HCS473_13 Datasheet, PDF (40/68 Pages) Microchip Technology – KEELOQ® 3-Axis Transcoder

HCS473
FIGURE 5-2:
TYPICAL DECODER
OPERATION
Start
No _Transmission
Received
?
Yes
No
Does
Serial Number
Match
?
Yes
Decrypt Transmission
No
Is
Decryption
Valid
?
Yes
No
Is
Counter
Within 16
?
Execute
Yes ^Command
and
Update
Counter
No
Is
No
Counter
Within 32K
?
Yes
Save Counter
in Temp Location
5.3 Synchronization with Decoder
The technology features a sophisticated
synchronization technique (Figure 5-3) which does not
require the calculation and storage of future codes. If
the stored counter value for that particular transmitter
and the counter value that was just decrypted are
within a window of 16 codes, the counter is stored and
the command is executed. If the counter value was not
within the single operation window, but is within the
double operation window of 32K codes (when using a
16-bit counter), the transmitted synchronization value
is stored in temporary location and it goes back to wait-
ing for another transmission.
When the next valid transmission is received, it will
check the new value with the one in temporary storage.
If the two values are sequential, it is assumed that the
counter had just gotten out of the single operation ‘win-
dow’. Since it is now back in sync, the new synchroni-
zation value is stored and the command executed.
If a transmitter has somehow gotten out of the double
operation window, the transmitter will not work and
must be relearned. Since the entire window rotates
after each valid transmission, codes that have been
used are part of the ‘blocked’ (32K) codes and are no
longer valid. This eliminates the possibility of grabbing
a previous code and retransmitting to gain entry.
Note:
The synchronization method described in
this section is only a typical implementation
and because it is usually implemented in
firmware, it can be altered to fit the needs
of a particular system
FIGURE 5-3:
SYNCHRONIZATION
WINDOW (16-BIT
COUNTER)
Entire Window
rotates to eliminate
use of previously
used codes
Blocked
(32K Codes)
Current
Position
Double
Operation
(32K Codes)
Single Operation
Window (16 Codes)
5.4 Inductive Communication
Communication between a base station and a HCS473
transponder occurs via magnetic coupling between the
transponder coil and base station coil. The base station
coil forms part of a series RLC circuit. The base station
communicates to the transponder by switching the 125
kHz signal to the series RLC circuit on and off. Thus,
the base station magnetic field is switched on and off.
The transponder coil is connected in parallel with a res-
onating capacitor (125 kHz) and the HCS473.
When the transponder is brought into the base station
magnetic field, it magnetically couples with this field
and draws energy from it. This loading effect can be
observed as a decrease in voltage across the base sta-
tion resonating capacitor. The KEELOQ transponder
communicates to the base station by “shorting out” its
parallel LC circuit. This detunes the transponder and
removes the load, which is observed as an increase in
voltage across the base station resonating capacitor.
The base station capacitor voltage is the input to the
base station AM demodulator circuit. The demodulator
extracts the transponder data for further processing by
the base station software.
DS40035D-page 40
Preliminary
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