DS2422 datasheet(9/48 Pages) DALLAS

9 / 48 page

DS2422

9 of 48

Figure 4. Hierarchical Structure for 1-Wire Protocol

1-Wire net

Other

Devices

BUS

Master

DS2422

Available

Commands:

Command

Level:

Data Field

Affected:

1-Wire ROM Function

Commands

DS2422-specific

Memory Function

Commands

Read ROM

Match ROM

Search ROM

Conditional Search ROM

Skip ROM

Resume

Overdrive Skip

Overdrive Match

64-bit ROM, RC-Flag

64-bit ROM, RC-Flag, Alarm Flags,

Search Conditions

RC-Flag

RC-Flag, OD-Flag

64-bit ROM, RC-Flag, OD-Flag

Write Scratchpad

Read Scratchpad

Copy Scratchpad w/PW

Read Memory w/PW

Read Memory w/PW &

w/CRC

Clear Memory w/PW

Forced Conversion

Start Mission w/PW

Stop Mission w/PW

256-bit Scratchpad, Flags

256-bit Scratchpad

512 byte Data Memory, Registers,

Flags, Passwords

Memory, Registers, Passwords

Mission Time Stamp, Mission Samples

Counter, Start Delay, Sample

Rate Register, Alarm Flags,

Passwords

Memory addresses 020C to 020Fh

Flags, Timestamp

Flags

PARASITE POWER

The block diagram (Figure 3) shows the parasite-powered circuitry. This circuitry “steals” power whenever the I/O

input is high. I/O provides 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 conserved; and 2) if the

battery is exhausted for any reason, the ROM may still be read.

64-BIT LASERED ROM

Each DS2422 contains a unique ROM code that is 64 bits long. The first 8 bits are a 1-Wire family code. The next

48 bits are a unique serial number. The last 8 bits are a CRC of the first 56 bits. See Figure 5 for details. The

1-Wire CRC is generated using a polynomial generator consisting of a shift register and XOR gates as shown in

Figure 6. The polynomial is X

8 + X5 + X4 + 1. Additional information about the Dallas 1-Wire CRC is available 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 8

th 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 8 bits of CRC returns the shift register to all 0s.

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