18B20 查看數據表（PDF） - Maxim Integrated

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18B20

Programmable Resolution 1-Wire Digital Thermometer

Maxim Integrated 

DS18B20

The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search

[ECh] command. Any DS18B20s with a set alarm flag will respond to the command, so the master can

determine exactly which DS18B20s have experienced an alarm condition. If an alarm condition exists

and the TH or TL settings have changed, another temperature conversion should be done to validate the

alarm condition.

POWERING THE DS18B20

The DS18B20 can be powered by an external supply on the VDD pin, or it can operate in “parasite power”

mode, which allows the DS18B20 to function without a local external supply. Parasite power is very

useful for applications that require remote temperature sensing or that are very space constrained.

Figure 1 shows the DS18B20’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 DS18B20 while the bus is high,

and some of the charge is stored on the parasite power capacitor (CPP) to provide power when the bus is

low. When the DS18B20 is used in parasite power mode, the VDD pin must be connected to ground.

In parasite power mode, the 1-Wire bus and CPP can provide sufficient current to the DS18B20 for most

operations as long as the specified timing and voltage requirements are met (see the DC Electrical

Characteristics and AC Electrical Characteristics). However, when the DS18B20 is performing

temperature conversions or copying data from the scratchpad memory to EEPROM, the operating current

can be as high as 1.5mA. This current can cause an unacceptable voltage drop across the weak 1-Wire

pullup resistor and is more current than can be supplied by CPP. To assure that the DS18B20 has sufficient

supply current, it is necessary to 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 4. The 1-Wire

bus must be switched to the strong pullup within 10µs (max) after a Convert T [44h] or Copy Scratchpad

[48h] command is issued, and the bus must be held high by the pullup for the duration of the conversion

(tCONV) or data transfer (tWR = 10ms). No other activity can take place on the 1-Wire bus while the pullup

is enabled.

The DS18B20 can also be powered by the conventional method of connecting an external power supply

to the VDD pin, as shown in Figure 5. The advantage of this method is that the MOSFET pullup is not

required, and the 1-Wire bus is free to carry other traffic during the temperature conversion time.

The use of parasite power is not recommended for temperatures above +100°C since the DS18B20 may

not be able to sustain communications due to the higher leakage currents that can exist at these

temperatures. For applications in which such temperatures are likely, it is strongly recommended that the

DS18B20 be powered by an external power supply.

In some situations the bus master may not know whether the DS18B20s on the bus are parasite powered

or powered by external supplies. The master needs this information to determine if the strong bus pullup

should be used during temperature conversions. To get this information, the master can issue a Skip ROM

[CCh] command followed by a Read Power Supply [B4h] command followed by a “read time slot”.

During the read time slot, parasite powered DS18B20s will pull the bus low, and externally powered

DS18B20s will let the bus remain high. If the bus is pulled low, the master knows that it must supply the

strong pullup on the 1-Wire bus during temperature conversions.

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