DS1821
DETAILED PIN DESCRIPTION
PIN
PIN
PIN
8-PIN
PR35 TO-220 SOIC SYMBOL
DESCRIPTION
1
2
2/TAB
3
2
1
GND
DQ
Ground.
Data input/output pin for 1-WireTM programming
operation; Thermostat output pin in normal operation.
VDD pin. +5V nominal.
3
1
8
VDD
DS1821S (8-pin SOIC): All pins not specified in this table are not to be connected.
OVERVIEW
The block diagram of Figure 1 shows the major components of the DS1821. The DS1821 has two
operating modes: 1-WireTM and thermostat.
The part arrives from the factory in 1-Wire mode. In this mode, the DQ pin of the DS1821 is configured
as a 1-Wire communication port which would be connected to a microprocessor. The microprocessor will
write data into the high and low temperature trigger registers, TH and TL, respectively, to set up the
temperature limits for thermostat operation. In this mode, the result of the last temperature measurement
made by the DS1821 may also be read directly by the microprocessor.
Once temperature limits have been set and thermometer operation has been verified, the user may convert
the DS1821 from a temperature sensor into a thermostat by writing to a bit in the status register. The part
will then be configured for thermostat operation; this will also become the default power-up state for the
device on the subsequent power up.
In thermostat mode, the DQ line becomes the thermostat output. This open drain output will go to its
active state (programmable on/off) when the temperature of the DS1821 goes above the limit set in the
TH register, and will remain active until the temperature goes below the limit programmed into the TL
register.
If the user wishes to establish communications with the DS1821 once it has been placed in thermostat
mode (for example, to change temperature trip point limits), this may be done by dropping V while
holding the DQ line high, then clocking the DQ line 16 times. The part will then be placed intoDD1-Wire
mode, and will allow the I/O functions of the device to operate, and reads from or writes to the memory
are possible. This does not change the power-up state of the device, unless the user writes the
configuration bit to do so.
2 of 16