DS1921 datasheet(3/44 Pages) DALLAS

3 / 44 page

DS1921H/Z

3 of 44

DS1921H/Z BLOCK DIAGRAM Figure 1

Internal

Timekeeping &

Control Reg. &

Counters

3V Lithium

General-Purpose

SRAM

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

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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