HCS473_13 datasheet(40/68 Pages) MICROCHIP

40 / 68 page

HCS473

DS40035D-page 40

Preliminary

 2000-2013 Microchip Technology Inc.

FIGURE 5-2:

TYPICAL DECODER

OPERATION

5.3

Synchronization with Decoder

The

technology

features

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.

FIGURE 5-3:

SYNCHRONIZATION

WINDOW (16-BIT

COUNTER)

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.

Transmission

Received

Does

Serial Number

Match

Decrypt Transmission

Decryption

Valid

Counter

Within 16

Counter

Within 32K

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

Yes

and

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

Blocked

Entire Window

rotates to eliminate

use of previously

used codes

Current

Position

(32K Codes)

Double

Operation

(32K Codes)

Single Operation

Window (16 Codes)

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