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HCS300-ISN Datasheet(PDF) 7 Page - Microchip Technology |
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HCS300-ISN Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 20 page  © 2001 Microchip Technology Inc. DS21137F-page 7 HCS300 3.6 CONFIG (Configuration Word) The Configuration Word is a 16-bit word stored in EEPROM array that is used by the device to store information used during the encryption process, as well as the status of option configurations. The following sections further explain these bits. TABLE 3-2: CONFIGURATION WORD 3.6.1 DISCRIMINATION VALUE (DISC0 TO DISC9) The discrimination value aids the post-decryption check on the decoder end. It may be any value, but in a typical system it will be programmed as the 12 Least Significant bits of the serial number. Values other than this must be separately stored by the receiver when a transmitter is learned. The discrimination bits are part of the information that form the encrypted portion of the transmission (Figure 4-2). After the receiver has decrypted a transmission, the discrimination bits are checked against the receiver’s stored value to verify that the decryption process was valid. If the discrimi- nation value was programmed as the 12 LSb’s of the serial number then it may merely be compared to the respective bits of the received serial number; saving EEPROM space. 3.6.2 OVERFLOW BITS (OVR0, OVR1) The overflow bits are used to extend the number of possible synchronization values. The synchronization counter is 16 bits in length, yielding 65,536 values before the cycle repeats. Under typical use of 10 operations a day, this will provide nearly 18 years of use before a repeated value will be used. Should the system designer conclude that is not adequate, then the overflow bits can be utilized to extend the number of unique values. This can be done by programming OVR0 and OVR1 to 1s at the time of production. The encoder will automatically clear OVR0 the first time that the synchronization value wraps from 0xFFFF to 0x0000 and clear OVR1 the second time the counter wraps. Once cleared, OVR0 and OVR1 cannot be set again, thereby creating a permanent record of the counter overflow. This prevents fast cycling of 64K counter. If the decoder system is programmed to track the overflow bits, then the effective number of unique synchronization values can be extended to 196,608. 3.6.3 BAUD RATE SELECT BITS (BSL0, BSL1) BSL0 and BSL1 select the speed of transmission and the code word blanking. Table 3-3 shows how the bits are used to select the different baud rates and Section 5.7 provides detailed explanation in code word blanking. TABLE 3-3: BAUD RATE SELECT 3.6.4 LOW VOLTAGE TRIP POINT SELECT (VLOW SEL) The low voltage trip point select bit is used to tell the HCS300 what VDD level is being used. This information will be used by the device to determine when to send the voltage low signal to the receiver. When this bit is set to a one, the VDD level is assumed to be operating from a 5V or 6V VDD level. If the bit is set low, then the VDD level is assumed to be 3.0 volts. FIGURE 3-1: VLOW CHARACTERISTICS Bit Number Bit Description 0 Discrimination Bit 0 1 Discrimination Bit 1 2 Discrimination Bit 2 3 Discrimination Bit 3 4 Discrimination Bit 4 5 Discrimination Bit 5 6 Discrimination Bit 6 7 Discrimination Bit 7 8 Discrimination Bit 8 9 Discrimination Bit 9 10 Overflow Bit 0 (OVR0) 11 Overflow Bit 1 (OVR1) 12 Low Voltage Trip Point Select (VLOW SEL ) 13 Baud rate Select Bit 0 (BSL0) 14 Baud rate Select Bit 1 (BSL1) 15 Reserved, set to 0 BSL1 BSL0 Basic Pulse Element Code Words Transmitted 0 0 400 µsAll 0 1 200 µs 1 out of 2 1 0 100 µs 1 out of 2 1 1 100 µs 1 out of 4 -40 050 85 2.0 1.6 1.8 1.4 2.2 2.4 2.6 3.6 3.8 4.0 4.2 VLOW Temp (C) Volts (V) VLOW sel = 0 VLOW sel = 1 |
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