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DS1921H/Z BLOCK DIAGRAM Figure 1
DS1921H/Z
1-Wire
Port
IO
ROM
Function
Control
64-Bit
Lasered
ROM
Parasite
Powered
Circuitry
Memory
Function
Control
General-Purpose
SRAM
256-Bit
Scratchpad
32.768kHz
Oscillator
Temperature
Sensor
3V Lithium
Control
Logic
Internal
Timekeeping &
Control Reg. &
Counters
Register Page
Alarm Time Stamp
and Duration
Logging Memory
Histogram Memory
Datalog
Memory
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the IO input is high. IO will provide 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 DS1921 is
solely operated by battery energy. As a consequence, if the battery is exhausted, all memory data is lost
including the data of the last mission, and no new mission can be started. Application Note 5057:
OneWireViewer Tips and Tricks explains how to check the battery status.
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 + X 5 + X 4 + 1. Additional information about the Maxim 1-Wire Cyclic Redundancy Check is
available in Application Note 27.
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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