MLX90109 100 modulation (On-Off Keying) DownloadLink 3890

Application Note
100% modulation (On-Off Keying)
1 Scope
This application note explains how to use the MLX90109 transceiver to obtain 100% modulation
of the magnetic field (On-Off keying modulation). The MLX90109 datasheet will help to
understand the basic operating principles and specifications before reading this document.
2 Related Melexis Products
MLX90109 and MLX90125 (dedicated antenna for MLX90109).
3 General Introduction
On-Off keying modulation consists of switching On and Off the current through the transceiver’s
antenna MLX90109. This can be achieved by switching the modulation pin MODU (pin 4)
between VModuMin and VDD.
According to the datasheet of MLX90109 transceiver, VModuMin is fixed to 0,8V to get maximum
reading performance.
Field Condition
ON
OFF
Voltage on Modu pin
0,8 Volts
VDD
Switching ON the magnetic field depends on the internal driver of MLX90109 and will take less
than 5 carrier periods as far as the “Auto Start Up Impedance” condition is respected.
Switching OFF the magnetic field will depend on the quality factor (Q) of the parallel antenna
connected to the transceiver. It will take about Q carrier periods in a free swing condition.
Depending on the antenna Q and the protocol used, this fall time can be a limitation that will
prevent the modulation depth from reaching 100%.
An additional transistor can be used to reduce the fall time of the field and hence insure
compatibility with On-Off keying fast protocol.
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Author: DRU
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Application Note
100% modulation (On-Off Keying)
3.1 Application Schematic
Resistors R1 and R2 are calculated to obtain 0,8Volts on MODU pin. High values are chosen to
reduce power consumption.
R1
39kOhm
R2
8,2kOhm
The antenna used in the description below has the following characteristics (MLX90125CZA-A)
L
73,7uH
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Author: DRU
Qant
24
Zant (125khz)
1,4kOhms
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Ctune (125khz)
22nF
Rev.001
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Application Note
100% modulation (On-Off Keying)
3.2 Pictures of 100% Modulation
The picture below shows general behaviour of MLX90109 during a 100% modulation pattern.
8.6V
9.8V
9.68V
5V
0.8V
The pictures below shows fall and rise time details.
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Author: DRU
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Application Note
100% modulation (On-Off Keying)
4 Reducing the switch-off time of the antenna
4.1 Description
Protocols that use On-Off keying modulation (E.g. HITAG protocol) need to shut down the
magnetic field pretty quickly, in order to minimize the time during which no energy is delivered to
the transponder (E.g. HITAG2 protocol uses field Off pulses of typically 48us).
The fall time with the MLX90109 mainly depends on the quality factor of the parallel antenna.
With the antenna MLX90125CZA-A, used for the measurements, the coil swing-out will take
approximately:
t shutdwon = Q ⋅
1
f carrier
= 24 ⋅ 8µs = 192µs .
The application circuit below shows how to reduce the fall time with a transistor and a schottky
diode in parallel of the MLX90109 antenna.
4.2 Fast decay circuit
Transistor T1 will be controlled with the inverted command of the microcontroller port used for
field management (100% modulation). When Off (5V on the grid), T1 is blocked and the additional
system is disconnected from the antenna. D1 avoids conduction of the parasitic diode of
transistor T1.
When On (0V on the grid), T1 is open. This will short the antenna for negative swings and
dissipate the remaining energy through transistor T1.
The fall time will hence be reduced.
D1
T1
VDD
VDD
MLx90125 CZA-A
Ctune
1
2
3
4
R1
8
7
6
5
MLx90109
R2
Microcontroller Port used for 100% modulation
R2 floating
R2 connected
=>
=>
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Power down (VMODU = VDD)
Vantmax (VMODU = 0,8V)
T1
BS250 (P-MOS)
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D1
BAT43 (schottky)
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May-2004
Application Note
100% modulation (On-Off Keying)
4.3 Scope pictures
Fall time without transistor T1 (200us).
Fall time with transistor T1 (20us).
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Author: DRU
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Application Note
100% modulation (On-Off Keying)
5 HITAG2 protocol emulation
5.1 Short description
Hitag2 is a protocol based on a reader talk first protocol. This means that the transponder will
only reply on request of the reader (START command).
Commands are sent to the transponder by switching On/Off the current through the reader
antenna (100% modulation or On/Off keying). Depending if the field is On/Off, these times will be
indicated with respectively low and high field.
All data sent by the reader will start with a low field condition. After a fixed low field time the field
is switched on again and the length of this high field period will make the distinction between ‘0’
and ‘1’ bit (see table below).
0
0
1
0
1
stop
Data
Reader Antenna
T[0]
Symbol
tlow
T[0]
T[1]
tstop
Description
Low field time
Logic ‘0’ pulse length
Logic ‘1’ pulse length
High field for stop condition
Remark
T0 = Carrier period time = 1 / 125khz
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Author: DRU
T[1]
Duration
4 to 10 T0
18 to 22 T0
26 to 32 T0
>36 T0
tlow
tstop
Application configuration
6 T0
22 T0
28 T0
>36 T0
= 8µs nominal
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100% modulation (On-Off Keying)
5.2 Communication
After switching On the magnetic field, the reader must wait tPowerUp before sending a START
command. The reader sends a START command “11000 + Stop condition” and waits for a reply
from the transponder.
After receiving a START command, the transponder will reply in Manchester 4kbaud with a
header of 5 bits to 1 followed by 32 memory data bits.
Field ON
waits T power Up
START command
Waits for response
Header
32 bits memory
5.3 Scope pictures
The picture below shows a START command sent by the reader.
1
Without fast decay
1
0
0
0
Stop
1
1
0
0
0
Stop
With fast decay
As can be seen above, the additional transistor T1 allows for a 100% modulation depth with a
good switch off time of the antenna.
The picture below shows the response of the transponder measured on the reader antenna after
START request.
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Application Note
100% modulation (On-Off Keying)
START request
Reply of the
transponder in
Manchester
4kBaud
Remark:
Some special attention must be paid to decode the data from the MLX90109, especially when the
transponder does not send continuously its identification number (e.g. reader talk first
applications). For that purpose, please refer to the application note MLX90109, Reading
Operation.
6 Conclusion
Using two inexpensive components, the MLX90109 is capable to properly manage custom
protocols, like HITAG2 that require the use of short period on / off keying.
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Author: DRU
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May-2004