Wireless Gas Sensor

Application Note AN8000.11
XE8000 Wireless Gas Sensor
AN8000.11
Application Note
Wireless Gas Sensor
Rev 1 February 2006
www.semtech.com
1
Application Note AN8000.11
XE8000 Wireless Gas Sensor
Abstract
This application note describes a wireless gas sensor module. The application is based on an 8 bit micro controller
with a 16+10 bit ZoomingADC™ on chip and a 433MHz transceiver. The sensor is a semi-conducting metal oxide
layer gas sensor.
The system consists of two modules; one mobile station with the sensor and one base station for the
communication with a PC. As the mobile station is battery powered ultra low power consumption is required.
Therefore, the low power micro controller and the low power transceiver used in this application are perfectly
suitable.
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2
Application Note AN8000.11
XE8000 Wireless Gas Sensor
Table of contents
1
INTRODUCTION ......................................................................................................................................5
2
GENERAL DESCRIPTION.......................................................................................................................5
2.1 BLOCK DESCRIPTION .............................................................................................................................5
2.2 FUNCTIONAL DESCRIPTION .....................................................................................................................6
2.3 COMPONENTS DESCRIPTION ...................................................................................................................6
2.3.1
Gas Sensor ................................................................................................................................6
2.3.2
XE8805/05A micro-controller......................................................................................................6
2.3.3
XE1201A Transceiver (300 - 500 MHz) ......................................................................................6
3
SENSOR INTERFACE .............................................................................................................................7
3.1 MSGS3000 CHARACTERISTICS ..............................................................................................................7
3.1.1
Sensor Supply Voltages .............................................................................................................8
3.2 XE8805/05A DAC CHARACTERISTICS ....................................................................................................9
3.2.1
Implementation of the DAC_bias function..................................................................................9
3.3 XE8805/05A ZOOMINGADCTM CHARACTERISTICS .................................................................................10
3.3.1
Configuration of the ZoomingADCTM for Gas Sensor Application .............................................10
3.4 HARDWARE INTERFACE ........................................................................................................................12
3.5 SOFTWARE DESCRIPTION .....................................................................................................................13
3.5.1
Mobile sensor system...............................................................................................................13
3.5.2
Base System Flowchart ............................................................................................................14
3.6 RF TRANSCEIVER INTERFACE ...............................................................................................................15
3.7 PROTOCOL DESCRIPTION .....................................................................................................................15
3.8 HARDWARE INTERFACE ........................................................................................................................16
3.8.1
XE1201A - XE8805/05A interface ............................................................................................16
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
Table of Figures
FIGURE 1: GLOBAL DESCRIPTION ................................................................................................5
FIGURE 2: MSGS3000 SCHEMATIC .............................................................................................7
FIGURE 3 BELOW SHOWS THE SENSOR TRANSFER FUNCTION FOR ALCOHOL. ..................................7
FIGURE 3: MSGS3000 TRANSFER FUNCTION FOR ALCOHOL ........................................................7
FIGURE 4: VH AND VS VOLTAGE PATTERN ...................................................................................8
FIGURE 5: GENERAL BLOCK DIAGRAM .........................................................................................9
FIGURE 7: ZOOMINGADCTM BLOCK DIAGRAM .............................................................................10
FIGURE 8: ZOOMINGADCTM DECIMAL OUTPUT VERSUS VOLTAGE INPUT ........................................11
FIGURE 9: HARDWARE INTERFACE BETWEEN XE88LC05 AND MSGS3000 GAS SENSOR .............12
FIGURE 10: FLOWCHARTS OF MOBILE SENSOR APPLICATION ........................................................13
FIGURE 11: FLOWCHART FOR BASE SYSTEM APPLICATION ...........................................................14
FIGURE 12: RF FRAME CONTENTS ............................................................................................15
FIGURE 13: XE1201A PIN OUT AND DESCRIPTION ......................................................................16
FIGURE 14: XE1201A – XE88LC05 INTERFACE .......................................................................17
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4
Application Note AN8000.11
XE8000 Wireless Gas Sensor
1 Introduction
The purpose of this application note is to explain the different steps needed, to build a wireless gas sensor based
on XE8805/05A capabilities. This application note demonstrates XE8805/05A performances as both sensing
machine and RF communication driver. To implement the gas sensor function, we used a semi-conductor gas
sensor. For the RF communication the XE1201A (UHF) radio transceiver is used.
The system can be built with two XE8805/05A ProStart modules and two XE1201A RF modules from Semtech and
the MSG3000 gas sensor from Microsens.
2 General Description
2.1
Block Description
Figure 1 shows a wireless gas sensing machine. This system is composed of two different parts:
-
The mobile station which includes the gas sensor
The base station which is connected to a PC
The mobile system includes the MSGS3000 gas sensor, a XE1201A transceiver and a XE8805/05A microcontroller.
The base system includes a transceiver XE1201A, a XE88LC05 micro-controller and a PC.
GAS SENSOR
PB0
ANTENNA
2
DAB
XE1201
ZoomingADC
MSGS3000
8
XE88LC05
MOBILE SYSTEM
XE88LC05
ANTENNA
2
RS 232
XE1201
UART Tx
UART Rx
8
PC station
BASE SYSTEM
Figure 1: Global Description
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
2.2
Functional Description
The mobile system micro-controller activates the gas sensor every five seconds. After the gas sensor has been
stimulated, it measures the relevant gas quantities. As soon as the measurement is finished, the value is
captured by the micro-controller’s ZoomingADC™. Finally, the data is sent to the base system via the XE1201A
transceiver.
The RF unit at the base system is waiting for a matching data frame. The arriving data will then be send to the
PC com port via a UART connection. The PC application calculates and displays the data using the following
equation:
Signal = Offset + Gain * Code + Gain2 * (Code) 2
Code is the digital value coming from the ZoomingADC. The user can customise the Offset, Gain and Gain2
parameters on the PC.
2.3
Components description
2.3.1
Gas Sensor
A semi-conducting metal oxide layer characterises the low power MSG3000 sensor. The measurements of
specific oxidising or reducing gases are based on reversible conductivity change of the sensing element at
an appropriate working temperature.
For more information about this product, consult http://www.microsens.ch , the microsens web site.
2.3.2
XE8805/05A micro-controller
The XE8805/05A is an ultra low-power micro-controller unit, associated with a versatile analog-to-digital
converter including programmable offset and gain pre-amplifier. This acquisition chain, the ZoomingADCTM
also includes an analog multiplexer (AMUX) allowing selection of four differential inputs channels or seven
common referenced signal paths.
As the XE88LC05 has several sources of interrupts and events, it can directly read the XE1201A data
output and synchronised clock.
2.3.3
XE1201A Transceiver (300 - 500 MHz)
The XE1201A is a half-duplex FSK transceiver for operation in the 433 MHz ISM band (optimised) and in
the 300-500 MHz band. The range of the XE1201A is between 200 and 300 meters. The modulation used
is the Continuous Phase, 2 level Frequency Shift Keying (CPFSK). The direct conversion (zero-IF) receiver
architecture enables on-chip channel filtering.
The XE1201A includes a bit synchroniser so that glitch free data with synchronised clock can directly be
read by a low cost / low complexity micro-controller. The transmitted power level can also be controlled via
the bus.
For more information on XE8000 micro-controller series and XE1200 RF transceiver series, please consult
the Semtech website.
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3 Sensor Interface
3.1
MSGS3000 characteristics
The MSGS3000 contains two main parts. One part is the sensitive element, which consists of a semi-conducting
metal oxide layer. The other part is an integrated heater where the thin sensitive metal oxide layer is deposited.
The gas selectivity of the sensor is determined by superficial or bulk doping of the semi-conducting metal oxide
with metal catalysts. Different types of gases can be detected by adjusting the working temperature of the
sensor.
Figure 2 below shows the schematic of the gas sensor, with the heating and sensing parts.
VH
Heating
Sensing
VS
Figure 2: MSGS3000 schematic
Figure 3 below shows the Sensor transfer function for Alcohol.
Alcohol: 0.4mg/l Air at 34°C
Saturated with Water
3
2
Air at 34°C
Saturated with
Water
1
0
0
10
20
30
40
50
60
Figure 3: MSGS3000 Transfer Function for Alcohol
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.1.1
Sensor Supply Voltages
An appropriate working temperature has to be achieved in order to measure specific oxidising or reducing
gases. This particular working temperature is achieved with the voltage VH connected to the heater of the
sensor. Figure 4 below, shows the pattern of a specific heating voltage VH to measure the alcohol
concentration in air.
To create this voltage pattern the DAC function of the XE8805/05A is used. The power supply voltage VS
for the sensor is generated by the digital output pin PB0. See figure 4 below.
VH[V]
DAC Bias
2.10
0.68
300
100
t[ms]
VS[V]
3.0
PortB0
450
t[ms]
Figure 4: VH and VS voltage pattern
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.2
XE8805/05A DAC characteristics
The XE8805/05A has two digital to analog converters (DAC)s: a signal DAC able to pass a 4KHz signal with
10bits precision, and a bias DAC, able to output 10mA to bias a resistive bridge sensor. Both are DACs
formed from a generic DAC and an amplifier. This makes current and voltage drive possible and gives the
user freedom to choose the preferred filtering scheme. For more details on the DAC performances, consult the
TN8000.03 on the Semtech website.
3.2.1
Implementation of the DAC_bias function
The DAC peripheral is used together with the onboard amplifier in order to deliver the current required by
the integrated heater of the MSG3000.
The figure below shows the block diagram of the peripheral. It consists of a control block that manages all
communication with the CPU, sets the configuration of the peripheral and implements the different test
modes. The DAC converts the digital data in an analog output signal with an amplifier that can output up to
10mA.
C
O
N
T
R
O
L
DAB_R_P
DAB_OUT
D
DAB_R_M
A
DAB_A1_P
DAB_AO_P
+ Amp
DAB_AO_M
XE8805/05A
DAB_A1_M
Figure 5: General block diagram
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
XE8805/05A ZoomingADCTM characteristics
3.3
XE8805/05A ZoomingADCTM is appropriate for this type of measurement, thanks to its offset compensation
capabilities.
See below Figure 5 that shows XE88LC05 ZoomingADCTM block diagram.
Figure 7: ZoomingADCTM block diagram
In our case, the output of the sensor is connected to the first (AC_A0–AC_A1) input channel of ZoomingADCTM and
all PGAs are enabled. The AD converter is used to convert the differential input signal into a 16 bits 2’s
complement output format. For more details on ZoomingADCTM performances, see AN8000.05 on the Semtech
website.
3.3.1
Configuration of the ZoomingADCTM for Gas Sensor Application
The following parameters have to be known in order to be able to calculate the configuration of the
acquisition chain.
•
Power supply of sensor
Vsupply = 3 V
•
Reference voltage of ZoomingADC
Vref = 3V
•
Full scale span of sensor output
Vout = 0.7V
Calculation of the necessary gain to cover ADC full scale:
1.5V
Vref/2
Gain = | Vout (max) | =
≅ 4.29
0.35V
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
Calculation of the offset compensation
ZoomingADC output code
(decimal)
The output of the sensor is 0V if the air is not saturated with alcohol and 0.7V when the air is saturated with alcohol.
In order to obtain the full scale output range of the ZoomingADC™ an offset has to be applied so that an input
voltage of 0V equals to an output code of –32768 and an input voltage of 0.7V equals to an output of 32767.
40000
30000
20000
10000
0
-10000 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-20000
-30000
-40000
Sensor output voltage (V)
Figure 8: ZoomingADCTM decimal output versus voltage input
The graphic above is shifted to Vout/2 = 0.35V. Therefore, an offset has to be calculated to remove this shift.
VinADC = GD1 * GD 2 * GD3 * Vin − (Goff 3 + Goff 2 * GD3) * Vref
This formula gives the voltage at the input of the ADC. The offset is given at the input of the acquisition chain. The
gain has to be multiplied by the input voltage. Therefore VinADC = 0V because the offset has to be corrected when
Vin equals 0.35V
Vin * Gain = (Goff 3 + Goff 2 * GD3) * Vref
This formula equals
Vin * Gain = Goff 3 * Vref
with Goff2 = 0 since the offset of the PGA2 is not enabled.
Goff 3 =
Vin * Gain 0.35V * 4.29
≅
≅ 0.5
Vref
3V
Goff 3 = 0.5 ⇒ Goff 3 =
PGA3 _ off (5 : 0)
12
⇒ PGA3 _ off = 6 = 0 x06 H
0x06H is the value which has to be written into the “RegACCfg4” register
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.4
Hardware Interface
Figure 4 below shows the hardware interface between the MSGS3000 gas sensor and the XE8805/05A microcontroller.
The sensor voltage Vs is generated by the digital output PB0. To fulfil the current requirements of the sensors
heating element, the DAC output DAC DAB_OUT has to be connected to the amplifier input DAB_A1_P. The
amplified DACoutput DAB_A0_M is connected directly to the sensors heating element. The gas sensor output
voltage VO is captured by the ZoomingADC™ input AC_A1.
XE8805/05A
680kΩ
VSS
3 3nF
VDDA_1
AC_R0
AC_R1
3
AC_A0
4
MSGS3000
V0
AC_A1
2
VS
PB0
1
VH
DAB_OUT
DAB_A1_P
DAB_A0_P
1nF-10nF
DAB_A0_M
DAB_A1_M
DAB_R_P
DAB_R_M
Figure 9: Hardware Interface between XE8805/05A and MSGS3000 Gas Sensor
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.5
Software description
3.5.1
Mobile sensor system
Condition 1 : SendData = TRUE
Initialisations
Condition 2 : Measure = TRUE
Condition 3 : Counter = 5
Condition
1
Yes
No
Transceiver activation
transmission
Condition
2
Yes
No
Gas sensor
SendData
and data = FALSE
Measure = FALSE
aquisition
SendData = TRUE
Measure = FALSE
HALT
1 Hz
IRQ
Counter++
Condition
3
Yes
No
SendData = FALSE
Measure = TRUE
SendData = FALSE
Measure = FALSE
RETI
Figure 10: Flowcharts of mobile sensor application
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.5.2
Base System Flowchart
Condition 1 : ReceiveData = TRUE
Sensing Machine 2
Software
Microcontroller
Initialisation
Condition
1
Yes
No
Put received Data on
UART
Figure 11: Flowchart for base system application
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.6
RF Transceiver Interface
The RF link is achieved with the Semtech RF transceiver XE1201A. The programming interface between the
micro controller and the XE1201A is established via a 3-wire bus. In our case, XE1201A (300–500 MHz band)
is used to handle RF link depending on the frequency and on the range one wants to use for RF
communication.
3.7
Protocol Description
Figure 12 below describes RF frame contents.
Preamble
(16 to 32 bits)
Start word
DATA
(8 bits)
(16 bits)
bits(15:8)
Stop word
(8 bits)
bits(7:0)
Figure 12: RF Frame contents
Preamble is a sequence of “0” and “1” use to synchronise data and clock at transceiver output.
Start word defines the beginning of RF transmission.
Data 16 bit value from the ZoomingADC™
Stop word defines the end of RF transmission.
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
3.8
Hardware Interface
3.8.1 XE1201A - XE8805/05A interface
Figure 13 below shows XE1201A pins characteristics.
Figure 13: XE1201A pin out and description
The XE1201A has separated pins for received data (RXD) and transmitted data (TXD) and a chip enable pin (EN).
RXD is connected to PA[0] to wake up the micro-controller when data is received (event generation).
CLKD is connected to PA[1] for event generation on each rising edge, of the received clock.
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
Figure 14 below shows XE1201A – XE8805/05A interface.
RF TL
VDD B
TL
A
EN
RF RF RF
OU GN B
T
D
RF
A
Q0
DE
DVDD
AVDD
TPA
TPB
XTAL
XE1201A
AGND
SC
LO
SD GN TK
D
A
IO
TK
B
TK
C
S
W
A
S
W
B
RX
TX
XTAL
PC[5]
DGND
PC[2]
RXD
PA[0]
CLKD
PA[1]
VRTXD
EF
PC[7]
XE8805/05A
PC[6]
PC[0]
PC[1]
For more visibility, the power supply is not illustrated
Figure 14: XE1201A – XE8805/05A interface
Note:
RF transceivers need specific design rules. That’s why in this application, we used the RF module XE1201
which is an existing product. These modules respect some RF design rules in order to ensure RF
transceiver expected behaviour.
You can find a XE1201A reference design on the Semtech web site:
http://www.semtech.com
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Application Note AN8000.11
XE8000 Wireless Gas Sensor
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Contact Information
Semtech Corporation
Wireless and Sensing Products Division
200 Flynn Road, Camarillo, CA 93012
Phone (805) 498-2111 Fax : (805) 498-3804
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