DS18B20X
DETAILED PIN DESCRIPTIONS Table 1
SYMBOL DESCRIPTION
GND
DQ
Ground.
Data Input/Output pin. Open-drain 1-wire interface pin. Also
provides power to the device when used in parasite power mode
(see “Parasite Power” section.)
VDD
Optional VDD pin. VDD must be grounded for operation in
parasite power mode.
OVERVIEW
Figure 1 shows a block diagram of the DS18B20X, and pin descriptions are given in Table 1. The 64-bit
ROM stores the device’s unique serial code. The scratchpad memory contains the 2-byte temperature
register that stores the digital output from the temperature sensor. In addition, the scratchpad provides
access to the 1-byte upper and lower alarm trigger registers (TH and TL), and the 1-byte configuration
register. The configuration register allows the user to set the resolution of the temperature-to-digital
conversion to 9, 10, 11, or 12 bits. The TH, TL and configuration registers are nonvolatile (EEPROM), so
they will retain data when the device is powered down.
The DS18B20X uses Dallas’ exclusive 1-wire bus protocol that implements bus communication using
one control signal. The control line requires a weak pullup resistor since all devices are linked to the bus
via a 3-state or open-drain port (the DQ pin in the case of the DS18B20X). In this bus system, the
microprocessor (the master device) identifies and addresses devices on the bus using each device’s unique
64-bit code. Because each device has a unique code, the number of devices that can be addressed on one
bus is virtually unlimited. The 1-wire bus protocol, including detailed explanations of the commands and
“time slots,” is covered in the 1-WIRE BUS SYSTEM section of this datasheet.
Another feature of the DS18B20X is the ability to operate without an external power supply. Power is
instead supplied through the 1-wire pullup resistor via the DQ pin when the bus is high. The high bus
signal also charges an internal capacitor (CPP), which then supplies power to the device when the bus is
low. This method of deriving power from the 1-wire bus is referred to as “parasite power.” As an
alternative, the DS18B20X may also be powered by an external supply on VDD.
DS18B20X BLOCK DIAGRAM Figure 1
VPU
PARASITE POWER
4.7K
MEMORY CONTROL
DS18B20X
CIRCUIT
LOGIC
DQ
TEMPERATURE SENSOR
64-BIT ROM
AND
1-wire PORT
INTERNAL VDD
ALARM HIGH TRIGGER (TH)
REGISTER (EEPROM)
GND
VDD
SCRATCHPAD
CPP
ALARM LOW TRIGGER (TL)
REGISTER (EEPROM)
CONFIGURATION REGISTER
(EEPROM)
POWER
SUPPLY
SENSE
8-BIT CRC GENERATOR
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