is recommended.
A 200 millisecond interval is recommended between
ranging cycles (Figure 2) to allow the transducer to
clear.
The ECHO line requires a pull-up resistor (4.7 kΩ was
used in this application).
There must be a common ground between the
PIC16C74 circuitry and the ranging module.
Some applications may not need the resources of the
higher end PIC16CXX devices. It is still possible to do
this application using a device that does not contain a
CCP module (for ECHO timing). The capture function
can be implemented in firmware. The effect of a
firmware implementation is that the resolution of the
ECHO time would be 3 Tcy cycles versus 1 Tcy cycle
for the CCP module.
Also, the firmware
implementation would not allow other tasks to be
performed while the capture function was occurring.
Refer to Appendix A for general ranging module
specifications.
1995 Microchip Technology Inc.
DS00597A-page 3
AN597
APPENDIX A: POLAROID MODULE
SPECIFICATIONS
Note:
This appendix contains general specifications from the Polaroid Ultrasonic Ranging
System Manual. Please refer to the
current Polaroid Ultrasonic Ranging
System Manual for current information
regarding
ranging
module
design
considerations.
Resolution
a)
b)
c)
d)
Accuracy: (again, you must have a well defined target)
Temperature Compensate
DESIGN CONSIDERATIONS IN
ULTRASONICS
a)
Range: (with user custom designed processing
electronics)
Farther
a)
b)
c)
Above all, know the target and range well, and
design a system with them in mind.
Use a higher transmit frequency.
Look at phase differences of a given cycle of the
transmitted signal and received echo (as
opposed to using and integration technique).
Increase the clock frequency of the timer.
b)
Use an acoustic horn to “focus” the sound
(narrowing the beamwidth).
Use two transducers – 1 receiver and
1 transmitter – facing each other.
Lower the transmitting frequency (which will
decrease the attenuation in air).
c)
Use a second small target, as a reference, at a
known distance in the ranging path (such as a
1/4” rod several feet away), process both
echoes, then normalize the second distance
with respect to the first, since t1/d1 = t2/d2.
Incorporate a temperature sensing integrated
circuit to drive a VCO to do the distance interval
clocking.
To increase sensitivity of detection circuit
change the value of C4 from 3300 pF to 1000 pF
on the 6500 Series Ranging Module.
Beam Width:
Closer
Increase
a)
a)
b)
c)
a)
Use a shorter transmit signal (such as four
cycles).
Use two transducers – one to transmit, one to
receive (eliminates waiting for damping time).
Use an acoustic lens (to disperse the signal).
Decrease the transmitting frequency.
Use several transducers to span an area.
Decrease
a)
b)
TABLE 1:
Use an acoustic horn (to focus the sound).
Increase the transmitting frequency.
RECOMMENDED OPERATING CONDITIONS
Supply Voltage, VCC
High-level input voltage, VIH
BINH, INIT
Low-level input voltage, VIL
BINH, INIT
Min.
Max.
Unit
4.5
6.8
V
2.1
ECHO and OSC output voltage
V
0.6
V
6.8
V
Delay time, power up to INIT high
5
ms
Recycle period
80
ms
Operating free-air temperature, TA
0
DS00597A-page 4
40
°C
1995 Microchip Technology Inc.
AN597
TABLE 2:
ELECTRICAL CHARACTERISTICS OVER RECOMMENDED RANGES OF SUPPLY
VOLTAGE AND OPERATING FREE-AIR TEMPERATURE (UNLESS OTHERWISE
NOTED)
Parameter
Test Conditions
Min.
Typ.
Max.
Unit
Input current
BINH, INIT
V1 = 2.1V
1
mA
High-level output current, IOH
ECHO, OSC
VOH = 5.5V
100
µA
Low-level output voltage, VOL
ECHO, OSC
IOL = 1.6 mA
0.4
V
Transducer bias voltage
TA = 25°C
200
V
Transducer output voltage (peak-to-peak)
TA = 25°C
400
V
Number of cycles for XDCR output to reach 400V
C= 500 pF
Internal blanking interval
7
2.38*
Frequency during 16-pulse transmit period
OSC output
49.4*
XMIT output
49.4*
Frequency after 16 pulse transmit
period
OSC output
93.3*
XMIT output
0
ms
kHz
kHz
During transmit period
2000
After transmit period
100
mA
Supply current, ICC
* These typical values apply for a 420 kHz ceramic resonator.
1995 Microchip Technology Inc.
DS00597A-page 5
AN597
Please check the Microchip BBS for the latest version of the source code. For BBS access information,
see Section 6, Microchip Bulletin Board Service information, page 6-3.
APPENDIX B: FIRMWARE LISTING
MPASM 01.02 Released
XDCR.ASM
11-14-1994
LOC OBJECT CODE
LINE SOURCE TEXT
VALUE
0000 0190
0001 3005
0002 0097
0003
0004
0005
0006
0007
0185
0186
0187
0188
0189
0008 1683
0009 3007
000A 009F
000B
000C
000D
000E
000F
0010
0011
0012
0185
0186
3004
0087
0188
0189
1283
0012 1010
0013 018C
0001
0002
0003
0004
0005
0006
0007
0008
0009
0010
0011
0012
0013
0014
0015
0016
0017
0029
0030
0031
0032
0033
0034
0035
0036
0037
0038
0039
0040
0041
0042
0043
0044
0045
0046
0047
0048
0049
0050
0051
0052
0053
0054
0055
0056
0057
0058
0059
0060
0061
0062
0063
0064
0065
0066
0067
0068
DS00597A-page 6
;
;
;
;
;
;
;
;
;
;
;
;
;
;
9:29:15
PAGE
1
XDCR.ASM
This routine continually executes ranging cycles in the
following order:
1)
2)
3)
4)
Timers and Flags are cleared
Ranging Cycle Executes
Distance is Calculated (to 0.5 inch)
HW is re-initialized for next cycle
The processor uses a 4MHz oscillator, so all timing
calculations are referenced to that. The calculated
distance is a 16-bit result in the ACCbHI:ACCbLO registers.
LIST P=16C74, F=INHX8M
;
;******************
; Bank 0 Registers
;******************
;
; TMR1 is off, Prescaler is 1 for a capture timeout of 65 msec
clrf
T1CON
; Set to capture on every rising edge
movlw
0x05
movwf
CCP1CON
; Clear the Ports
clrf
PORT_A
clrf
PORT_B
clrf
PORT_C
clrf
PORT_D
clrf
PORT_E
;
;******************
; Bank 1 Registers
;******************
;
bsf
STATUS,RP0
; Set RP0
; Port A is Digital, Port E is Digital
movlw
0x07
movwf
ADCON1
; Configure CCP1 (RC2) as an input, and all other ports
; as Outputs, (RE0 = INIT, RE1 = BINH)
clrf
TRIS_A
clrf
TRIS_B
movlw
0x04
movwf
TRIS_C
clrf
TRIS_D
clrf
TRIS_E
bcf
STATUS,RP0
; Clear RP0
Xdcr
;
; Initialize Timers and Flags
;
bcf
T1CON,0
; Disable TMR1
clrf
PIR1
; Clear Timer1 Overflow Flag & Timer1 Capture Flag
1995 Microchip Technology Inc.
AN597
0014
0015
0016
0017
0018
0019
001A
001B
001C
001C
001D
001E
001F
0020
0021
0022
018E
018F
0195
0196
1409
1410
21F3
1489
0069
clrf
TMR1L
; Clear TMR1L
0070
clrf
TMR1H
; Clear TMR1H
0071
clrf
CCPR1L
; Clear CCPR1L
0072
clrf
CCPR1H
; Clear CCPR1H
0073
bsf
PORT_E,0
; Set INIT High on Ranging Module
0074
bsf
T1CON,0
; Enable TMR1
0075
call
DEL_9
; Delay 0.9 msec for transducer to stabilize
0076
bsf
PORT_E,1
; Enable Transducer to Receive (BINH)
0077
chk_t1
190C 0078
btfsc
PIR1,2
; Check for Capture
2822 0079
goto
chk_done
; Jump if Capture
1C0C 0080
btfss
PIR1,0
; Check for TMR1 Overflow
281C 0081
goto
chk_t1
; Loop if nothing happened
1010 0082
bcf
T1CON,0
; Turn off TMR1
2833 0083
goto
ovr_flo
; Capture event did not occur
0084
chk_done
0085 ;
0086 ; Calculate distance to 0.5 inch resolution
0087 ;
0022 1010 0088
bcf
T1CON,0
; Turn off TMR1
0023 0815 0089
movf
CCPR1L,W
; Move LSB into W
0024 00A2 0090
movwf
ACCbLO
; Move LSB into ACCbLO
0025 0816 0091
movf
CCPR1H,W
; Move MSB into W
0026 00A3 0092
movwf
ACCbHI
; Move MSB into ACCbHI
0027 304A 0093
movlw
0x4A
; Move 75usec/0.50in into W
0028 00A0 0094
movwf
ACCaLO
; Move LSB into ACCaLO
0029 01A1 0095
clrf
ACCaHI
; Clear MSB (ACCaHI)
002A 208F 0096
call
D_divF
; Call 16-bit/8-bit routine
0097
; which is described in
0098
; Application Note 544
002B 3025 0099
movlw
0x25
; Check remainder to see if
002C 0224 0100
subwf
ACCcLO,W
; we should round up...
002D 1803 0101
btfsc
STATUS,CARRY
; If Remainder < (0.5 * Divisor), skip
002E 0AA2 0102
incf
ACCbLO,F
; Round up
002F 1903 0103
btfsc
STATUS,Z
; Check low byte for wrap around
0030 0AA3 0104
incf
ACCbHI,F
; If LSB wrapped, increment high byte
0031 1D03 0105
btfss
STATUS,Z
; Check high byte for wrap around
0032 2835 0106
goto
done
; High byte didn't wrap
0033
0107
ovr_flo
0033 01A2 0108
clrf
ACCbLO
0034 01A3 0109
clrf
ACCbHI
0035
0110
done
0035 21FD 0111
call
DEL_100
; Wait 100 msec before clearing HW.
0036 1009 0112
bcf
PORT_E,0
; Disable INIT
0037 1089 0113
bcf
PORT_E,1
; Disable BINH
0038 21FD 0114
call
DEL_100
; Wait 100 msec before enabling HW.
0039 2812 0115
goto
Xdcr
0116
0120
0149
0150
end
0151
MEMORY USAGE MAP ('X' = Used, '-' = Unused)
0000 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXXXX-----0040 : ---------------- ---------------- ---------------- ---------------All other memory blocks unused.
Errors
:
0
Warnings :
0
Messages :
0
1995 Microchip Technology Inc.
DS00597A-page 7
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Printed in the USA, 6/6/95
1995, Microchip Technology Inc.
*Information contained in this publication regarding device applications and the like is intended by way of suggestion only. No representation of warranty is given and no liability is assumed by Microchip Technology Inc. with respect to
the accuracy or use of such information.Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip.The Microchip logo and name are trademarks of
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DS00597A-page 8
1995 Microchip Technology Inc.