DS1994 Просмотр технического описания (PDF) - Maxim Integrated
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DS1994 Datasheet PDF : 23 Pages
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Figure 1. DS1994 BLOCK DIAGRAM
1-WIRE
PORT
1-W
ROM
FUNCTION
CONTROL
64-BIT
LASERED
ROM
PARASITE-
POWERED
CIRCUITRY
DS1994
MEMORY
FUNCTION
CONTROL
256-BIT
SCRATCHPAD
3V LITHIUM
32.768 kHz
OSCILLATOR
SRAM
16 PAGES
oF 256-BITS
PAGES
TIMEKEEPING
FUNCTIONS
HOLDING REGISTERS
INTERNAL REGISTERS
& COUNTERS
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry steals power whenever
the data input is high. The data line provides sufficient power as long as the specified timing and voltage
requirements are met. The advantages of parasite power are two-fold: 1) by parasiting off this input,
battery power is not consumed for 1-Wire ROM function commands, and 2) if the battery is exhausted for
any reason, the ROM may still be read normally. The remaining circuitry of the DS1994 is solely
operated by battery energy.
64-BIT LASERED ROM
Each DS1994 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code.
The next 48 bits are a unique serial number. The last 8 bits are a CRC of the first 56 bits. (See Figure 2.)
The 1-Wire CRC is generated using a polynomial generator consisting of a shift register and XOR gates,
as shown in Figure 3. The polynomial is X8 + X5 + X4 + 1. Additional information about the 1-Wire
Cyclic Redundancy Check is available in Application Note 27: Understanding and Using Cyclic
Redundancy Checks with Maxim iButton Products. The shift register bits are initialized to zero. Then
starting with the least significant bit of the family code, 1 bit at a time is shifted in. After the 8th bit of the
family code has been entered, then the serial number is entered. After the 48th bit of the serial number has
been entered, the shift register contains the CRC value. Shifting in the 8 bits of CRC should return the
shift register to all zeros.
Figure 2. 64-BIT LASERED ROM
MSB
LSB
8-Bit CRC Code
48-Bit Serial Number
8-Bit Family Code (04h)
MSB
LSB MSB
LSB MSB
LSB
3 of 23
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