DS18S20-PAR

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PARASITE POWER

The DS18S20-PAR’s parasite power circuit allows the DS18S20-PAR to operate without a local external

power supply. This ability is especially useful for applications that require remote temperature sensing or

that are very space constrained. Figure 1 shows the DS18S20-PAR’s parasite-power control circuitry,

which “steals” power from the 1-wire bus via the DQ pin when the bus is high. The stolen charge powers

the DS18S20-PAR while the bus is high, and some of the charge is stored on the parasite power capacitor

long as the specified timing and voltage requirements are met (refer to the DC ELECTRICAL

CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this data sheet).

However, when the DS18S20-PAR is performing temperature conversions or copying data from the

scratchpad memory to EEPROM, the operating current can be as high as 1.5 mA. This current can cause

an unacceptable voltage drop across the weak 1-wire pullup resistor and is more current than can be

provide a strong pullup on the 1-wire bus whenever temperature conversions are taking place or data is

being copied from the scratchpad to EEPROM. This can be accomplished by using a MOSFET to pull

the bus directly to the rail as shown in Figure 2. The 1-wire bus must be switched to the strong pullup

within 10

µs (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus

No other activity can take place on the 1-wire bus while the pullup is enabled.

SUPPLYING THE DS18S20-PAR DURING TEMPERATURE CONVERSIONS

Figure 2

OPERATION – MEASURING TEMPERATURE

The core functionality of the DS18S20-PAR is its direct-to-digital temperature sensor. The temperature

sensor output has 9-bit resolution, which corresponds to 0.5

°C steps. The DS18S20-PAR powers-up in a

low-power idle state; to initiate a temperature measurement and A-to-D conversion, the master must issue

a Convert T [44h] command. Following the conversion, the resulting thermal data is stored in the 2-byte

temperature register in the scratchpad memory and the DS18S20-PAR returns to its idle state. The

DS18S20-PAR output data is calibrated in degrees centigrade; for Fahrenheit applications, a lookup table

or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended two’s

complement number in the temperature register (see Figure 3).

The sign bits (S) indicate if the

temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. Table 2

gives examples of digital output data and the corresponding temperature reading.

Resolutions greater than 9 bits can be calculated using the data from the temperature, COUNT REMAIN

and COUNT PER °C registers in the scratchpad. Note that the COUNT PER °C register is hard-wired to

4.7K

1-Wire Bus

Micro-

processor

DS18S20-PAR

GND

DQ

To Other

1-Wire Devices