DS18S20

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The initialization sequence required to begin any communication with the DS18S20 is shown in Figure

10. A reset pulse followed by a presence pulse indicates the DS18S20 is ready to send or receive data

given the correct ROM command and memory function command.

The bus master transmits (TX) a reset pulse (a low signal for a minimum of 480 ms). The bus master

then releases the line and goes into a receive mode (RX). The 1–Wire bus is pulled to a high state via the

5k pullup resistor. After detecting the rising edge on the DQ pin, the DS18S20 waits 15–60

µs and then

transmits the presence pulse (a low signal for 60–240

µs).

MEMORY COMMAND FUNCTIONS

The following command protocols are summarized in Table 2, and by the flowchart of Figure 9.

Write Scratchpad [4Eh]

This command writes to the scratchpad of the DS18S20, starting at the TH register. The next 2 bytes

written will be saved in scratchpad memory at address locations 2 and 3. Writing may be terminated at

any point by issuing a reset.

Read Scratchpad [BEh]

This command reads the contents of the scratchpad. Reading will commence at byte 0, and will continue

through the scratchpad until the ninth (byte 8, CRC) byte is read. If not all locations are to be read, the

master may issue a reset to terminate reading at any time.

Copy Scratchpad [48h]

This command copies the scratchpad into the E

bytes in nonvolatile memory. If the bus master issues read time slots following this command, the

DS18S20 will output 0 on the bus as long as it is busy copying the scratchpad to E

when the copy process is complete. If parasite-powered, the bus master has to enable a strong pullup for

at least 10 ms immediately after issuing this command. The DS18S20 EEPROM is rated for a minimum

of 50,000 writes and 10 years data retention at T = +55°C.

Convert T [44h]

This command begins a temperature conversion.

No further data is required.

The temperature

conversion will be performed and then the DS18S20 will remain idle. If the bus master issues read time

slots following this command, the DS18S20 will output 0 on the bus as long as it is busy making a

temperature conversion; it will return a 1 when the temperature conversion is complete. If parasite-

after issuing this command.

Recall E2 [B8h]

This command recalls the temperature trigger values stored in E

happens automatically upon power–up to the DS18S20 as well, so valid data is available in the scratchpad

as soon as the device has power applied. With every read data time slot issued after this command has

been sent, the device will output its temperature converter busy flag: 0=busy, 1=ready.

Read Power Supply [B4h]

With every read data time slot issued after this command has been sent to the DS18S20, the device will

signal its power mode: 0=parasite power, 1=external power supply provided.