Datasheet

DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
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OVERVIEW
Figure 1 shows a block diagram of the DS1825, 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 (T
H
and T
L
), 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. It is also used for the hard-
wired address programmed by the AD0-AD3 pins. The T
H
, T
L
, and configuration registers are NV (EEPROM), so
they will retain data when the device is powered down.
The DS1825 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 through a 3-state or
open-drain port (the DQ pin in the case of the DS1825). 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 data sheet.
Another feature of the DS1825 is the ability to operate without an external power supply. Power is instead supplied
through the 1-Wire pullup resistor through the DQ pin when the bus is high. The high bus signal also charges an
internal capacitor (C
PP
), 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 DS1825 can also be powered
by an external supply on V
DD
.
Figure 1. DS1825 BLOCK DIAGRAM
Power
Supply
Sen se
64-Bit ROM
And
1-wire Port
S
C
R
A
T
C
P
A
D
16-bit Temp Reg
8-bit TH Register
8-bit TL Register
8-bit Config. Reg
8-bit CRC Gen
Memory
Control Logic
V DD
GND
DQ
V PULLUP
AD0-AD3
4.7k
Parasite
Power
Circuit
Cpp
Address Pin
Input Latch