DS28EA00U+T&R【PDF 22/29 页】IC THERMOMETER 1-WIRE 8-USOP

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1-Wire Signaling

The DS28EA00 requires strict protocols to ensure data

integrity. The protocol consists of four types of signaling

on one line: reset sequence with reset pulse and pres-

ence pulse, write-zero, write-one, and read-data.

Except for the presence pulse, the bus master initiates

all falling edges. The DS28EA00 can communicate at

two different speeds, standard speed and overdrive

speed. If not explicitly set into the overdrive mode, the

DS28EA00 communicates at standard speed. While in

overdrive mode the fast timing applies to all waveforms.

To get from idle to active, the voltage on the 1-Wire line

needs to fall from V

PUP

below the threshold V

. To get

from active to idle, the voltage needs to rise from

ILMAX

past the threshold V

. The time it takes for the

voltage to make this rise is seen in Figure 12 as ? and

its duration depends on the pullup resistor (R

PUP

) used

and the capacitance of the 1-Wire network attached.

The voltage V

ILMAX

is relevant for the DS28EA00 when

determining a logical level, not triggering any events.

Figure 12 shows the initialization sequence required to

begin any communication with the DS28EA00. A reset

pulse followed by a presence pulse indicates the

DS28EA00 is ready to receive data, given the correct

ROM and control function command. If the bus master

uses slew-rate control on the falling edge, it must pull

down the line for t

RSTL

+ t

to compensate for the edge.

A t

RSTL

duration of 480祍 or longer exits the overdrive

mode, returning the device to standard speed. If the

DS28EA00 is in overdrive mode and t

RSTL

is no longer

than 80祍, the device remains in overdrive mode. If the

device is in overdrive mode and t

RSTL

is between 80祍

and 480祍, the device resets, but the communication

speed is undetermined.

After the bus master has released the line, it goes into

receive mode. Now the 1-Wire bus is pulled to V

PUP

through the pullup resistor, or in the case of a DS2482-

x00 or DS2480B driver, by active circuitry. When the

threshold V

is crossed, the DS28EA00 waits for t

PDH

and then transmits a presence pulse by pulling the line

low for t

PDL

. To detect a presence pulse, the master

must test the logical state of the 1-Wire line at t

MSP

The t

RSTH

window must be at least the sum of t

PDHMAX

PDLMAX

, and t

RECMIN

. Immediately after t

RSTH

expired, the DS28EA00 is ready for data communica-

tion. In a mixed population network, t

RSTH

should be

extended to minimum 480祍 at standard speed and

48祍 at overdrive speed to accommodate other 1-Wire

devices.

Read/Write Time Slots

Data communication with the DS28EA00 takes place in

time slots, which carry a single bit each. Write time slots

transport data from bus master to slave. Read time

slots transfer data from slave to master. Figure 13 illus-

trates the definitions of the write and read time slots.

All communication begins with the master pulling the

data line low. As the voltage on the 1-Wire line falls

below the threshold V

, the DS28EA00 starts its inter-

nal timing generator that determines when the data line

is sampled during a write time slot and how long data is

valid during a read time slot.

1-Wire Digital Thermometer with

Sequence Detect and PIO

DS28EA00

Maxim Integrated

RESISTOR

MASTER

DS28EA00

RSTL

PDL

RSTH

PDH

MASTER Tx "RESET PULSE"

MASTER Rx "PRESENCE PULSE"

PUP

IHMASTER

ILMAX

?/DIV>

REC

MSP

Figure 12. Initialization Procedure Reset and Presence Pulses

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