DS1925 datasheet(30/46 Pages) MAXIM

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1-Wire Bus System

The 1-Wire bus is a system that has a single bus master

and one or more slaves. In all instances, the DS1925 is

a slave device. The bus master is typically a microcon-

troller. The discussion of this bus system is broken down

into three topics: hardware configuration, transaction

sequence, and 1-Wire signaling (signal types and timing).

The 1-Wire protocol defines bus transactions in terms of

the bus state during specific time slots that are initiated

on the falling edge of sync pulses from the bus master.

Hardware Configuration

The 1-Wire bus has only a single line by definition; it is

important that each device on the bus be able to drive

it at the appropriate time. To facilitate this, each device

attached to the 1-Wire bus must have open drain or three-

state outputs. The 1-Wire port of the DS1925 is open

drain with an internal circuit equivalent to that shown in

Figure 10.

A multidrop bus consists of a 1-Wire bus with multiple

slaves attached. At standard speed the 1-Wire bus has

a maximum data rate of 16.3kbps. The speed can be

boosted to 142kbps by activating the overdrive mode. The

DS1925 maximum data rate in standard speed mode is

15.4kbps and 125kbps in overdrive mode. The value of

the pullup resistor primarily depends on the network size

and load conditions. The DS1925 requires a pullup resis-

tor of maximum 2.2kΩ at any speed.

The idle state for the 1-Wire bus is high. If for any reason a

transaction needs to be suspended, the bus must be left in

the idle state if the transaction is to resume. If this does not

occur and the bus is left low for more than 16µs (overdrive

speed) or more than 120µs (standard speed), one or more

devices on the bus can be reset. With the DS1925 the

bus must be left low for no longer than 12µs at overdrive

to ensure that no DS1925 on the 1-Wire bus performs a

reset. The DS1925 communicates properly when used

in conjunction with a DS2480B or DS2490 1-Wire driver

and adapters that are based on these driver chips. During

designated periods the DS2480 or DS2490 strong pullup

feature must be enabled to provide sufficient power.

Table 26. Memory Access Conflicts and Remedy

Figure 10. Hardware Configuration

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 30

100Ω

MOSFET

DATA

DS1925 1-Wire PORT

DS9490

Rx = RECEIVE

Tx = TRANSMIT

COMMAND

INDICATION OF INTERFERENCE

REMEDY

XPC Commands

The data read changes to all FFh bytes or

all bytes received are FFh, including the

CRC at the end of the command flow.

Wait 20ms, 1-Wire reset, address the device, and repeat

the command.

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