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HCS301-IP Datasheet(PDF) 7 Page - Microchip Technology |
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HCS301-IP Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 20 page  © 1996 Microchip Technology Inc. Preliminary DS21143A-page 7 HCS301 3.6.5 LOW VOLTAGE TRIP POINT SELECT The low voltage trip point select bit is used to tell the HCS301 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 9.0 volt or 12.0 volt VDD level. If the bit is set low, then the VDD level is assumed to be 6.0 volts. Refer to Figure 3-1 for voltage trip point. The LED current limiting resistor value also affects the LED current: VLOW is tested at 3.5V and 13.0V. FIGURE 3-1: TYPICAL VOLTAGE TRIP POINTS (BY CHARACTERIZATION) •VLOW = 1 (high limiting). •VLOW = 0 (low limiting). -40 20 40 100 8.5 7.5 8.0 7.0 9.0 2.5 3.0 3.5 4.0 VLOW Temp (C) Volts (V) VLOW sel = 1 VLOW sel = 0 4.5 -20 0 60 80 5.0 5.5 Max Min Max Min 4.0 TRANSMITTED WORD 4.1 Transmission Format The HCS301 transmission is made up of several parts (Figure 4-1). Each transmission is begun with a preamble and a header, followed by the encrypted and then the fixed data. The actual data is 66 bits which consists of 32 bits of encrypted data and 34 bits of fixed data. Each transmission is followed by a guard period before another transmission can begin. Refer to Table 8-4 for transmission timing requirements. The encrypted portion provides up to four billion changing code combinations and includes the button status bits (based on which buttons were activated) along with the synchronization counter value and some discrimination bits. The fixed portion is comprised of the status bits, the function bits and the 28-bit serial number. The fixed and encrypted sections combined increase the number of combinations to 7.38 x 1019. 4.2 Synchronous Transmission Mode Synchronous transmission mode can be used to clock the code word out using an external clock. To enter synchronous transmission mode, the program- ming mode start-up sequence must be executed as shown in Figure 4-3. If either S1 or S0 is set on the fall- ing edge of S2 (or S3), the device enters synchronous transmission mode. In this mode, it functions as a nor- mal transmitter, with the exception that the timing of the PWM data string is controlled externally and that 16 extra bits are transmitted at the end with he code word. The button code will be the S0, S1 value at the falling edge S2 or S3. The timing of the PWM data string is controlled by supplying a clock on S2 or S3 and should not exceed 20 KHz. The code word is the same as in PWM mode with 16 reserved bits at the end of the word. The reserved bits can be ignored. When in syn- chronous transmission mode S2 or S3 should not be toggled until all internal processing has been com- pleted as shown in Figure 4-4. 4.3 Code Word Organization The HCS301 transmits a 66-bit code word when a button is pressed. The 66-bit word is constructed from a Fixed Code portion and an Encrypted Code portion (Figure 4-2). The Encrypted Data is generated from 4 button bits, 2 overflow counter bits, 10 discrimination bits and the 16-bit sync value (Figure 8-5). The Fixed Code Data is made up from two status bits, four button bits and the 28-bit serial number. The four button bits and the 28-bit serial number may be encrypted with the Envelope Key if the envelope encryption is enabled by the user. |
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