Application Note 131
February 2011
An Introduction to Acoustic Thermometry
An Air Filled Olive Jar Teaches Signal Conditioning
Jim Williams and Omar Sanchez-Felipe
Introduction
We occasionally lecture to university engineering students.
A goal of these lectures is to present technology in a novel,
even charming, way. This hopefully entices the student
towards the topic; an aroused curiosity is fertile ground
for education. One such lecture investigates acoustic thermometry as an example of signal conditioning techniques.
This subject has drawn enough interest that it is presented
here for wider dissemination and as supplementary material for future acoustic thermometry lectures.
Acoustic Thermometry
Acoustic thermometry is an arcane, elegant temperature
measurement technique. It utilizes sound’s temperature
dependent transit time in a medium to measure temperature. The medium may be a solid, liquid or gas. Acoustic
thermometers function in environments that conventional
sensors cannot tolerate. Examples include extreme temperatures, applications where the sensor would be subjected to destructive physical abuse and nuclear reactors.
Gas path acoustic thermometers respond very quickly to
temperature changes because they have essentially no
thermal mass or lag. An acoustic thermometer’s “body”
is the measurand. Additionally, an acoustic thermometer’s
reported “temperature” represents the total measurement
path transit time as opposed to a conventional sensor’s
single point determination. As such, an acoustic thermometer is blind to temperature variations within the measurement path. It reports the measurement path’s delay as its
“temperature,” whether or not the path is iso-thermal.
A pleasant surprise is that the sonic transit time in a gas
path thermometer is almost entirely insensitive to pressure
and humidity, leaving temperature as the sole determinant.
Additionally, sonic speed in air varies predictably as the
square root of temperature.
Practical Considerations
A practical acoustic thermometer demonstration begins
with selecting a sonic transducer and a dimensionally stable
measurement path. A wideband ultrasonic transducer is
desirable to promote fast, low jitter, hi-fidelity response
free of resonances and other parasitics. The electrostatic
type specified in Figure 1 meets these requirements. A
single transducer serves as both transmitter and receiver.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
BOTTLE CAP AND
STIFFENING PLATE
GLASS BOTTLE ≈ 6"
ULTRASONIC TRANSDUCER
(SENSCOMP #604142)
TRANSDUCER MOUNTS
SONIC MEASURMENT PATH ≈ 12"
AN131 F01
HEADER AND
OUTPUT WIRES
(ALL CAP VIEWS ARE X-RAY)
Figure 1. Ultrasonic Transducer Rigidly Mounts Within Stiffened Cap Affixed to Bottle. Structure Defines Fixed Length Measurement
Path Essentially Independent of Physical Variables. Sonic Transit Time at 75°F ≈ 900µs with ≈1µs/°F Variation
an131f
AN131-1
Application Note 131
The device is rigidly mounted within the stiffened metal
cap of a glass enclosure, promoting measurement path
dimensional stability. The enclosure and its cap are conveniently furnished by a bottle of “Reese” brand “Cannonball” olives (Figure 2). After removing the olives and
their residue, the bottle and cap are baked out at 100°C
prior to joining. The transducer leads pass through the
cap via a coaxial header. This arrangement, along with
the glass enclosure’s relatively small thermal expansion
coefficient, yields a path distance stable against temperature, pressure and mechanically induced changes. Path
length, including enclosure bottom bounce and return
to transducer, is about 12". This sets a two-way trip
time of around 900µs (speed of sound in air ≈ 1.1 ft/ms).
At 75°F, this path’s temperature dependent variation is approximately 1µs/°F. A desired 0.1°F resolution mandates
mechanical and electronic induced path length uncertainty
inside 100ns; approximately 0.001" dimensional stability
referred to the 12" path length. Considering likely error
sources, this is a realistic goal.
Overview
Figure 2. Photograph Details Cap Assembly, Partially Views
Glass Enclosed Measurement Path. Ultrasonic Transducer Visible
Within Cap. Stiffening Plate, Bonded to Cap Top, Prevents
Ambient Pressure or Temperature Changes from Deforming
Thin Metal Cap, Promoting Measurement Path Length Stability.
Coaxial Header Provides Transducer Connections
Note 1. See Appendix A, “Measurement Path Calibration” for details on
determining calibration constants.
Figure 3 is a simplified overview of the acoustic thermometer. The transducer, which can be considered a capacitor,
is biased at 150VDC. The start pulse clock drives it with
a short impulse, launching an ultrasonic event into the
measurement path. Simultaneously, the width decoding
flip-flop is set high. The sonic impulse bounces off the
enclosure bottom, returns to the transducer and impinges
on it. The resulting minuscule mechanical displacement
causes the transducer to give up charge (Q = ΔC•V),
which appears as a voltage at the receiver amplifier input.
The trigger converts the amplifier’s output excursion into
a logic compatible level which resets the flip-flop. The
flip-flop output width represents the measurement path’s
temperature dependent sonic transit time. The microprocessor, equipped with the measurement path’s temperature/
delay calibration constants, calculates the temperature and
supplies this information to the display.1 The start pulse
generator has a second output which gates the trigger
output off during nearly the entire measurement cycle.
The trigger output only passes during the immediate time
vicinity when a return pulse is expected. This discriminates
against unwanted sonic events originating outside the
measurement path, eliminating false triggers. A second
gating, sourced from the width decoding flip-flop, shuts
down the 150V bias supply switching regulator during the
measuring interval. Return pulse amplitude is under 2mV
at the transducer and the high gain, wideband receiver
amplifier is vulnerable to parasitic inputs. Shutting down
the 150V bias supply during the measurement prevents
its switching harmonics from corrupting the amplifier.
Figure 4 describes the system’s event sequence. A measurement cycle begins with a start pulse (A) driving the
transducer and setting the flip-flop (B) high. After the sonic
impulse’s transit time, the amplifier responds (C, diagram
right), tripping the trigger (D, diagram right) which resets
the flip-flop. Gate signals (E and F) protect the trigger
from unwanted sonic events and start pulse artifacts and
shut off the high voltage regulator during measurement.
an131f
AN131-2
Application Note 131
150V GATING
150VDC
BIAS
MEASUREMENT PATH
SHUTDOWN
WIDEBAND
RECEIVER
AMPLIFIER
A ≈ 20,000
+
–
TRANSDUCER
TRIGGER
TRIGGER GATING
V+
WIDTH DECODE
START PULSE
CLOCK
S
FLIP-FLOP
Q
Q
R
AN131 F03
WIDTH OUT = TEMPERATURE DEPENDENT
TRANSIT TIME
MICROPROCESSOR
DISPLAY
MEMORY
(CALIBRATION CONSTANTS)
Figure 3. Conceptual Signal Conditioning for Acoustic Thermometer. Start Clock Launches Acoustic Pulse into
Measurement Path, Sets Width Decode Flip-Flop High. Acoustic Paths Return Pulse, Amplified by Receiver, Trips Trigger,
Resetting Flip-Flop. Resultant “Q” Width Output, Representing Path’s Temperature Dependent Transit Time, is Converted
to Temperature Reading by Microprocessor. Gating High Voltage Supply and Trigger Prevents Spurious Outputs
an131f
AN131-3
Application Note 131
A
START
B
FLIP-FLOP
C
AMPLIFIER
OUTPUT
D
TRIGGER
OUTPUT
E
TRIGGER
GATING
F
150VDC
SHUTDIWN
GATING
AN131 F03
Figure 4. Figure 3’s Event Sequence. Start Pulse (A) Drives Transducer, Sets Flip-Flop (B) High.
Sonic Pulse Return Activates Amplifier (C, Extreme Right), Causing Trigger Output (D) to Reset
Flip-Flop (B). Trigger Gating (E) Prevents Erroneous Trigger Response to Start Pulse Caused
Amplifier Output (C, Extreme Left) and External Sonic Events. Gating (F) Turns Off 150V Switching
Converter During Measurement, Precluding Amplifier Output Corruption
an131f
AN131-4
Application Note 131
BAV-21
10k
RECEIVER
3pF
0.22µF
ACOUSTIC PATH
LENGTH ≈ 12"
20k
+
–
15k
20k*
A1
LT1122
+
–
20k*
BAV-99
A2
LT1220
500Ω
20k*
1k*
–
+
A3
LT1220
1.3k
+
–
15µF 15µF
+
+
GATED TRIGGER
15V
TYPICAL = 8.0V
10k
1k*
BAV-21
5V
C1
LT1011
10k
10µF
15V, 10µs START PULSE
15V
START PULSE GENERATOR
TRIG OUT
LTC6993-1
GND
1k
Q2
2N3904
V+
360Ω
A
SET
Q1
2N2369
5V
DIV
1N4148
1N4148
1k
499k*
TRIG OUT
5V
LTC6993-1
V+
GND
B
SET
RECEIVER-TRIGGER
GATING
CK
182k
DIV
S
976k
519k*
150V BIAS SUPPLY
5V
5V
DATA OUTPUT
WIDTH GENERATOR
D
74HC74
Q
820µH
1/6 74HC04
R
BAV-21
5V
OUTPUT WIDTH = TEMPERATURE DEPENDENT DELAY
1N914
1N914
MICROPROCESSOR
WIDTH IN HV BIAS GATING
ATMEL AT-MEGA32U4 BOARD C7
B4
100Hz
OUT
OUT
LTC6991
V+
S
D7
1µF
10V
10M*
DIV
LT1072
VC
G
0.1µF
D6
Q3
VN2222L
100k
FB
0.0068µF
0.001µF
84.5k*
1k
LATCH
R
G
Q3
MPS-A42
SW
START PULSE
INPUT
5V
150V
390Ω
47µF
MSB
LSB
487k*
HP5082-7300 (AVAGO)
DP
COUNT
RESET
W 7490
R
D
IN 7490
R
D
IN 7490
R
AN131 F03
* = 1% FILM
A1, A2, A3, C1 = ±15V
ALL OTHERS = 5V
= SENSCOMP #604142
Figure 5. Detailed Circuitry Closely Follows Figure 3’s Concept. Start Pulse Generator is Comprised of 100Hz Clock, One-Shot
Multivibrators and Q1-Q2 Driver Stage. A ≈ 20,000 Receiver Amplifier Splits Gain Among Three Stages, Biases Trigger Comparator.
Flip-Flop Output Width Feeds Microprocessor Which Calculates and Displays Temperature. Capacitive Coupling Isolates High Voltage
DC Transducer Bias, Diode Clamping Prevents Destructive Overloads. Switching Regulator Controls High Voltage Via Cascode. Gating
Obviates Switching Regulator Noise Originated Interference, Minimizes External Sonic Corruption
an131f
AN131-5
Application Note 131
Detailed Circuitry
Figure 5’s detailed schematic closely follows Figure 3’s
concepts. An LTC®6991 oscillator furnishes the 100Hz
clock. LTC®6993-1 monostable “A” provides a 10µs width
to the Q1-Q2 driver, which capacitively couples the start
pulse (Trace A, Figure 6) to the transducer. Simultaneously,
the monostable sets the flip-flop (E) high. The flip-flop
high output shuts down the LT®1072 based high voltage
converter during the measurement. Monostable “B” produces a pulse (B) which gates off C1’s trigger output for
a time just shorter than the fastest expected sonic return.
The launched sonic pulse travels down the measurement
path, bounces, returns, and impinges on the transducer.
The transducer, biased at 150VDC, releases charge (Q =
ΔC•V) which appears as a voltage at the receiver amplifier.
The cascaded amplifier, with an overall gain of ≈ 17,600,
produces A2’s output (C) and a further amplified version at
A3. C1 triggers (D) at the first event that exceeds its negative input threshold, resetting the flip-flop. The resultant
flip-flop width, representing the temperature dependent
transit time, is read by the microprocessor which determines the temperature and displays it.2
Figure 7 studies receiver amplifier operation at the critical return impulse trip point. The returning sonic pulse is
viewed at A2 (Trace A). A3 (B) adds gain, softly saturating the signals leading response. C1’s trigger output (C)
responds to multiple triggers but the flip-flop output (D)
remains high after the initial trigger, securing transit time
data.
Note 2. Complete processor software code appears in Appendix B,
“Software Code.”
A = 5V/DIV
A = 20V/DIV
B = 10V/DIV
C = 20V/DIV
B = 20V/DIV
D = 10V/DIV
E = 10V/DIV
C = 5V/DIV
D = 5V/DIV
200µs.DIV
AN131 F06
Figure 6. Figure 5’s Waveforms Include Start Pulse (A),
Trigger Gate (B), A2 Output (C), Trigger (D) and Flip-Flop Q
Output Width (E). Amplifier Output Causes Multiple Trigger
Transitions, But Flip-Flop Maintains Pulse Width Integrity.
A2-Trigger Outputs at Photo Right, Due to Acoustic Pulse
Second Bounce, Are Inconsequential
20µs DIV
AN131 F07
Figure 7. Receiver Amplifier—Trigger Operating Detail at Trip
Point. Returning Sonic Pulse Viewed at A2 Output (Trace A) After
X440 Gain. A3’s Output (B) Adds X40, Softly Saturating Signals
Leading Response. C1’s Output (C) Responds to Multiple Triggers
But Flip-Flop Output (D) Remains High After Initial Trigger
an131f
AN131-6
Application Note 131
Gating prevents high voltage supply switching harmonics from producing spurious amplifier-trigger outputs.
Figure 8 shows gate-off detail. The flip-flop output (Trace A)
going high shuts down high voltage switching (B) at the
measurement onset. This state persists during the entire
transit time, preventing erroneous amplifier-trigger outputs. Figure 9’s flip-flop fall (Trace A) combines with LT1072
VC pin associated components, producing delayed high
voltage turn-on (B) after the vulnerable, small amplitude
return pulse trip point. This assures a clean, noise-free
trigger.
Several circuit attributes aid performance. As mentioned,
gating the trigger output prevents sonic interference from
outside sources. Similarly, gating the 150V converter off
prevents its harmonics from corrupting the high gain,
wideband receiver amplifier. Additionally, the 150V sup-
ply value is a gain term, making its regulation loss during
the measurement a potential concern. Practically, the 1µF
output capacitor decays only 30mV in this time, or about
0.02%. This small variation is constant, insignificant and
may be ignored. Deriving the trigger trip point and the start
pulse from the same supply allows trigger voltage to vary
ratiometrically with received signal amplitude, enhancing
stability. Finally, the transducer used is wideband, highly
sensitive and free from resonances, promoting repeatable,
jitter-free operation.3 All of the above directly contribute to
the circuit’s < 100ns (0.1°F) resolution of the ≈ 1ms path
length – less than 100ppm uncertainty. Absolute accuracy
from 60°F to 90°F is within 1°F referenced to Appendix
A’s calibration.
Note 3. Readers rich in years will recognize the specified transducer
descends from 1970’s era Polaroid SX-70 automatic focus cameras (every
baby boomer had to have one).
A = 5V/DIV
A = 5V/DIV
B = 100V/DIV
B = 100V/DIV
20µs DIV
AN131 F08
Figure 8. High Voltage Bias Supply Gate-Off Detail. Flip-Flop
Output (Trace A) Going High Shuts Down LT1072 Switching
Regulator, Turning Off 150V Flyback Events (B). Off-Time
Extends During Measurement Interval, Precluding Receiver
Amplifier Corruption
20µs DIV
AN131 F09
Figure 9. High Voltage Bias Supply Gate-On Detail. Flip-Flop
Output (Trace A) Drops Low at Sonic Pulse Return. Components
at LT1072 Switching Regulator VC Pin Delay Bias Supply
Turn-On (B). Sequencing Ensures Bias Supply is Off During
Amplifier-Trigger’s Vulnerable Response to Sonic Pulse Arrival
an131f
AN131-7
Application Note 131
Triggering on later bounces offers the potential benefit
of easing timing tolerances and merits consideration.
Figure 10 shows multiple sonic bounces, discernible in
A2’s output, decaying into acoustic dispersion induced
noise contained within the glass enclosure. Triggering
on a later bounce would relax timing margins, but incurs
unfavorable signal-to-noise characteristics. Signal processing techniques could overcome this, but the increased
resolutions utility would have to justify the effort.
References
1.Lynnworth, L.C. and Carnevale, E.H., “Ultrasonic
Thermometry Using Pulse Techniques.” Temperature:
Its Measurement and Control in Science and Industry,
Volume 4, p. 715-732, Instrument Society of America
(1972).
2.Mi, X.B. Zhang, S.Y. Zhang, J.J. and Yang, Y.T., “Automatic Ultrasonic Thermometry.” Presented at Fifteenth
Symposium on Thermophysical Properties, June 22-27,
2003, Boulder, Colorado, U.S.A.
3. Williams, Jim, “Some Techniques For Direct Digitization
of Transducer Outputs,” Linear Technology Corporation,
Application Note 7, p.4-6, Feb. 1985.
A = 1V/DIV
4.Analog Devices Inc., “Multiplier Applications Guide,”
“Acoustic Thermometer,” Analog Devices Inc., p. 11-13,
1978.
500µs DIV UNCALIBRATED
AN131 F10
Figure 10. Multiple Sonic Bounces, Discernable in A2’s Output,
Decay into Acoustic Dispersion Induced Noise Contained Within
Glass Enclosure. Triggering on Later Bounce Would Ease
Timing Tolerances, But Incur Signal-to-Noise Degradation
an131f
AN131-8
Application Note 131
Appendix A
Measurement Path Calibration
Theoretically, temperature calibration constants can be
calculated from measurement path length. In practice,
it is difficult to determine path length to the required accuracy. Enclosure, transducer and mounting dimensional
uncertainties necessitate calibration vs known temperatures. Figure A1 shows the calibration arrangement. The
enclosure is placed in a controllable thermal chamber
equipped with an accurate thermometer iso-thermal with
the enclosure. Ten evenly spaced temperature points from
60°F to 90°F are generated by stepping chamber set-point.
The enclosure has a 30 minute time constant to 0.25°F
settling, so adequate time must be allowed for each step
to stabilize before readings are taken. Readings consist
of noting the counter indicated pulse width at each temperature and recording the data. This information is then
loaded into the microprocessor memory. See Appendix B,
“Software Code.”
HP-2804A THERMOMETER
THERMAL CHAMBER
ACOUSTIC THERMOMETER
ENCLOSURE
ACOUSTIC THERMOMETER CIRCUITRY
FLIP-FLOP PULSE WIDTH OUT
HP-5335A COUNTER
(PULSE WIDTH MODE)
AN131 A1
Figure A1. Calibration Arrangement Consists of Thermometer, Counter, Acoustic Conditioning Circuitry and Enclosure in Thermal
Chamber. Temperature is Stepped Every 3°F Between 60°F and 90°F Allowing 30 Minute Stabilization Time Per Point
an131f
AN131-9
Application Note 131
APPENDIX B
The software code for the Atmel AT-Mega 32U4 microprocessor, combined with the calibration constants stored in
its memory (see Appendix A), enables the processor to
calculate and display the sensed temperature. This code,
written by Omar Sanchez-Felipe of LTC, appears below.
//OLIVER.C:
//
//
Hot temps from a jar of olives.
//
//
The microprocessor is the Atmel ATmega32U4. See ‘atmel.com’ for its
//
datasheet. The code is compiled with the current version of the
//
‘avr-gcc’ compiler obtainable at ‘winavr.sourceforge.net’.
//
//
Calibration data:
//
Pulse len (usecs)
Temp (deg)
//
877.40
84.2
//
884.34
76.0
//
887.40
72.4
//
892.56
66.4
//
897.60
60.7
//
#include <avr/io.h>
#include <avr/wdt.h>
//
Some useful defs
#define BYTE
#define WORD
#define DWORD
#define BOOL
#define NULL
#defineTRUE
#defineFALSE
unsigned char
unsigned short
unsigned long
unsigned char
(void *)0
1
0
#define BSET(r, n)
#define BCLR(r, n)
#define BTST(r, n)
#define DDOUT(r, v)
#define DDIN(r, v)
#define SETBITS(r, v)
#define CLRBITS(r, v)
r |= (1<<n)
r &= ~(1<<n)
(r & (1<<n))
(r |= (v))
(r &= ~(v))
(r |= (v))
(r &= ~(v))
// set/clr/tst nth bit of reg
#define SYS_CLK
#defineCLKPERIOD
#define POSEDGE
#defineNEGEDGE
#define TMRGATE
#defineBCDRSET
#defineBCDPULS
#defineDPYLATCH
16000000L
625
1
0
PORTC7
PORTD7
PORTB4
PORTD6
// clock (Hz)
//
//
//
//
config (set) bits for OUT
config (clear) bits for IN
set multiple bits on register
clear “”
// edge definitions for timerwait
// I/O pins
an131f
AN131-10
Application Note 131
//
Calibration table and entry.
//
Using small units of time and temp allows all calcs
//
to be done in fixed point.
structcalpoint
{
DWORD pulse;
// pulse duration, in tenths of nsecs
WORD temp;
// temperature, in tenths of degrees
WORD slope;
// slope (in tenths of nsecs/tenths of degree)
};
struct calpoint caltab[] =
// the calibration table
{
{8774000L, 842, 0},
{8843400L, 760, 0},
{8874000L, 724, 0},
{8925600L, 664, 0},
{8976000L, 607, 0}
};
#define NCALS
(sizeof(caltab)/sizeof(struct calpoint))
999
#defineTEMPERR
voidspin(WORD);
BOOL
timerwait(BYTE v, long tmo, WORD *p);
voidsetdpy(WORD);
voiddobackgnd(void);
WORDdotemp();
intmain()
{
WORD temp, i;
// set master clock divisor
CLKPR = (1<<CLKPCE); CLKPR = 0;
// clear WDT
MCUSR = 0;
WDTCSR |= (1<<WDCE) | (1<<WDE);
WDTCSR = 0x00;
// init the
BCLR(PORTD,
BCLR(PORTD,
DDOUT(DDRD,
BCLR(PORTB,
DDOUT(DDRB,
display I/O pins
BCDRSET);
DPYLATCH);
((1<<BCDRSET) | (1<<DPYLATCH)));
BCDPULS);
(1<<BCDPULS));
// init pulse-width counter (timer #3)
TCCR3A = 0x00;
TCCR3B = 0x01;
// no clk prescaling
DDIN(DDRC, (1<<DDC7));
// PC7 is gate
BSET(PORTC, PORTC7);
// enable pullup
// compute the slope for each entry in the cal table
// the first slope is never used, so we leave it at 0
// note we’re inverting the sign of the slope
for (i = 1; i < NCALS; i++)
caltab[i].slope =
(caltab[i].pulse - caltab[i-1].pulse) /
(caltab[i-1].temp - caltab[i].temp);
setdpy(0);
an131f
AN131-11
Application Note 131
for (;;)
{
temp = dotemp();
setdpy(temp);
spin(1000);
}
} // main
//
//
//
void
{
// compute temperature
// set the display
// spin a second and repeat
Set the display LED’s to specified count by brute-force
incrementing the BCD counter that feeds it.
setdpy(WORD cnt)
// reset BCD counter
BSET(PORTD, BCDRSET);
asm(“nop”); asm(“nop”);
BCLR(PORTD, BCDRSET);
while (cnt--)
{
BSET(PORTB, BCDPULS);
asm(“nop”); asm(“nop”);
BCLR(PORTB, BCDPULS);
asm(“nop”); asm(“nop”);
}
// latch current val (to avoid flicker)
BCLR(PORTD, DPYLATCH);
asm(“nop”); asm(“nop”);
BSET(PORTD, DPYLATCH);
}
//
Set up the edge detector to trigger on either a positive or
//
negative edge, depending on the ‘edge’ flag, and then wait for
//
it to happen. See defines for POSEDGE/NEGEDGE above.
//
if param ‘tmo’ is TRUE, a timeout is set so that we dont wait
//
forever if no pulse materializes. Returns the counter value
//
at which the edge occurs via pointer ‘p’.
//
#define tcnTMO(SYS_CLK/20000L)
// approx 1 msecs
BOOL
timerwait(BYTE edge, long tmo, WORD *p)
{
if (edge == NEGEDGE)
BCLR(TCCR3B, ICES3); // falling edge
else BSET(TCCR3B, ICES3); // rising edge
BSET(TIFR3, ICF3);
tmo *= tcnTMO;
while (!BTST(TIFR3, ICF3))
{if (tmo-- < 0)
return FALSE;
}
*p = ICR3; // return current counter
return TRUE;
}
an131f
AN131-12
Application Note 131
//
Measure the next POSITIVE pulse and map into temperature.
//
WORDdotemp()
{
WORD strt, end, i;
DWORD dur, temp;
strt = end = dur = 0;
// wait for any ongoing pulse to complete
if (!timerwait(NEGEDGE, 5000, &strt))
return 0;
// now catch the first positive pulse
if (!timerwait(POSEDGE, 5000, &strt))
return 0;
// and wait for it to complete
if (!timerwait(NEGEDGE, 5000, &end))
return 0;
dur = (end - strt);
dur *= CLKPERIOD; // duration now in tenths of nsecs
// compute temp. If warmer than highest calibrated temp
// or cooler than lowest calibrated temp, return TEMPERR
//
if (dur < caltab[0].pulse || dur > caltab[NCALS-1].pulse)
temp = TEMPERR;
else { // an entry will always be found, but we (re)init
// temp to avoid complaints from the compiler
temp = TEMPERR;
for (i = 1; i < NCALS; i++)
{if (dur <= caltab[i].pulse)
{temp = caltab[i].temp +
(caltab[i].pulse - dur) /
caltab[i].slope;
break;
}
}
}
return temp;
}
//---------------------------------------------------------------//
Spin for ‘ms’ millisecs. Spin constant is empirically determined.
//
#define SPINC (SYS_CLK / 21600L)
volatile WORD
spinx;
void
spin(WORD ms)
{
WORDi;
while (ms--)
for (i = 0; i < SPINC; i++) spinx = i*i;
}
//END
an131f
AN131-13
Application Note 131
#
#
#
#
CC MCU GCC MAKEFILE:
GMAKE file for the “oliver” code running on the ATmega32U4.
= avr-gcc.exe
= atmega32u4
#
These are common to compile, link and assembly rules
#
The ‘no-builtin’ opt keeps gcc from assuming defs for putchar() and
#others
#
COMMON = -mmcu=$(MCU) -fno-builtin
CF
CF
##CF
= $(COMMON)
+= -Wall -gdwarf-2
+= -Wall -gdwarf-2 -O0
AF
AF
AF
= $(COMMON)
+= $(CF)
+= -x assembler-with-cpp -Wa,-gdwarf2
LF
LF
= $(COMMON)
+= -Wl,-Map=$(TMP)$(APP).map
#
weird intel flags
#
HEX_FLASH_FLAGS = -R .eeprom
HEX_EEPROM_FLAGS= -j .eeprom
HEX_EEPROM_FLAGS+= --set-section-flags=.eeprom=”alloc,load”
HEX_EEPROM_FLAGS+= --change-section-lma .eeprom=0 --no-change-warnings
#------------------------------------------------------------------#
MAKE DIRECTIVES
#
#
The “target directory” TMP is standard for the intermediate files
#
(.obj) and the final product.
#------------------------------------------------------------------TMP = ./tmp/
APP
ELF
OBJS = oliver
= $(TMP)$(APP).elf
= $(TMP)oliver.o
all:
$(TMP) $(ELF) $(TMP)$(APP).hex $(TMP)$(APP).eep size
$(TMP):
rm -rf .\tmp
mkdir .\tmp
$(TMP)oliver.o: oliver.c
$(CC) $(INCLUDES) $(CF) -O1 -c
$< -o $*.o
oliver.asm: oliver.c
$(CC) $(INCLUDES) $(CF) -O1 -S
$< -o oliver.asm
an131f
AN131-14
Application Note 131
#
Linker --------------------------------------------------------$(ELF): $(OBJS)
$(CC) $(LF) $(OBJS) $(LINKONLYOBJS) $(LIBDIRS) $(LIBS) -o $(ELF)
%.hex: $(ELF)
avr-objcopy -O ihex $(HEX_FLASH_FLAGS)
$< $@
%.eep:$(ELF)
-avr-objcopy $(HEX_EEPROM_FLAGS) -O ihex $< $@ || exit 0
%.lss:$(ELF)
avr-objdump -h -S $< > $@
size:$(ELF)
@echo
@avr-size -C --mcu=${MCU} $(ELF)
#
clean:
Misc
-----------------------------------------------------------
-rm -rf $(OBJS) $(TMP)$(APP).elf ./dep/* $(TMP)$(APP).hex $(TMP)$(APP).eep
$(TMP)$(APP).map $(TMP)$(APP).d
#end
an131f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN131-15
Application Note 131
an131f
AN131-16
Linear Technology Corporation
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