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HCS200 Datasheet(PDF) 8 Page - Microchip Technology |
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HCS200 Datasheet(HTML) 8 Page - Microchip Technology |
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8 / 16 page  HCS200 DS40138A-page 8 Preliminary © 1996 Microchip Technology Inc. 5.0 SPECIAL FEATURES 5.1 Code Word Completion Code word completion is an automatic feature that makes sure that the entire code word is transmitted, even if the button is released before the transmission is complete. The HCS200 encoder powers itself up when a button is pushed and powers itself down after the command is finished, if the user has already released the button. If the button is held down beyond the time for one transmission, then multiple transmissions will result. If another button is activated during a transmission, the active transmission will be aborted and the new code will be generated using the new button information. 5.2 Blank Alternate Code Word Federal Communications Commission (FCC) part 15 rules specify the limits on fundamental power and harmonics that can be transmitted. Power is calculated on the worst case average power transmitted in a 100 ms window. It is therefore advantageous to minimize the duty cycle of the transmitted word. This can be achieved by minimizing the duty cycle of the individual bits and by blanking out consecutive words. Blank Alternate Code Word (BACW) is used for reducing the average power of a transmission (Figure 5-1). This is a selectable feature that is determined in conjunction with the baud rate selection bit BSL0. Using the BACW allows the user to transmit a higher amplitude transmission if the transmission length is shorter. The FCC puts constraints on the aver- age power that can be transmitted by a device, and BACW effectively prevents continuous transmission by only allowing the transmission of every second word. This reduces the average power transmitted and hence, assists in FCC approval of a transmitter device. 5.3 Seed Transmission In order to increase the level of security in a system, it is possible for the receiver to implement what is known as a secure learn function. This can be done by utilizing the seed value on the HCS200 which is stored in EEPROM and can only be transmitted when all three button inputs are pressed at the same time (Table 5-1). Instead of the normal key generation method being used to create the encryption key, this seed value is used. 5.4 VLOW: Voltage LOW indicator The VLOW bit is transmitted with every transmission (Figure 4-2 and Figure 8-5) and will be transmitted as a zero if the operating voltage is above the voltage trip point. The VLOW signal is transmitted so the receiver can give an indication to the user that the transmitter battery is low. FIGURE 5-1: BLANK ALTERNATE CODE WORD (BACW) TABLE 5-1: PIN ACTIVATION TABLE External Internal S2 S1 S0 S3 S2 S1 S0 Standby after CC 0 0 00000 Hop 0 0 10001 0 1 00010 0 1 10011 1 0 01100 1 0 11110 1 1 01110 Fixed 1 1 11111 One Code Word BACW Disabled (All words transmitted) BACW Enabled (1 out of 2 transmitted) A 2A 100ms 100ms 100ms 100ms Amplitude |
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Similar Description - HCS200 |
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