Principle and Application of New Digital Temperature Sensor ADT75

Introduction
ADT75 is a new digital temperature sensor produced by ADI that integrates temperature sensor, 12-bit A/D converter, programmable over-temperature alarm and SMBus/I2C bus interface. Its rated operating temperature range is -55～+125℃, and it can accurately and sensitively detect digital temperature, with a maximum temperature error of ±1℃ and a temperature measurement resolution of 0.0625℃; its operating voltage range is 3～5.5V, and its typical power consumption is 79μW at 3.3V. Its typical operating current value is only 3μA in shutdown mode.

1 Pins and structural principle of ADT75
The pin arrangement of ADT75 is shown in Figure 1, and the pin description is listed in Table 1.

ADT75 adopts 8-pin MOSP and SOIC packages, and its internal structure is shown in Figure 2.

The working process of ADT75 is as follows: after the on-chip temperature sensor collects the temperature, it generates an accurate voltage proportional to the absolute temperature and compares it with the internal reference voltage; then it is input into the accurate digital regulator and converted into data with an effective accuracy of 12 bits. The data is compared with the limit value. If the measured value exceeds the limit, the OS/ALERT pin outputs the limit information.
In normal mode, the temperature conversion takes 60 ms, and then the analog conversion circuit is automatically turned off. After 40 ms, the analog circuit is powered on and the conversion of the next temperature value begins. Therefore, a temperature conversion cycle is 100 ms.

2 Register structure of ADT75
ADT75 contains 6 registers: 1 address pointer register, 4 data registers and 1 single-step mode register. Among the data registers, the configuration register is the only 8-bit register, and the other 3 are 16-bit; the temperature value register is the only read-only register, and the other 3 are readable and writable. The single-step mode register is also readable and writable. After power-on, the initial value of the address pointer register is 0x00, and the pointer points to the temperature value register. The register description of ADT75 is listed in Table 2.

(1) Address Pointer Register
This 8-bit read-only register stores the address pointing to a data register. Single-step mode can be selected. Bits P0 and P1 select the data register to read/write data. Write 0x04 to bits P0, P1, and P2 to select single-step mode. The remaining bits of the address pointer register are all 0. The address selection of the register is listed in Table 3.

[page]

(2) Temperature Value Register
This 16-bit read-only register stores the temperature value measured by the chip's internal temperature sensor. The temperature is stored in two's complement form, with the highest bit being the sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits.
(3) Configuration Register
This 8-bit readable and writable register provides multiple configuration modes for the ADT75: shutdown, overtemperature interrupt, single step, SMBus alarm enable, OS/ALERT pin polarity, and overtemperature error queue.
(4) THYST Constant Register
This 16-bit readable and writable register stores the temperature hysteresis limit value for two interrupt modes. This limit value is stored in two's complement form, with the highest bit being the temperature value sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits. The default value of the limit value THYST is +75°C.
(5) TOS Constant Register
This 16-bit readable and writable register stores the overtemperature limit value for two interrupt modes. This temperature limit value is stored in two's complement form, with the highest bit being the temperature value sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits. The default value of the limit value TOS is +80℃.

3 Application Examples of ADT75
3.1 Hardware Design
Under the influence of external temperature field, the fiber length, cross-sectional structure, refractive index distribution characteristics of fiber core and cladding in the fiber delay line system will change. Therefore, the phase and mode birefringence characteristics of the optical carrier signal propagating in the fiber will change with the change of temperature, thereby affecting the delay of the microwave signal finally demodulated. In order to reduce the influence of temperature change on the delay of microwave signal, it is necessary to design a temperature control system to control the temperature of the system.
The hardware design circuit mainly includes two parts: digital signal processor TMS320F2812 and digital temperature sensor ADT75.
TMS320F2812 launched by TI is used as the core control chip. Its external crystal oscillator frequency is 30 MHz, which can be multiplied by the PLL on the chip, and the maximum main frequency can reach 150 MHz; it has a fast running speed and can process the collected temperature signal in real time.
TMS320F2812 does not have an I2C bus design, but has 56 GPIO ports, so the GPIO port is used to simulate the I2C bus timing to control ADT75. This hardware circuit has simple structure, low power consumption and strong practicability. The interface circuit between ADT75 and TMS320F2812 is shown in Figure 3.

The GPIOB0 pin of TMS320F2812 is used as the clock signal line of the I2C bus, and the GPIOB1 pin is used as the serial data line of the I2C bus. The power supply voltage is 5 V, the 10 kΩ resistor is an open-drain pull-up resistor, and the 0.1 μF capacitor acts as a decoupling. This design only collects the temperature of the optical fiber delay line system in real time, and does not require an over-temperature alarm, so the OS/ALERT pin is reserved. The address of ADT75 is 7 bits, the upper 4 bits are 1001, and the lower 3 bits are determined by the address pins A0~A2. Since there is only one ADT75, all its 3 address pins can be grounded, and the chip address can be determined as 1001000. The temperature control system performs PID calculations inside the TMS320F2812 according to the measured temperature, and then adjusts the internal temperature of the optical fiber delay line through an external temperature control device to stabilize it within a certain set range. [page]

(2) Temperature Value Register
This 16-bit read-only register stores the temperature value measured by the chip's internal temperature sensor. The temperature is stored in two's complement form, with the highest bit being the sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits.
(3) Configuration Register
This 8-bit readable and writable register provides multiple configuration modes for the ADT75: shutdown, overtemperature interrupt, single step, SMBus alarm enable, OS/ALERT pin polarity, and overtemperature error queue.
(4) THYST Constant Register
This 16-bit readable and writable register stores the temperature hysteresis limit value for two interrupt modes. This limit value is stored in two's complement form, with the highest bit being the temperature value sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits. The default value of the limit value THYST is +75°C.
(5) TOS Constant Register
This 16-bit readable and writable register stores the overtemperature limit value for two interrupt modes. This temperature limit value is stored in two's complement form, with the highest bit being the temperature value sign bit. When reading this register, read the upper 8 bits first and then the lower 8 bits. The default value of the limit value TOS is +80℃.

3 Application Examples of ADT75
3.1 Hardware Design
Under the influence of external temperature field, the fiber length, cross-sectional structure, refractive index distribution characteristics of fiber core and cladding in the fiber delay line system will change. Therefore, the phase and mode birefringence characteristics of the optical carrier signal propagating in the fiber will change with the change of temperature, thereby affecting the delay of the microwave signal finally demodulated. In order to reduce the influence of temperature change on the delay of microwave signal, it is necessary to design a temperature control system to control the temperature of the system.
The hardware design circuit mainly includes two parts: digital signal processor TMS320F2812 and digital temperature sensor ADT75.
TMS320F2812 launched by TI is used as the core control chip. Its external crystal oscillator frequency is 30 MHz, which can be multiplied by the PLL on the chip, and the maximum main frequency can reach 150 MHz; it has a fast running speed and can process the collected temperature signal in real time.
TMS320F2812 does not have an I2C bus design, but has 56 GPIO ports, so the GPIO port is used to simulate the I2C bus timing to control ADT75. This hardware circuit has simple structure, low power consumption and strong practicability. The interface circuit between ADT75 and TMS320F2812 is shown in Figure 3.

The GPIOB0 pin of TMS320F2812 is used as the clock signal line of the I2C bus, and the GPIOB1 pin is used as the serial data line of the I2C bus. The power supply voltage is 5 V, the 10 kΩ resistor is an open-drain pull-up resistor, and the 0.1 μF capacitor acts as a decoupling. This design only collects the temperature of the optical fiber delay line system in real time, and does not require an over-temperature alarm, so the OS/ALERT pin is reserved. The address of ADT75 is 7 bits, the upper 4 bits are 1001, and the lower 3 bits are determined by the address pins A0~A2. Since there is only one ADT75, all its 3 address pins can be grounded, and the chip address can be determined as 1001000. The temperature control system performs PID calculations inside the TMS320F2812 according to the measured temperature, and then adjusts the internal temperature of the optical fiber delay line through an external temperature control device to stabilize it within a certain set range. [page]

(2) Transmitting the ADT75 address
Before reading the temperature value, the address must be sent to the slave device. The 7-bit address of ADT75 is 0x48. Since it is reading data (the read/write bit is 1), the 8-bit address command transmitted is 0x91. When transmitting data, when SCL is 0, the data on SDA is allowed to change; when it is 1, the data on SDA remains unchanged. After the 8-bit address transmission is completed, the host releases SDA (set SDA=1) and waits for the response signal from the slave.
(3) Detecting the response bit of ADT75
After the I2C bus transmits 8 bits of data, the slave sends a low-level response signal to the host, indicating that the slave is working normally and can receive the next byte of data. When detecting the response bit of ADT75, you should pay attention to setting the GPIOB1 port to input.

EALLOW:
GpioMuxRegs. GPBDIR. bit. GPIOB1=0;
EDIS;
If SDA=0, TMSS20F2812 starts to read the high byte of data from ADT75; SDA=1 indicates that ADT75 is busy or damaged, and the data reading ends.
(4) Reading data
When the response signal of ADT75 is detected to be 0, the temperature value is read. The data transmission of the I2C bus is in bytes. First, the high byte of the temperature value (the integer part of the temperature value, the highest bit is the sign bit) is read, and the read data is stored in retc. Every time 1 bit of data is received, retc is shifted left by 1 bit. If SDA=1, retc is incremented by 1; if SDA=0, retc remains unchanged.

After the high 8-bit data is transmitted, TMS320F2812 sends a low-level response signal to ADT75, which is completed by the Mack() function. At this time, the data transmission direction of the GPIOB1 port needs to be changed to output:
EALLOW:
GpioMuxRegs. GPBDIR. bit. GPIOB1=1;
EDIS:
Then start receiving the low byte of the temperature value (the decimal part of the temperature value). After the reading is successful, TMS320F2812 sends a non-response bit, indicating that the reading of this temperature value has ended and enters the stop state.
(5) End data transmission
The end of data transmission is completed by the Stop() function, and the end conditions are as follows:

Delay(50);
So far, the whole process of reading a temperature value is over. During the debugging process, it was found that when setting a breakpoint and running single-step, the low-level response signal sent by ADT75 could not be detected, and the response signal was always 1; if the breakpoint was not set and the execution was continuous, the low-level response signal could be detected. This is the difference between ADT75 and other I2C bus devices (such as E2PROM chip AT24C256). Pay attention to this detail during the debugging process.
The temperature conversion cycle of ADT75 is 100 ms. In this design, the temperature value is read every approximately 250 ms to monitor the temperature changes of the optical fiber delay line system.

Conclusion
In the hardware design of the optical fiber delay line system, ADT75 can fully meet the requirements of real-time temperature acquisition, and the temperature measurement is accurate and sensitive. Due to the use of the I2C bus interface, the temperature detection circuit structure is simple, occupies a small space, the serial interface occupies less resources of TMS320F2812, has high reliability, low power consumption, and is not easily affected by environmental interference. Experiments have proved that both the design and operation have achieved satisfactory results.