HCS473_13 Datasheet, PDF (8/68 Pages) Microchip Technology – KEELOQ® 3-Axis Transcoder

HCS473
2.3.3 TRANSPONDER INTERFACE
The transponder interface on the HCS473 consists of
the following:
• The internal transponder circuitry has separate
power supply (VDDT) and ground (VSST) connec-
tions.
- The VDDT pin supplies power to the transpon-
der circuitry and also outputs a regulated volt-
age if the LCX antenna input is receiving a
strong signal; transponder is placed in a
strong LF field.
- The VSST pin supplies the ground reference
to the transponder circuitry and must be con-
nected to the VSS pin.
• LF input amplifier and envelope detector to detect
and shape the incoming low frequency excitation
signal.
• Three sensitive transponder inputs with over-volt-
age protection (LCX, LCY, LCZ).
• Incoming LF energy rectification and regulation on
the LCX input to supplement the supply voltage in
low-battery transponder instances.
• 10V zener input protection from excessive
antenna voltage resulting when proximate to very
strong magnetic fields.
• LCCOM pin used to bias the transponder reso-
nant circuits for best sensitivity.
• LF antenna clamping transistors for inductive
responses back to the transponder reader. The
antenna ends are shorted together, ‘clamped’,
dissipating the oscillatory energy. The reader
detects this as a momentary load on its excitation
antenna.
• Damping transistors to increase LF communica-
tion reliability when using high Q-factor LC anten-
nae.
The LCCOM pin functions to bias the LCX, LCY, and
LCZ AGC amplifier inputs. The amplifier gain control
sets the optimum level of amplification in respect to the
incoming signal strength. The signal then passes
through an envelope detector before interpretation in
the logic circuit.
A block diagram of the transponder circuit is shown in
Figure 2-8.
FIGURE 2-8:
HCS473 TRANSPONDER
CIRCUIT
Rectifier/
Regulator
VCCT
LCX
LCY
LCZ
LCCOM
Noise
Filter
Signal In
Damp/Clamp
Control
2.3.4 INTERNAL EEPROM
The HCS473 has an on-board non-volatile EEPROM
which is used to store:
• configuration options
- encryption keys
- serial number
- vehicle ID’s
- baud rates
- ... see Section 3.1.4 and Section 3.2.1
• 64 bits of user memory
• synchronization counter.
All options are programmable during production, but
many of the security related options are programmable
only during production and are further read protected.
The user area allows storage of general purpose infor-
mation and is accessible only through the transponder
communication path.
During every EEPROM write, the device ensures that
the internal programming voltage is at an acceptable
level prior to performing the EEPROM write.
DS40035D-page 8
Preliminary
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