Rainbow Electronics DS1821 - Manual (Page 2)

DS1821
Page 2 of 17
DETAILED PIN DESCRIPTIONS Table 1
PR35 8-PIN
SOIC*
SYMBOL DESCRIPTION
1 2 GND
Ground pin.
2 1 DQ Open drain data input/output pin – 1-wire operation; Open drain
thermostat output pin –thermostat operation.
3 8 V
DD Power supply pin.
*All pins not specified in this table are “No Connect” pins.
OVERVIEW
Figure 1 shows a block diagram of the DS1821 and pin descriptions are given in Table 1. The DS1821
can operate as a standalone thermostat with user-programmable trip-points or as 8-bit temperature sensor
with a 1-wire digital interface. The open-drain DQ pin functions as the thermostat output for thermostat
operation and as the data I/O pin for 1-wire communications. The 1-wire interface provides user access to
the nonvolatile (EEPROM) thermostat trip-point registers (T
H
and T
L
), the status/configuration register,
and the temperature register.
When configured as standalone thermostat, temperature conversions start immediately at power-up. In
this mode, the DQ pin becomes active when the temperature of the DS1821 exceeds the limit
programmed into the T
H
register, and remains active until the temperature drops below the limit
programmed into the T
L
register.
The DS1821 uses Dallas’ exclusive 1-wire bus protocol that implements bus communication with one
control signal. This system is explained in detail in the 1-WIRE BUS SYSTEM section of this datasheet.
DS1821 BLOCK DIAGRAM Figure 1
TEMPERATURE SENSOR FUNCTIONALITY
The core functionality of the DS1821 is its proprietary direct-to-digital temperature sensor, which
provides 8-bit (1°C increment) centigrade temperature readings over the range of -55°C to +125°C.
A block diagram of the temperature measurement circuitry is shown in Figure 2. This circuit measures
the temperature by counting the number of clock cycles generated by an oscillator with a low temperature
coefficient (temp-co) during a gate period determined by a high temp-co oscillator. The low temp-co
V
DD
4.7K
DQ
GND
TH REGISTER
TL REGISTER
CONFIGURATION REGISTER
AND CONTROL LOGIC
TEMPERATURE SENSOR
1-WIRE
INTERFACE
AND
I/O CONTROL
V
DD
POWER
SUPPLY
SENSE
DIGITAL
COMPARATOR/
LOGIC
DS1821