DS2890

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1-WIRE BUS SYSTEM

The 1-Wire bus is a system, which has a single bus master and one or more slaves. In all instances the

DS2890 is a slave device. The bus master is typically a microcontroller. 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. For a more

detailed protocol description, refer to Chapter 4 of the Book of DS19xx iButton Standards.

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 3-state outputs. The 1-Wire port of the DS2890 is open drain with an internal circuit equivalent

to that shown in Figure 9. A multi-drop bus consists of a 1-Wire bus with multiple slaves attached. At

regular speed the 1-Wire bus has a maximum data rate of 16.3 kbits per second. The speed can be boosted

to 142 kbits per second by activating the Overdrive Mode. For a discrete bus master interface as in

Figure 12, the 1-Wire bus requires a pull-up resistor with a minimum value of 2.2 k

Ω. Depending on

1-Wire communication speed, regular or overdrive, and bus load characteristics, the optimal pull-up

resistor value will be in the 1.5 k

Ω to 5 kΩ range. Figure 13 shows a DS2480B bus master configuration

with an interface to the host CPU serial port. Among many features, the DS2480B simplifies the 1-Wire

interface design, generates slew-rate controlled 1-Wire waveforms, and off-loads 1-Wire timing

generation overhead required in a discrete solution.

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 16 µs (Overdrive Speed) or more than 120 µs (regular speed), one or more devices on the

bus may be reset.

Figure 12. HARDWARE CONFIGURATION

RX

TX

Open Drain

Port Pin

5 µA

Typ.

DS2890 1-WIRE PORT

RX = RECEIVE

TX = TRANSMIT

BUS MASTER

DATA

RX

TX

MOSFET

100

Ω

See

Text

NOTE:

Depending on 1-Wire communication speed, regular or overdrive, and bus load characteristics, the

optimal pull-up resistor value will be in the 1.5 k

Ω to 5 kΩ range.