DS2404 View Datasheet(PDF) - Dallas Semiconductor -> Maxim Integrated

Part Name

Description

Manufacturer

DS2404

EconoRAM Time Chip

Dallas Semiconductor -> Maxim Integrated 

DS2404

More information on how to arbitrate port access is found in section “Device Operation Modes” later in

this document.

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power

whenever the I/O, RST , or VCC pins are high. When using the 1-Wire port in battery operate mode,

RST and VCC provide no power since they are low. However, I/O 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 these pins, battery power is conserved and 2) the ROM may be read in absence of

normal power. For instance, in battery-operate mode, if the battery fails, the ROM may still be read

normally.

In battery-backed mode, if VCC fails, the port switches in the battery but inhibits communication. The

ROM may still be read normally over the 1-Wire port if RST is low.

64-BIT LASERED ROM

Each DS2404 contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family

code (DS2404 code is 04h). The next 48 bits are a unique serial number. The last eight 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 Dallas 1-

Wire Cyclic Redundancy Check is available in Application Note 27, “Understanding and Using Cyclic

Redundancy Checks with Dallas Semiconductor iButton Products”.

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.

64-BIT LASERED ROM Figure 2

CRC

SERIAL NUMBER

8-BITS

48-BIT UNQUE NUMBER

MSB

DS2404 FAMILY CODE

04h

LSB

1-WIRE CRC CODE Figure 3

4 of 28

020998

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