Datasheet DS18B20 - 7

link to page 5 link to page 2 link to page 3 link to page 3 link to page 7 link to page 7 DS18B20 Programmable Resolution 1-Wire Digital Thermometer or equal to TL or higher than or equal to TH, an alarm con- by CPP. To assure that the DS18B20 has sufficient supply dition exists and an alarm flag is set inside the DS18B20. current, it is necessary to provide a strong pullup on the This flag is updated after every temperature measure- 1-Wire bus whenever temperature conversions are tak- ment; therefore, if the alarm condition goes away, the flag ing place or data is being copied from the scratchpad to will be turned off after the next temperature conversion. EEPROM. This can be accomplished by using a MOSFET The master device can check the alarm flag status of to pull the bus directly to the rail as shown in Figure 6. The all DS18B20s on the bus by issuing an Alarm Search 1-Wire bus must be switched to the strong pullup within [ECh] command. Any DS18B20s with a set alarm flag will 10µs (max) after a Convert T [44h] or Copy Scratchpad respond to the command, so the master can determine [48h] command is issued, and the bus must be held high exactly which DS18B20s have experienced an alarm by the pullup for the duration of the conversion (tCONV) condition. If an alarm condition exists and the T or data transfer (t H or TL WR = 10ms). No other activity can take settings have changed, another temperature conversion place on the 1-Wire bus while the pullup is enabled. should be done to validate the alarm condition. The DS18B20 can also be powered by the conventional method of connecting an external power supply to the
Powering the DS18B20
VDD pin, as shown in Figure 7. The advantage of this The DS18B20 can be powered by an external supply on method is that the MOSFET pullup is not required, and the VDD pin, or it can operate in “parasite power” mode, the 1-Wire bus is free to carry other traffic during the tem- which allows the DS18B20 to function without a local perature conversion time. external supply. Parasite power is very useful for applica- The use of parasite power is not recommended for tem- tions that require remote temperature sensing or that are peratures above +100°C since the DS18B20 may not be very space constrained. Figure 3 shows the DS18B20’s able to sustain communications due to the higher leak- parasite-power control circuitry, which “steals” power from age currents that can exist at these temperatures. For the 1-Wire bus via the DQ pin when the bus is high. The applications in which such temperatures are likely, it is stolen charge powers the DS18B20 while the bus is high, strongly recommended that the DS18B20 be powered by and some of the charge is stored on the parasite power an external power supply. capacitor (CPP) to provide power when the bus is low. When the DS18B20 is used in parasite power mode, the In some situations the bus master may not know whether V the DS18B20s on the bus are parasite powered or pow- DD pin must be connected to ground. ered by external supplies. The master needs this informa- In parasite power mode, the 1-Wire bus and CPP can pro- tion to determine if the strong bus pullup should be used vide sufficient current to the DS18B20 for most operations during temperature conversions. To get this information, as long as the specified timing and voltage requirements the master can issue a Skip ROM [CCh] command fol- are met (see the DC Electrical Characteristics and AC lowed by a Read Power Supply [B4h] command followed Electrical Characteristics). However, when the DS18B20 by a “read time slot”. During the read time slot, parasite is performing temperature conversions or copying data powered DS18B20s will pull the bus low, and externally from the scratchpad memory to EEPROM, the operating powered DS18B20s will let the bus remain high. If the current can be as high as 1.5mA. This current can cause bus is pulled low, the master knows that it must supply an unacceptable voltage drop across the weak 1-Wire the strong pullup on the 1-Wire bus during temperature pullup resistor and is more current than can be supplied conversions. VPU
DS18B20 DS18B20
V V PU V DD (EXTERNAL GND DQ V PU DD GND DQ VDD SUPPLY) µP µP 4.7kΩ 4.7kΩ 1-Wire BUS TO OTHER 1-Wire BUS TO OTHER 1-Wire DEVICES 1-Wire DEVICES Figure 6. Supplying the Parasite-Powered DS18B20 During Figure 7. Powering the DS18B20 with an External Supply Temperature Conversions www.maximintegrated.com Maxim Integrated │

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