Datasheet

DS18S20

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SKIP ROM [CCh]

The master can use this command to address all devices on the bus simultaneously without sending out

any ROM code information. For example, the master can make all DS18S20s on the bus perform

simultaneous temperature conversions by issuing a Skip ROM command followed by a Convert T [44h]

command.

Note that the Read Scratchpad [BEh] command can follow the Skip ROM command only if there is a

single slave device on the bus. In this case, time is saved by allowing the master to read from the slave

without sending the device’s 64-bit ROM code. A Skip ROM command followed by a Read Scratchpad

command will cause a data collision on the bus if there is more than one slave since multiple devices will

attempt to transmit data simultaneously.

ALARM SEARCH [ECh]

The operation of this command is identical to the operation of the Search ROM command except that

only slaves with a set alarm flag will respond. This command allows the master device to determine if

any DS18S20s experienced an alarm condition during the most recent temperature conversion. After

every Alarm Search cycle (i.e., Alarm Search command followed by data exchange), the bus master must

return to Step 1 (Initialization) in the transaction sequence. See the Operation—Alarm Signaling section

for an explanation of alarm flag operation.

DS18S20 FUNCTION COMMANDS

After the bus master has used a ROM command to address the DS18S20 with which it wishes to

communicate, the master can issue one of the DS18S20 function commands. These commands allow the

master to write to and read from the DS18S20’s scratchpad memory, initiate temperature conversions and

determine the power supply mode. The DS18S20 function commands, which are described below, are

summarized in Table 2 and illustrated by the flowchart in Figure 15.

CONVERT T [44h]

This command initiates a single temperature conversion. Following the conversion, the resulting thermal

data is stored in the 2-byte temperature register in the scratchpad memory and the DS18S20 returns to its

low-power idle state. If the device is being used in parasite power mode, within 10µs (max) after this

command is issued the master must enable a strong pullup on the 1-Wire bus for the duration of the

conversion (t

CONV

) as described in the Powering the DS18S20 section. If the DS18S20 is powered by an

external supply, the master can issue read-time slots after the Convert T command and the DS18S20 will

respond by transmitting 0 while the temperature conversion is in progress and 1 when the conversion is

done. In parasite power mode this notification technique cannot be used since the bus is pulled high by

the strong pullup during the conversion.

WRITE SCRATCHPAD [4Eh]

This command allows the master to write 2 bytes of data to the DS18S20’s scratchpad. The first byte is

written into the T

(byte 3 of the scratchpad). Data must be transmitted least significant bit first. Both bytes MUST be

written before the master issues a reset, or the data may be corrupted.

READ SCRATCHPAD [BEh]

This command allows the master to read the contents of the scratchpad. The data transfer starts with the

least significant bit of byte 0 and continues through the scratchpad until the 9th byte (byte 8 – CRC) is

read. The master may issue a reset to terminate reading at any time if only part of the scratchpad data is

needed.