DS1822+ Maxim Integrated Products, DS1822+ Datasheet - Page 8

• Manufacturer Part Number: DS1822+
• Description: Board Mount Temperature Sensors IC THERMOMETER ECONO DIG
• Manufacturer: Maxim Integrated Products

Specifications of DS1822+

• Temperature Threshold: Programmable
• Full Temp Accuracy: +/- 2 C
• Package / Case: TO-92
• Digital Output - Bus Interface: 1-Wire
• Digital Output - Number Of Bits: 9 bit to 12 bit
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 3 V
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 55 C
• Output Type: Digital
• Supply Current: 1.5 mA
• Ic Output Type: Digital
• Sensing Accuracy Range: ± 2°C
• Supply Voltage Range: 3V To 5.5V
• Resolution (bits): 12bit
• Sensor Case Style: TO-92
• No. Of Pins: 3
• Accuracy %: 2°C
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

CONFIGURATION REGISTER
Byte 4 of the scratchpad memory contains the configuration register, which is organized as
illustrated in Figure 8. The user can set the conversion resolution of the DS1822 using the R0 and R1
bits in this register as shown in Table 3. The power-up default of these bits is R0 = 1 and R1 = 1 (12-bit
resolution). Note that there is a direct tradeoff between resolution and conversion time. Bit 7 and bits 0–4
in the configuration register are reserved for internal use by the device and cannot be overwritten.
CONFIGURATION REGISTER Figure 8
THERMOMETER RESOLUTION CONFIGURATION Table 3
CRC GENERATION
CRC bytes are provided as part of the DS1822’s 64-bit ROM code and in the 9
memory. The ROM code CRC is calculated from the first 56 bits of the ROM code and is contained in the
most significant byte of the ROM. The scratchpad CRC is calculated from the data stored in the
scratchpad, and therefore it changes when the data in the scratchpad changes. The CRCs provide the bus
master with a method of data validation when data is read from the DS1822. To verify that data has been
read correctly, the bus master must recalculate the CRC from the received data and then compare this
value to either the ROM code CRC (for ROM reads) or to the scratchpad CRC (for scratchpad reads). If
the calculated CRC matches the read CRC, the data has been received error free. The comparison of CRC
values and the decision to continue with an operation are determined entirely by the bus master. There is
no circuitry inside the DS1822 that prevents a command sequence from proceeding if the DS1822 CRC
(ROM or scratchpad) does not match the value generated by the bus master.
The equivalent polynomial function of the CRC (ROM or scratchpad) is:
The bus master can recalculate the CRC and compare it to the CRC values from the DS1822 using the
polynomial generator shown in Figure 9. This circuit consists of a shift register and XOR gates, and the
shift register bits are initialized to 0. Starting with the least significant bit of the ROM code or the least
significant bit of byte 0 in the scratchpad, one bit at a time should shifted into the shift register. After
shifting in the 56
polynomial generator will contain the recalculated CRC. Next, the 8-bit ROM code or scratchpad CRC
from the DS1822 must be shifted into the circuit. At this point, if the recalculated CRC was correct, the
shift register will contain all 0s. Additional information about the Dallas 1-Wire cyclic redundancy check
CRC = X
th
8
bit from the ROM or the most significant bit of byte 7 from the scratchpad, the
bit 7
R1
+ X
0
0
1
1
0
5
+ X
R0
bit 6
R1
0
1
0
1
4
+ 1
bit 5
R0
Resolution
10-bit
11-bit
12-bit
9-bit
bit 4
1
8 of 21
bit 3
Max Conversion Time
1
93.75ms
187.5ms
375ms
750ms
bit 2
1
bit 1
1
(t
(t
(t
(t
CONV
CONV
CONV
CONV
/8)
/4)
/2)
bit 0
)
th
1
byte of the scratchpad
DS1822