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HCS473 Datasheet(PDF) 11 Page - Microchip Technology |
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HCS473 Datasheet(HTML) 11 Page - Microchip Technology |
11 / 68 page 2000-2013 Microchip Technology Inc. Preliminary DS40035D-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 simultane- ously 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 VDD. The HCS473 control logic wakes and delays a nominal switch debounce time (TDB) 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 TPU 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 KEELOQ the- ory and implementation, please refer to the KEELOQ introductory Technical Brief (TB003). |
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