DS1821

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DS1821 FUNCTION COMMAND SET Table 4

Command

Description

Protocol

1-Wire Bus Activity

After Command is Issued

TEMPERATURE CONVERSION COMMANDS

Read

Temperature

Reads last converted temperature

value from temperature register.

AAh

Master receives 8-bit temperature

value from DS1821.

Start Convert T Initiates temperature conversions.

EEh

None

Stop Convert T Halts temperature conversions.

22h

None

THERMOSTAT and STATUS/CONFIGURATION COMMANDS

Write TH

01h

DS1821.

Write TL

02h

DS1821.

Read TH

A1h

DS1821.

Read TL

A2h

DS1821.

Write Status

Writes data to the

status/configuration register.

0Ch

Master transmits 8-bit

status/configuration value to DS1821.

Read Status

Reads data from the

status/configuration register.

ACh

Master receives 8-bit

status/configuration value from

DS1821.

HIGH-RESOLUTION COMMANDS

Read Counter

Reads data from the counter

register

A0h

Master receives 9-bit counter value

from DS1821.

Load Counter

Loads slope accumulator data into

the counter register

41h

None

1-Wire SIGNALING

The DS1821 uses a strict 1-Wire communication protocol to insure data integrity. Several signal types are

defined by this protocol: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. All of these

signals, with the exception of the presence pulse, are initiated by the bus master.

INITIALIZATION PROCEDURE: RESET AND PRESENCE PULSES

All communication with the DS1821 begins with an initialization sequence that consists of a reset pulse

from the master followed by a presence pulse from the DS1821. This is illustrated in Figure 7. When the

DS1821 sends the presence pulse in response to the reset, it is indicating to the master that it is on the bus

and ready to operate given an appropriate function command.

When the bus is released, the 5k pullup resistor pulls the 1-Wire bus high. When the DS1821 detects this

rising edge, it waits 15–60 µs and then transmits a presence pulse by pulling the 1-Wire bus low for 60–

240 µs.