DS1904

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hazards such as dirt, moisture, and shock. Accessories permit the DS1904 to be mounted on almost any

surface including printed circuit boards and plastic key fobs. The DS1904 adds functions such as

calendar, time and date stamp, stopwatch, hour meter, interval timer, and logbook to any type of

electronic device or embedded application that uses a microcontroller.

OVERVIEW

The DS1904 has two main data components: 1) 64-bit lasered ROM, and 2) real-time clock counter

(Figure 1). The real-time clock utilizes an on-chip oscillator that is connected to a 32.768 kHz crystal.

The hierarchical structure of the 1-Wire protocol is shown in Figure 2. The bus master must first provide

one of four ROM function commands: 1) Read ROM, 2) Match ROM, 3) Search ROM, 4) Skip ROM.

The protocol for these ROM functions is described in Figure 7. After a ROM function command is

successfully executed, the real-time clock functions become accessible and the master may then provide

one of the real-time clock function commands. The protocol for these commands is described in Figure 5.

All data is read and written least significant bit first.

BLOCK DIAGRAM Figure 1

1 Hz

DIVIDER

DATA

ROM

CONTROL

FUNCTION

64-BIT

ROM

LASERED

CLOCK

FUNCTION

CONTROL

OSCILLATOR

CONTROL

READ/WRITE BUFFER

RTC COUNTER (32-BIT)

32.768 kHz

OSCILLATOR

LID

CONTACT

LITHIUM

3V

64-BIT LASERED ROM

Each DS1904 contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family

code. The next 48 bits are a unique serial number. The last eight bits are a CRC of the first 56 bits. (See

Figure 3.) 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

Dallas Semiconductor 1-Wire Cyclic Redundancy Check is available in the Book of DS19xx iButton

Standards. The shift register bits are initialized to zero. Then starting with the least significant bit of the

family code, one 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 eight bits of CRC should return the shift register to all zeros. The

64-bit ROM and ROM Function Control section allow the DS1904 to operate as a 1-Wire device and

follow the 1-Wire protocol detailed in the section "1-Wire Bus System".