DS18B20温度传感器：1-Wire接口与特性详解
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更新于2024-07-25
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DS18B20是一款专为简化温度监控和传感应用设计的1-Wire接口温度传感器。这款创新的产品具有以下关键特性：
1. **独特的单线接口**：DS18B20仅需一个I/O端口（DQ）进行通信，节省了硬件资源，对于嵌入式系统设计极其方便。
2. **独一无二的64位序列代码**：每个传感器都有一个内置在ROM中的唯一标识符，这使得设备间的区分和识别变得容易。
3. **多点连接能力**：由于采用单线接口，可以轻松构建多路温度测量网络，适合分布式系统应用，如工业自动化、智能家居或环境监测等。
4. **无外部组件需求**：该传感器内置电源管理功能，可以直接从数据线路获取3.0V到5.5V的电压，无需额外的电源组件。
5. **宽温范围**：DS18B20能够测量-55°C至+125°C（-67°F至+257°F）的温度，特别适用于各种环境条件下的温度检测。
6. **高精度**：在-10°C至+85°C的范围内，提供0.5°C的精度，确保了测量结果的准确性。
7. **可选分辨率**：用户可以根据需求选择9至12位的温度分辨率，实现不同精度等级的测量。
8. **快速转换**：将温度转换为12位数字信号的过程能在750毫秒内完成，响应速度较快。
9. **非易失性报警设置**：支持用户定义的温度警报界限，当温度超出预设范围时，传感器会自动触发报警。
10. **报警搜索功能**：通过发送特定命令，可以搜索并定位到报警状态的设备，便于故障诊断和处理。
11. **多种封装形式**：DS18B20提供8引脚SO（150 mil）、8引脚SOP和3引脚TO-92三种封装选项，适应不同的电路板布局。
12. **软件兼容性**：该传感器与DS1822等其他型号的1-Wire设备兼容，可以无缝集成到现有的1-Wire软件生态系统中。
DS18B20凭借其小巧、低功耗、易集成和多功能的特点，在工业控制、家用电器、环境监控等多个领域展现了强大的应用潜力。开发人员在设计中只需要简单的硬件配置和相应的软件编程，即可实现精准的温度测量和报警功能。
DS18B20
5 of 21
OPERATION — ALARM SIGNALING
After the DS18B20 performs a temperature conversion, the temperature value is compared to the user-
defined two’s complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see Figure 3).
The sign bit (S)
indicates if the value is positive or negative: for positive numbers S = 0 and for negative
numbers S = 1. The T
H
and T
L
registers are nonvolatile (EEPROM) so they will retain data when the
device is powered down. T
H
and T
L
can be accessed through bytes 2 and 3 of the scratchpad as explained
in the MEMORY section of this datasheet.
T
H
AND T
L
REGISTER FORMAT Figure 3
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
S 2
6
2
5
2
5
2
5
2
2
2
1
2
0
Only bits 11 through 4 of the temperature register are used in the T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers. If the measured temperature is lower than or equal to T
L
or higher than T
H
, an alarm
condition exists and an alarm flag is set inside the DS18B20. This flag is updated after every temperature
measurement; therefore, if the alarm condition goes away, the flag will be turned off after the next
temperature conversion.
The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search
[ECh] command. Any DS18B20s with a set alarm flag will respond to the command, so the master can
determine exactly which DS18B20s have experienced an alarm condition. If an alarm condition exists
and the T
H
or T
L
settings have changed, another temperature conversion should be done to validate the
alarm condition.
POWERING THE DS18B20
The DS18B20 can be powered by an external supply on the V
DD
pin, or it can operate in “parasite power”
mode, which allows the DS18B20 to function without a local external supply. Parasite power is very
useful for applications that require remote temperature sensing or that are very space constrained. Figure
1 shows the DS18B20’s parasite-power control circuitry, which “steals” power from the 1-Wire bus via
the DQ pin when the bus is high. The stolen charge powers the DS18B20 while the bus is high, and some
of the charge is stored on the parasite power capacitor (C
PP
) to provide power when the bus is low. When
the DS18B20 is used in parasite power mode, the V
DD
pin must be connected to ground.
In parasite power mode, the 1-Wire bus and C
PP
can provide sufficient current to the DS18B20 for most
operations as long as the specified timing and voltage requirements are met (refer to the DC
ELECTRICAL CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this data
sheet). However, when the DS18B20 is performing temperature conversions or copying data from the
scratchpad memory to EEPROM, the operating current can be as high as 1.5mA. This current can cause
an unacceptable voltage drop across the weak 1-Wire pullup resistor and is more current than can be
supplied by C
PP
. To assure that the DS18B20 has sufficient supply current, it is necessary to provide a
strong pullup on the 1-Wire bus whenever temperature conversions are taking place or data is being
copied from the scratchpad to EEPROM. This can be accomplished by using a MOSFET to pull the bus
directly to the rail as shown in Figure 4. The 1-Wire bus must be switched to the strong pullup within
10ms (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus must be
held high by the pullup for the duration of the conversion (t
conv
) or data transfer (t
wr
= 10ms). No other
activity can take place on the 1-Wire bus while the pullup is enabled.
The DS18B20 can also be powered by the conventional method of connecting an external power supply
to the V
DD
pin, as shown in Figure 5. The advantage of this method is that the MOSFET pullup is not
required, and the 1-Wire bus is free to carry other traffic during the temperature conversion time.
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