DS1205S 데이터 시트보기 (PDF) - Dallas Semiconductor -> Maxim Integrated

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DS1205S

MultiKey Chip

Dallas Semiconductor -> Maxim Integrated 

RESET/PRESENCE DETECT SEQUENCE Figure 17

tSYC

RESET PULSE

DRIVEN LOW

BY HOST

IDLE

STATE

DS1205S

FIRST BIT OF 1–WIRE

PROTOCOL

PRESENCE DETECT

DRIVEN BY 1–WIRE

DEVICE

tRSTL

tPDH

tPDL

tRSTH

1-Wire I/O Commands - Once the bus master has de-

tected a presence, it can issue one of the four different

1-wire I/O commands. These commands deal with the

laser-etched ROM code which has the following format.

Type ID

8 bits

Unique Serial Number CRC

48 bits

8 bits

All 1-wire commands are eight bits long. A list of these

commands are as follows:

CCh Pass Thru Mode

This command saves time by allowing direct ac-

cess to the DS1205S without identifying it by

ROM ID number. This command can only be used

when there is a single slave on the bus. If more

than one device is present, there will be bus con-

tention.

33h Read ROM Data

This command allows the bus master to read the

DS1205S’s unique 48-bit ID number and CRC.

This command can only be used if there is a

single DS1205S on the bus. If more than one is

present, there will be bus contention.

55H Match ROM Data

This mode allows the bus master to single out a

specific DS1205S on a multidrop bus. The bus

master selects the specific slave by the ROM ID

number for the transaction. This command can be

used with a single or multiple device on the bus.

F0h Search ROM Data

When a system is initially brought up, the bus

master might not know the number or types of de-

vices on the bus. By invoking the Search ROM

Data command the bus master can, by process of

elimination, find the ID numbers of all the devices

on the bus. Once this is known, the bus master

can then go back and read the device type that

corresponds to each ID number.

The ROM search process is the repetition of a simple

3-step routine: read a bit, read the complement of the

same bit, then write the desired value of that bit. The bus

master performs this simple 3-step routine on each bit of

the ROM. After one complete pass, the bus master

knows the contents of the ROM in one device. The bus

is reset and the process is repeated again, selecting a

different set of bit values. The bus master controls the

search according to what values are written as select

bits.

The following example of the ROM search process as-

sumes four different devices are connected to the same

1-wire bus. The ROM data of the four devices is as

shown:

ROM1

ROM2

ROM3

ROM4

00110101...

10101010...

11110101...

00010001...

The search process is as follows:

1. The bus master begins by resetting all devices pres-

ent on the 1-wire bus. After this, the bus master will

attempt to read the family code, serial number, and

CRC value for the part.

2. The bus master will then issue the Search ROM

Data command on the 1-wire bus.

3. The bus master performs two reads and receives

two 0 bits. This indicates that both 1 bits and 0 bits

exist as the first bit of the devices on the bus.

021798 11/17

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