AN1612, Shock and Mute Pager Applications Using Accelerometers

Freescale Semiconductor
Application Note
AN1612
Rev 5, 10/2006
Shock and Mute Pager Applications
Using Accelerometers
by: C.S. Chua
Sensor Application Engineering, Singapore, A/P
INTRODUCTION
30
20
ACCELEROMETER OUTPUT (G)
In the current design, whenever there is an incoming page,
the buzzer will “beep” until any of the buttons is depressed. It
can sometimes be quite annoying or embarrassing when the
button is not within your reach. This application note describes
the concept of muting the “beeping” sound by tapping the
pager lightly, which could be located in your pocket or
handbag. This demo board uses an accelerometer,
microcontroller hardware/software and a piezo audio
transducer. Due to the wide frequency response of the
accelerometer from d.c. to 400 Hz, the device is able to
measure both the static acceleration from the Earth's gravity
and the shock or vibration from an impact. This design uses a
40g accelerometer, which yields a minimum acceleration
range of -40g to +40g.
10
0
-10
-20
-30
TAPPING OF
ACCELEROMETER
-40
-50
-60
-70
-0.05
-0.03
-0.01
0
0.01
0.03
0.05
TIME (seconds)
CONCEPT OF TAP DETECTION
To measure the tapping of a pager, the accelerometer must
be able to respond in the range of hundreds of hertz. During
the tapping of a pager at the top surface, illustrated in
Figure 1, the accelerometer will detect a negative shock level
between -15g to -50g of force depending on the intensity.
Similarly, if the tapping action comes from the bottom of the
accelerometer, the output will be a positive value. Normally,
the peak impact pulse is in the order of a few milliseconds.
Figure 2 shows a typical waveform of the accelerometer under
shock.
TAPPING ACTION
FRONT VIEW
PCB
Figure 1. Tapping Action of Accelerometer
© Freescale Semiconductor, Inc., 2006. All rights reserved.
Figure 2. Typical Waveform of Accelerometer Under
Tapping Action
Therefore, we could set a threshold level, either by
hardware circuitry or software algorithm, to determine the
tapping action and mute the “beeping.” In this design, a
hardware solution is used because there will be minimal code
added to the existing pager software. However, if a software
solution is used, the user will be able to program the desire
shock level.
HARDWARE DESCRIPTION AND OPERATION
Since MMA1201P is fully signal-conditioned by its internal
op-amp and temperature compensation, the output of the
accelerometer can be directly interfaced with a comparator. To
simplify the hardware, only one direction (tapping on top of the
sensor) is monitored. The comparator is configured in such a
way that when the output voltage of the accelerometer is less
than the threshold voltage or Vref (refer to Figure 3), the
output of the comparator will give a logic 1, illustrated in
Figure 4. To decrease the VREF voltage or increase the
threshold impact in magnitude, turn the trimmer R2 anticlockwise.
For instance, if the threshold level is to be set to -20g, this
will correspond to a Vref voltage of 1.7 V.
+5.0 V
∆V
VREF = VOFFSET + ⎛ -------- × G THRESHOLD⎞
⎝ ∆G
⎠
VIN
3
VREF
2
8
+
4
+5.0 V
= 2.5 + (0.04 × [-20])
= 1.7 V
LM311N
U1
6
1 5
VOUT
7
100 k
R1
2
R2
+
100 k
C3
Under normal condition, VIN (which is the output of the
accelerometer) is at about 2.5 V. Since VIN is higher than Vref,
the output of the comparator is at logic 0. During any shock or
impact which is greater than -20g in magnitude, the output
voltage of the accelerometer will go below VREF. In this case,
the output logic of the comparator changes from 0 to 1.
When the pager is in silence mode, the vibrator produces
an output of about ±2g. This will not trigger the comparator.
Therefore, even in silence mode, the user can also tap the
pager to stop the alert. Refer to Figure 5 for the vibrator
waveform.
1.0 µ
1
Figure 3. Comparator Circuitry
2.0
1.5
VIBRATOR MOVEMENT (G)
6.0
5.0
VOUT (V)
4.0
3.0
2.0
1.0
0.5
0
-0.5
-1.0
-1.5
-2.0
-0.025
1.0
-0.015
-0.005
0
0.005
0.015
0.025
TIME (seconds)
0
-0.05
-0.03
-0.01
0
0.01
0.03
0.05
Figure 5. Vibrator Waveform
TIME (seconds)
Figure 4. Comparator Output Waveform
AN1612
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Freescale Semiconductor
Figure 6 is a schematic drawing of the whole demo and
Figure 7, Figure 8, and Figure 9 show the printed circuit board
and component layout for the shock and mute pager. Table 1
is the corresponding part list.
R4
MC78L05ACP
3
J2
1
2 C10
INPUT
0.33 µ
U5
OUTPUT
GND
2
+0.5 V
1
10 M
+0.5 V
C9
X1
C3
0.1 µ
R5
22 p
16
10 k
+0.5 V
S1
C5
10 n
+0.5 V
0.1 µ
C2
U1
8
VS
OUTPUT
6
5 RB
3
1.0 k
BYPASS
SELF-TEST
0.1 µ
C7
10 n
C8
4
R3
8
-
2 +
10 k
LM311N
U2
4
1
5
6
7
+5.0 V
C1 0.1 µ
GND
7
MMA1201P
R7
+5.0 V
R1
100 k
2
R2
1
100 k
C12
10 k
J1
R6
180 R
+
0.1 µ
D1
+5.0 V
R9
10 k
C4
22 p
4 MHz
OSC1
OSC2
17
VDD
10
18
19
RST
IRQ
22
23
TCAP1
TCAP2
41
VSS
TDO
SCLK
52
51
8
7
VRH
VRL
PLMA
PLMB
20
21
31
30
29
28
27
26
25
24
PA0
PA1
PA2
PA3
PA4
PA5
PA6
PA7
PC0
PC1
PC2/ECLK
PC3
PC4
PC5
PC6
PC7
49
48
47
46
45
44
43
42
39
38
37
36
35
34
33
32
PB0
PB1
PB2
PB3
PB4
PB5
PB6
PB7
PD0/AN0
PD1/AN1
PD2/AN2
PD3/AN3
PD4/AN4
PD5/AN5
PD6/AN6
PD7/AN7
14
13
12
11
9
5
4
3
TCMP1
TCMP2
2
1
+5.0 V
C6
10 n
C11 +
47 µ
U4
PIEZO
TRANSDUCER
MC68HC705B16CFN
U3
S2
Figure 6. Overall Schematic Diagram of the Demo
AN1612
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Freescale Semiconductor
3
SHOCK & MUTE
PAGER
+9V
U4
GND
D1
J2
U5
C9
R6
R1
C12
R3
U2
U1
C7 C8C2 C1
C6
R2
R14
C4
C5
R5
R8
S2
R9
R7
J1
U3
C3 X1
S1
C11
Figure 8. Solder Side of the PCB
Figure 7. Silk Screen of the PCB
Table 1. Bill of Material for the Shock and Mute Pager
Qty.
Value
References
Ceramic Capacitor
Device Type
4
0.1µ
C1, C2, C7, C9
Ceramic Capacitor
2
22p
C3, C4
Ceramic Capacitor
3
10n
C5, C6, C8
Solid Tantalum
1
0.33µ
C10
Electrolytic Capacitor
1
47µ
C11
Electrolytic Capacitor
1
1µ
C12
LED
1
5mm
D1
Header
1
2 way
J1
PCB Terminal Block
1
2 way
J2
Resistor ±5% 0.25W
1
100k
R1
Single Turn Trimmer
1
100k
R2
Resistor ±5% 0.25W
4
10k
R3, R5, R7, R9
Resistor ±5% 0.25W
1
10M
R4
Resistor ±5% 0.25W
1
180R
R6
Resistor ±5% 0.25W
1
1k
R8
Push Button
2
6mm
S1, S2
SOFTWARE DESCRIPTION
MMA1201P
1
—
U1
LM311N
1
—
U2
MC68HC705B16CFN
1
—
U3
Piezo Transducer
1
—
U4
MC78L05ACP
1
—
U5
Crystal
1
4MHz
X1
Upon powering up the system, the piezo audio transducer
is activated simulating an incoming page, if the pager is in
sound mode (jumper J1 in ON). Then, the accelerometer is
powered up and the output of the comparator is sampled to
obtain the logic level. The “beeping” will continue until the
accelerometer senses an impact greater than the threshold
level. Only then the alert is muted. However, when the pager
is in silence mode (jumper J1 is OFF), indicated by the blinking
red LED, the accelerometer is not activated. To stop the alert,
press the push-button S2.
To repeat the whole process, simply push the reset switch
S1.
Figure 9. Component Side of the PCB
AN1612
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Figure 10 is a flowchart for the program that controls the
system.
RECEIVE A
PAGE
Y
IS IT IN
SILENCE MODE?
N
TURN ON THE SHOCK SENSOR
N
IS BUTTON
ACTIVATED?
IS SHOCK
SENSOR ACTIVATED
OR BUTTON
ACTIVATED?
N
Y
Y
TURN OFF THE
SHOCK SENSOR
TURN OFF THE
BUZZER OR VIBRATOR
END
Figure 10. Main Program Flowchart
CONCLUSION
The shock and mute pager design uses a comparator to
create a logic level output by comparing the accelerometer
output voltage and a user-defined reference voltage. The
flexibility of this minimal component, high performance design
makes it compatible with many different applications, e.g. hard
disk drive knock sensing, etc. The design presented here uses
a comparator which yields excellent logic-level outputs and
output transition speeds for many applications.
AN1612
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Freescale Semiconductor
5
SOFTWARE SOURCE/ASSEMBLY PROGRAM CODE
******************************************************************************
*
*
******************************************************************************
*
*
*
Pager Shock & Mute Detection Version 1.0
*
*
*
*
The following code is written for MC68HC705B16 using MMDS05 software
*
*
Version 1.01
*
*
CASM05 - Command line assembler Version 3.04
*
*
P & E Microcomputer Systems, Inc.
*
*
*
*
Written by : C.S. Chua
*
*
9th January 1997
*
*
*
*
Software Description
*
*
*
*
J1 ON - Sound mode
*
*
Buzzer will turn off if the accelerometer is tapped or switch S2 is
*
*
depressed.
*
*
*
*
J1 OFF - Silence mode
*
*
LED will turn off if and only if S2 is depressed
*
*
*
******************************************************************************
******************************************
*
*
*
I/O Declaration
*
*
*
******************************************
PORTB
EQU
$01
; Port B
PLMA
EQU
$0A
; D/A to control buzzer
TCONTROL
EQU
$12
; Timer control register
TSTATUS
EQU
$13
; Timer Status Register
OCMPHI1
EQU
$16
; Output Compare Register 1 High Byte
OCMPLO1
EQU
$17
; Output Compare Register 1 Low Byte
TCNTHI
EQU
$18
; Timer Count Register High Byte
TCNTLO
EQU
$19
; Timer Count Register Low Byte
OCMPHI2
EQU
$1E
; Output Compare Register 2 High Byte
OCMPLO2
EQU
$1F
; Output Compare Register 2 Low Byte
******************************************
*
*
*
RAM Area ($0050 - $0100)
*
*
*
******************************************
ORG
$50
STACK
RMB
4
; Stack segment
TEMPTCNTLO
RMB
1
; Temp. storage of timer result (LSB)
TEMPTCNTHI
RMB
1
; Temp. storage of timer result (MSB)
******************************************
*
*
*
ROM Area ($0300 - $3DFD)
*
*
*
******************************************
ORG
$300
******************************************
*
*
*
Program starts here upon hard reset *
*
*
******************************************
RESET
CLR
PORTB
; Initialise Ports
LDA
#%01001000
; Configure Port B
STA
$05
LDA
TSTATUS
; Dummy read the timer status register so as to clear the OCF
CLR
OCMPHI2
CLR
OCMPHI1
LDA
OCMPLO2
JSR
COMPRGT
LDA
#$40
; Enable the output compare interrupt
STA
TCONTROL
LDA
#10
; Idle for a while before "beeping"
IDLE
JSR
DLY20
DECA
BNE
IDLE
CLI
; Interrupt begins here
BRSET
1,PORTB,SILENCE
; Branch if J1 is off
BSET
6,PORTB
; Turn on accelerometer
JSR
DLY20
; Wait till the supply is stable
TEST
BRSET
5,PORTB,MUTE
; Sample shock sensor for tapping
BRCLR
7,PORTB,MUTE
; Sample switch S2 for muting
JMP
TEST
MUTE
BCLR
6,PORTB
; Turn off accelerometer
SEI
CLR
PLMA
; Turn off buzzer
AN1612
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DONE
SILENCE
JMP
DONE
; End
BRSET
7,PORTB,SILENCE
; Sample switch S2 for stopping LED
SEI
BCLR
3,PORTB
; Turn off LED
JMP
DONE
; End
******************************************
*
*
*
Timer service interrupt
*
*
Alternates the PLMA data
*
*
and bit 3 of Port B
*
*
*
******************************************
TIMERCMP
BSR
COMPRGT
; Branch to subroutine compare register
BRSET
1,PORTB,SKIPBUZZER ; Branch if J1 is OFF
LDA
PLMA
EOR
#$80
; Alternate the buzzer
STA
PLMA
RTI
SKIPBUZZER
BRSET
3,PORTB,OFF_LED
; Alternate LED supply
BSET
3,PORTB
RTI
OFF_LED
BCLR
3,PORTB
RTI
******************************************
*
*
*
Subroutine reset
*
*
the timer compare register
*
*
*
******************************************
COMPRGT
LDA
TCNTHI
; Read Timer count register
STA
TEMPTCNTHI
; and store it in the RAM
LDA
TCNTLO
STA
TEMPTCNTLO
ADD
#$50
; Add C350 H = 50,000 periods
STA
TEMPTCNTLO
; with the current timer count
LDA
TEMPTCNTHI
; 1 period = 2 us
ADC
#$C3
STA
TEMPTCNTHI
; Save the next count to the register
STA
OCMPHI1
LDA
TSTATUS
; Clear the output compare flag
LDA
TEMPTCNTLO
; by access the timer status register
STA
OCMPLO1
; and then access the output compare register
RTS
******************************************
*
*
*
Delay Subroutine for 0.20 sec
*
*
*
*
Input: None
*
*
Output: None
*
*
*
******************************************
DLY20
STA
STACK+2
STX
STACK+3
LDA
#!40
; 1 unit = 0.7725 mS
OUTLP
CLRX
INNRLP
DECX
BNE
INNRLP
DECA
BNE
OUTLP
LDX
STACK+3
LDA
STACK+2
RTS
******************************************
*
*
*
This subroutine provides services
*
*
for those unintended interrupts
*
*
*
******************************************
SWI
RTI
; Software interrupt return
IRQ
RTI
; Hardware interrupt
TIMERCAP
RTI
; Timer input capture
TIMERROV
RTI
; Timer overflow interrupt
SCI
RTI
; Serial communication Interface Interrupt
ORG
$3FF2
; For 68HC05B16, the vector location
FDB
SCI
; starts at 3FF2
FDB
TIMERROV
; For 68HC05B5, the address starts at 1FF2
FDB
TIMERCMP
FDB
TIMERCAP
FDB
IRQ
FDB
SWI
FDB
RESET
AN1612
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AN1612
Rev 5
10/2006
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