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DS1921 Datasheet(PDF) 3 Page - Dallas Semiconductor |
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DS1921 Datasheet(HTML) 3 Page - Dallas Semiconductor |
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3 / 44 page  DS1921H/Z 3 of 44 DS1921H/Z BLOCK DIAGRAM Figure 1 Internal Timekeeping & Control Reg. & Counters 3V Lithium General-Purpose SRAM Register Page Alarm Time Stamp and Duration Logging Memory Datalog Memory Histogram Memory 32.768kHz Oscillator Control Logic Temperature Sensor 256-Bit Scratchpad Memory Function Control ROM Function Control 64-Bit Lasered ROM Parasite Powered Circuitry 1-Wire Port IO PARASITE POWER The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power when- ever the IO input is high. IO will provide sufficient power as long as the specified timing and voltage re- quirements are met. The advantages of parasite power are two-fold: 1) by parasiting off this input, lithium is conserved, and 2) if the lithium is exhausted for any reason, the ROM may still be read normally. 64-BIT LASERED ROM Each DS1921 contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family code. The next 36 bits are a unique serial number. The next 12 bits, called temperature range code, allow distinguishing the DS1921H and DS1921Z from each other and from other DS1921 versions. The last eight bits are a CRC of the first 56 bits. See Figure 3 for details. The 1-Wire CRC is generated using a polynomial generator consisting of a shift register and XOR gates as shown in Figure 4. The polynomial is X 8 + X5 + X4 + 1. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is avail- able in Application Note 27 and in the Book of DS19xx iButton Standards. The shift register bits are initialized to 0. Then starting with the least significant bit of the family code, one bit at a time is shifted in. After the eighth bit of the family code has been entered, then the serial number followed by the temperature range code is entered. After the range code has been entered, the shift register contains the CRC value. Shifting in the eight bits of CRC returns the shift register to all 0s. |
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