2

Analog Devices

iButton Description

The DS1904 RTC iButton is a rugged real-time clock module that can be accessed with minimal

hardware. Data is transferred serially via the 1-Wire protocol, which requires only a single data lead and

a ground return. The DS1904 contains a unique 64-bit factory-lasered ROM and a real-time

clock/calendar implemented as a binary counter. The durable MicroCan package is highly resistant to

environmental 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 32.768kHz crystal. The

hierarchical structure of the 1-Wire protocol is shown in Figure 2. The bus controller 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 controller 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

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 X8 + X5 + X4 + 1. Additional information about the

Analog Devices 1-Wire Cyclic Redundancy Check is available in the Book of iButton® Standards. The

ROM FUNCTION

CONTROL

64-BIT LASERED

ROM

CLOCK

FUNCTION

CONTROL

OSCILLATOR

CONTROL

DS1904

3V LITHIUM

DATA

LID

CONTACT

32.768kHz

OSCILLATOR

READ/WRITE BUFFER

RTC COUNTER (32-BIT)

DIVIDER

1Hz