Pérez et al.

: A demodulator of PWN signalsIngeniare

generated-for
Revista
a digital
Chilena
accelerometer
de Ingeniería,
is developed
vol. 14 Nº
using
2, 2006,
a microcontroller
pp. 119-123

A DEMODULATOR OF PWM SIGNALS GENERATED FOR A DIGITAL

ACCELEROMETER IS DEVELOPED USING A MICROCONTROLLER

UN DEMODULADOR DE SEÑALES PWM GENERADAS POR UN ACELERÓMETRO

DIGITAL ES DESARROLLADO USANDO UN MICROCONTROLADOR
Eduardo Pérez Lobato1 Marcelo Muñoz Tapia2 Jorge Ramírez Fernández3

Recibido 14 de marzo de 2005, aceptado 2 de enero de 2006

Received: March 14, 2005 Accepted: January 2, 2006

RESUMEN

Esta publicación presenta el uso de un microcontrolador para demodular dos señales PWM que están siendo generadas por
un acelerómetro digital, obtener sus anchos y enviarlas en forma serial al puerto paralelo de un computador de propósitos
generales.

Palabras clave: Acelerómetro, microcontrolador, control por computador, PWM.

ABSTRACT

This paper presents the use of a microcontroller to demodulate two Pulse Width Modulated (PWM) signals which are
being generated by a digital accelerometer, to obtain their pulse widths and transmit them serially to a parallel port of
a general purpose computer.

Keywords: Accelerometer, microcontroller, computer-controlled, PWM.

INTRODUCTION 2 and Figure 3 respectively. To demodulate PWM signals,

Karthaus and Fischer [1] used the envelope detection
Nowadays, there is a new generation of electronic technique and Pascual [2] developed a low pass filter.
accelerometers which are more precise, cheaper, This proposal deals with developing a PWM demodulator
smaller and with a wider range than their mechanical using a microcontroller as a single hardware piece.
counterparts. The scope of its use is very wide, which
usually involves force related applications, such as
mechanical stress, rotary torque, seismic events,
dynamic weight and so on. Thus, the introduction of
these tiny digital accelerometers opens the chances
of dynamically monitoring acceleration in a real time
fashion. In addition, if these devices are integrated with
general purpose computers, it is feasible to go beyond
just displaying them on an screen, calculating their
related energy and forces, etc., saving them up onto disk
for further analysis and distributing them through the
Internet using the Transmission Control Protocol/Internet
Protocol (TCP/IP). These accelerometers, as integrated
circuits, generate two PWM squared-waveforms as shown Figure 1. X-Y acceleration signals generated by the
in Figure 1, and the value of the accelerations AX and AY accelerometer.
can be calculated through the equations shown in Figure

1 Departamento de Ingeniería Industrial, Facultad de Ingeniería Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile,
perezlobato@uantof.cl
2 Centro de Computación, Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile, mmunoz@uantof.cl
3 Departamento de Geomensura, Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile, jramirez@unatof.cl
Ingeniare - Revista de Ingeniería, vol. 14 Nº 2, 2006 119
Ingeniare - Revista Chilena de Ingeniería, vol. 14 Nº 2, 2006

¤ T 1X ³ 1 each, TTL electrically leveled, one designated as the

AX  ¥ 0.5 * (1) ACCEL_X and the other as the ACCEL_Y.
¦ T2X µ́ 8
Microcontroller. The general purpose PIC-16F84 was
Equation that uses the T1X and T2X measured pulse chosen because its technical features suits with the
widths to calculate the X-acceleration value requested capabilities and its widespread use makes it a
simpler task to integrate it with further projects.
¤ T 1Y ³ 1 (2)
AY  ¥ 0.5 * Figure 2 depicts the block diagram between the computer
¦ T 2Y µ́ 8
parallel port, the microcontroller and the accelerometer,
giving details only of the inter-connection lines used and
Equation that uses the T1Y and T2Y measured pulse their related pin outs.
widths to calculate the Y-acceleration value

PWM DEMODULATOR REQUIREMENTS

To demodulate a PWM signal it is necessary to know

either the width of the high state pulse and the width of
the cycle. The present work presents the next strategy

1. Detect when a signal starts or finishes, either

if it is in the high or low state. To accomplish
this, detection either of the falling or raising
edges is needed.
2. Counting slices of time whenever a falling or
a raising edge has been detected.
Figure 2. The general layout shows up the interconnected
To implement the above strategy, a microcontroller signal lines among the computer, the
will be used, mostly because it has the following microcontroller and the accelerometer.
advantages

q Flexible detection either of the falling SEQUENCE TO DEMODULATE TWO PWM

or ra ising edges th rough embedded SIGNALS
programming.
To measure the width of the low and the high state of the
q Easiness to count slices of time through the ACCEL_X and ACCEL_Y signals, a time slice counting
internal programmable timer. is carried out, using the internal timer TMR0, which was
programmed to count slices of one microsecond, because
q Fast development of an interface between the both ACCEL_X and ACCEL_Y signals have a width time
computer parallel port and a microcontroller, of 200 micro seconds each, so the high and low time are
because electrical compatibility. 100 microseconds width. Thus, since TMR0 is an 8-bit
q Easy serial communication implementation, register, the maximum count is 255, or, 255 microseconds
which includes protocol developing, as maximum width time to record. The sequence to
handshaking control and data transmission accomplish it is described below and the block diagram
to demodulate the PWM signals is shown in Figure 3.
through embedded programming.
1. To measure the high state of the X-acceleration
PWM DEMODULATOR HARDWARE SETUP
signal, the software waits until a rising edge
of the ACCEL_X signal is detected. Once
Accelerometer. An ADXL-202 accelerometer was used detected, TMR0 is activated to count until a
as an acceleration sensor. It generates two PWM squared- falling edge of the ACCEL_X signal is detected
waveforms signals, 90º shifted, 1mV/g resolution, ± 2 g and then TMR0 is stopped and its content is
scope and, with a 5 KHz central operation frequency stored into the T1X register.

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 Pérez et al.: A demodulator of PWN signals generated for a digital accelerometer is developed using a microcontroller

2. To measure the low state of the X-acceleration

signal, the software waits until a falling edge Main Program
of the ACCEL_X signal is detected. Once
detected, TMR0 is activated to count until a
rising edge of ACCEL_X is detected and then ACCEL_X is High?
TMR0 is stopped an its content is stored into No

the TPX register. Clear internal timer TMR0 & make it count

3. To calculate the full width of the waveform, the

content of the registers T1X and TPX are added ACCEL_X is Low?
and the result is stored in T2X register. No
Store TMR0 into T1_X register
4. The above proceeding is repeated in order to Clear internal timer TMR0 & make it count
have the pulse widths of the ACCEL_Y signal
stored in the T1Y, TPY and T2Y registers.
ACCEL_X is High?
5. The content of T1X is moved to TTY and No

thenthe SEND_TTY subroutine is called. Store TMR0 into TP_X register

Add T1_X and TP_X and store the result onto
the T2_X register
6. The content of T2X is moved to TTY and then
the SEND_TTY subroutine is called.

7. The content of T1Y is moved to TTY and then ACCEL_Y is High?

the SEND_TTY subroutine is called. No
Clear internal timer TMR0 & make it count
8. The content of T2Y is moved to TTY and then
the SEND_TTY subroutine is called
ACCEL_Y is Low?
No
Store TMR0 into T1_Y register
SEQUENCE TO TRANSMIT 32 BITS TO THE
Clear internal timer TMR0 & make it count
COMPUTER PARALLEL PORT

The SEND_TTY subroutine transmits the 8-bit TTY

ACCEL_Y is High?
register to the parallel port of the computer, in a bit to bit No
fashion, so the next actions are performed and repeated Store TMR0 into TP_Y register
eight times. This subroutine is described below and the Add T1_Y and TP_Y and store result
block diagram is shown in Figure 4. into T2_Y register

Move T1_X onto TTY

q The PIC_READY line is raised to let SEND_TTY
computer know that the microcontroller is Move T2_X onto TTY
ready to send 32-bits. SEND_TTY
Move T1_Y onto TTY
q TTY is left-rotated, putting the most SEND_TTY
significant bit into the Carry bit of the
Move T2_Y onto TTY
microcontroller STATUS register.
SEND_TTY

q The Carry bit is outputted to the DATA

line.
Figure 3. The algorithm shows up the full steps sequence
q A falling edge detection of the IO_CLOCK of the PWM demodulator in order to obtain
line is waited. the AX and AY values and send them to the
parallel port of the computer.
The PIC_READY line is turned to low let computer know
that the microcontroller has already sent the eight-bits.

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Ingeniare - Revista Chilena de Ingeniería, vol. 14 Nº 2, 2006

q One chip demodulator means tiny space

and light weight, which makes it extremely
Send TTY portable, either to locate it wherever in the lab
or install it in remote sites, such as in seismic
applications.
Raise PIC_READY q Old mechanical accelerometers, pendulum style,
Counter fl 8 along with their heavy carcasses and gears, are
several times more expensive than the digital
ones.
Rotate left TTY register through carry
q Use of the parallel port instead of the serial port,
DATA fl Carry because a simpler protocol could be devised.
Counter fl Counter - 1

ACKNOWLEDGMENTS

The authors wish to thank to the Universidad de

IO_CLOCK is Antofagasta, which funded the Project PEI-1334-01,
High?
No named “Construcción de un acelerógrafo prototipo
Yes
inteligente para multipropósitos, sismo resistentes…”.
Besides we want to thank to the European Southern
IO_CLOCK is Observatory organization and Compañía Minera Zaldivar
Low? for help funding this work.
No
Yes REFERENCES

No [1] U. Karthaus, M. Fischer. “Fully Integrated Passive

Counter = 0
UHF RFID Transponder IC with 16.7 mW minimum
RF input power”. IEEE Journal Of Solid-State
Fall PIC_READY
Circuits. Vol. 38. Nº 10. October 2003.

[2] C. Pascual, Z. Song, P.T. Krein, D.V. Sarwate. “High-

Return
Fidelity PWM Inverter for Audio Amplification
Based On Real-Time DSP”. IEEE Transactions on
Power Electronics. Vol. 18, pp. 473 485. January
Figure 4. Com mu n icat ion rout i ne bet we en t he 2003.
microcontroller and the computer, along with
the related handshaking. [3] J.A. de Lima, S.F. Silva, A.S. Cordeiro. M. Verleysen.
“A CMOS/SOI Single-input PWM Discriminator for
Low-voltage Body-implanted Applications”. VLSI
CONCLUSIONS Design. Volume 15. Nº 1, pp. 469-476(8). January
2002.
Once built, this device was debugged and tested. There http://www.dice.ucl.ac.be /~verleyse /papers /
are some outcomes which are interesting to comment vlsidesign02jdl.pdf
on.
[4] C. Bolton. “PLD code creates PWM generators”.
q Developing, error correcting and modifying EDN, pp 108.110. August 8, 2002.
source code is by far a real easy task, taking http://www.edn.com/contents/images/80802di.pdf
into account it is the embedded hardware that is
under a correction process. Analogous process [5] J. Mahoney. “Circuit converts pulse width to
with either active or passive components is voltage”. EDN, pp.92-94. October 25, 2001.
clearly a far difficult task to make. http://www.edn.com/contents/images/102501di.pdf

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 Pérez et al.: A demodulator of PWN signals generated for a digital accelerometer is developed using a microcontroller

APPENDIX A. SOURCE CODE EMBEDDED

INTO THE MICROCONTROLLER Rise_edge_Y nop
btfsc PORTA,1
GoTo Rise_edge_Y
The next assembler list shows the assembler source code
movf TMR0,0
either of the PWM demodulator and the transmitter,
movwf T1Y
which was written onto the PIC16F84 with a EEPROM
clrf TMR0
Writer.
Fall_edge_Y nop
btfss PORTA,1
ASM GoTo Fall_edge_Y
T1X equ 0x10 bsf PORTA,2
T2X equ 0x11 movf TMR0,0
TPX equ 0x12 movwf TPY
T1Y equ 0x13 movf TPY,0
T2Y equ 0x14 addwf T1Y,0
TPY equ 0x15 movwf T2Y
cnt equ 0x16 movf T1X,0
TTY equ 0x17 movwf TTY
TOCS equ 0x01 Call PIC_to_PC
bcf OPTION_REG, PSA movf T2X,0
bcf OPTION_REG, TOCS movwf TTY
bcf OPTION_REG, PS2 Call PIC_to_PC
bcf OPTION_REG, PS1 movf T1Y,0
bsf OPTION_REG, PS0 movwf TTY
bcf STATUS,RP0 Call PIC_to_PC
clr PORTA movf T2Y,0
clrf PORTB movwf TTY
movlw 0x0B Call PIC_to_PC
movwf TRISA bcf PORTA,2
movlw 0x00 GoTo Init_X
movwf TRISB PIC_to_PC
Init_X nop nop
bcf PORTA,2 LowSt nop
btfss PORTA,0 btfss PORTA,3
GoTo Init_X GoTo LowSt
clrf TMR0 HiSt nop
Rise_edge_X nop btfsc PORTA,3
btfsc PORTA,0 GoTo HiSt
GoTo Rise_edge_X movlw 0x08
movf TMR0,0 movwf cnt
movwf T1X Loop rlf TTY,0
clrf TMR0 btfss STATUS,C
Fall_edge_X nop bcf PORTB,0
btfss PORTA,0 btfsc STATUS,C
GoTo Fall_edge_X bsf PORTB,0
movf TMR0,0 decfsz cnt,1
movwf TPX GoTo Loop
movf TPX,0 nop
addwf T1X,0 Return
movwf T2X ENDASM
Init_Y nop
btfss PORTA,1
GoTo Init_Y
clrf TMR0

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