HCS473 datasheet(13/68 Pages) MICROCHIP

13 / 68 page

 2002 Microchip Technology Inc.

Preliminary

DS40035C-page 11

HCS473

3.0

DEVICE OPERATION

HCS473 operation depends on how the device is acti-

vated. The device exits Low-power mode either when a

switch input is pulled high or when a signal is detected

on an LC antenna input pin. Once activated, the device

determines the source of the activation and enters

Encoder mode or Transponder mode.

A button input activation places the device into Encoder

mode. A signal detected on the transponder input

places the device into Transponder mode. Encoder

mode has priority over Transponder mode such that

communication on the transponder input would be

ignored or perhaps interrupted if it occurred simulta-

neously to a button activation; ignored until the button

input is released.

3.1

Encoder mode

3.1.1

ENCODER ACTIVATION

3.1.1.1

Button Activation

The main way to enter Encoder mode is when the

wake-up circuit detects a button input activation; button

input transition from GND to V

DD. The HCS473 control

logic wakes and delays a nominal switch debounce

time (T

DB) prior to sampling the button inputs. The but-

ton input states, cumulatively called the button status,

determine whether the HCS473 transmits a code hop-

ping or seed transmission.

The transmission begins a time T

PU after activation. It

consists of a stream of code words transmitted as long

as the switch input is held high or until a selectable

TSEL timeout occurs (see Section 3.1.4.16 for TSEL

options). A timeout returns the device to Low-power

mode, protecting the battery in case a button is stuck.

Additional button activations during a transmission will

immediately reset the HCS473, perhaps leaving the

current code word incomplete. The device will start a

new transmission which includes the updated button

status value.

Buttons removed during a transmission will have no

effect unless no buttons remain activated. If no button

activations remain, the minimum number of complete

code words will be completed (see Section 3.1.4.15 for

MTX options) and the device will return to Low Power

mode.

3.1.1.2

Proximity Activation

A second way to enter Encoder mode is if the proximity

activation option (PXMA) is enabled and the wake-up

circuit detects a wake-up sequence on an LC antenna

input pin. This form of activation is called Proximity

Activation as a code hopping transmission would be ini-

tiated when the device was proximate to a LF field.

3.1.2

TRANSMITTED CODE WORD

The HCS473 transmits a 69-bit code word in response

to a button activation or proximity activation, Figure 3-

1. The code word content varies with the two unique

transmission types; Hopping or Seed.

3.1.2.1

Hopping Code Word

Hopping code words are those transmitted during nor-

mal operation. Each Hopping code word contains a

preamble, header, 32 bits of encrypted data and up to

37 bits of fixed value data followed by a guard period

before another code word begins.

• The 32 bits of Encrypted Data include button sta-

tus bits, discrimination bits and the synchroniza-

tion counter value. The inclusion/omission of

overflow bits and size of both synchronization

counter and discrimination bit fields vary with the

CNTSEL option, Figure 3-2 and Section 3.1.4.5.

• The 37 bits of Fixed Code Data include queue

bits (if enabled), CRC bits, low voltage status and

serial number. The inclusion/omission of button

status and size of the serial number field vary with

the XSER option, Figure 3-2 and Section 3.1.4.3.

3.1.2.2

Seed Code Word

Seed code words are required when the system imple-

ments secure key generation. Seed transmissions are

activated when the button inputs match the value spec-

ified by the seed button code configuration option

(SDBT), Section 3.1.4.9.

Each Seed code word contains a preamble, header

and up to 69 bits of fixed data followed by a guard

period before another code word begins.

• The 69 bits of Fixed Code Data include queue

bits (if enabled), CRC bits, low voltage status, but-

ton status and the 60-bit seed value, Figure 3-2.

Note:

For additional information on K

EELOQ the-

ory and implementation, please refer to the

EELOQ

introductory

Technical

Brief

(TB003).

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