Abstract: The MC74 serial digital temperature sensor has advantages such as low price, high accuracy, and serial linear output. This paper introduces the main operating characteristics and working principle of the circuit, and provides the principle and part of the program code of a wireless digital temperature sensor circuit designed using the MC74. 1 Overview A wireless sensor can be constructed by connecting a common sensor with a low-power wireless transmitter controller and receiver. If the system is connected to the Internet, it can even form a wireless network sensor. This paper introduces a wireless digital temperature sensor composed of the MC74 integrated serial digital temperature sensor. The MC74 is an 8-bit serial interface integrated digital temperature sensor manufactured by ON Semiconductor. Its temperature data is converted from the thermal sensing unit. The MC74 has a temperature resolution of ±1°C and a conversion rate of 8 samples/second. Under normal operating conditions, the quiescent current is 200μA (typical), and under standby conditions, the quiescent current is 5μA (typical). Communication between other devices and the MC74 can be achieved through a 2-wire SM bus, an I2C-compatible serial bus. This bus can be used for multi-area monitoring. The SHDN bit in the CONFIG configuration register can be used to activate low-power standby mode. Its small size, low assembly cost, and ease of operation make the MC74 an ideal choice for thermal management in various systems. 2. Features of the MC74 Figure 1 shows the two package types of the MC74. Figure 2 shows the internal structure of the MC74. The characteristics of the MC74 are as follows: Digital temperature sensing function; Two packages: SOT-23 or TO-220; Output temperature is an 8-bit digital value; Simple SM bus, I2C serial interface; Solid-state (PN junction) temperature sensing; High accuracy: ±2°C at +25°C to +85°C; ±3°C at 0°C to +125°C; Supply voltage range: 2.7V to 5.5V; Low power consumption: Typical operating current is 200μA; Typical standby current is 5μA. Table 1 lists the pin functions of the MC74. [align=center] [/align] 3 MC74 Working Principle The internal structure of the MC74 is shown in Figure 2. It obtains temperature from a solid-state (PN junction) sensor and converts it into a digital signal. Its temperature resolution is ±1°C. It stores the converted temperature digital signal in an internal register and then reads it through the serial port. The system interface is a master-slave SM bus/I2C port, through which temperature data can be read at any time. The MC74 has a total of 8 programmable SM bus/I2C addresses. Multi-sensor configuration can be used simultaneously, that is, up to 8 similar MC74 sensors can be connected in a control system at the same time, and the circuit will enter a low-power wait mode when temperature acquisition is paused. 3.1 MC74 Low-Power Wait Mode After being allowed by the MC74, the host can set it to a low-power (IDD=5μA typical value) wait mode. In this mode, the A/D converter is stopped, the temperature data register is frozen, but the SM bus/I2C port is still operating normally. By setting the SHDN bit in the configuration register CONFIG, the MC74 can be put into a low-power wait mode: SHDN=0 is normal mode; SHDN=1 is low-power wait mode. 3.2 MC74's SM Bus/I2C Slave Address The MC74 has a default SM bus configured internally, with an I2C address of 1001101. The other seven addresses can be set by the user. Figure 3 shows the format of the MC74's read, write, and receive data. 3.3 MC74's Serial Port Operation The MC74 can be programmed and accessed through the serial clock input port SCLK and the bidirectional data port SDA of the 2-wire bidirectional serial port. All data transmission is under the control of the master in a master-slave architecture. Typically, the CPU or microcontroller acts as the master, providing the clock signal for all transmissions. The MC74 usually operates as a slave in a master-slave architecture. In serial protocol communication, all data transmission has two phases, with the most significant bit of each byte transmitted first. Access is initiated by a START condition, followed by a device address byte and one or more data bytes. The device address byte includes a read/write select bit. Each access must terminate with a STOP condition. An ACK acknowledges the reception of each byte. SDA can only be changed when SCLK is low. When SCLK is high, a change in SDA will act as a START and STOP condition. The relationship between the temperature and binary value of the MC74 is shown in Table 2. 3.4 MC74 8-bit Read-Only Temperature Register The binary value (two's complement) in this register represents the temperature of the airborne sensor after one exchange cycle. The data in the register is automatically updated after one interaction. The value of each cell in the temperature data register represents 1°C. This value uses two's complement arithmetic, so a reading of 00000000b is equivalent to 0°C. 4 Applications of the MC74 Temperature Sensor 4.1 Wireless Digital Temperature Sensor Composed of MC74 Figure 4 shows the circuit principle of the transmitter of a wireless digital temperature sensor composed of MC74. In the diagram, U1 is the PIC12C509AG microcontroller from the PIC series, and U2 is the MC74. The transmitter is based on the PIC12C509AG. It drives a loop antenna, which is etched onto the transmitter's circuit board. Essentially, it's a standard PIC12C509AG with a 433MHz ASK transmitter. The RF section of the PIC12C509AG includes a crystal oscillator, a complete phase-locked loop, mode control logic, and a power amplifier. As shown in Figure 4, the RF section and the PIC microcontroller section are logically separate, although they reside within the same physical packet. Typically, ASK modulation involves alternating the carrier amplitude in pulse-width modulation mode. Here, the Manchester encoding method is used. According to Manchester encoding (a low-to-high transition in the middle of a bit represents logic "0", and a high-to-low transition in the middle of a bit represents logic "1"), a synchronous bit stream is generated. The encoded clock signal does not undergo physical transitions at bit boundaries. Instead, logical transitions occur in the middle of each bit time. When powered on, the transmitter remains in sleep mode for most of the time to conserve power. During operation, the PIC12C509AG section uses the I2C bus to read the MC74. If the temperature changes, the new temperature will be sent. The transmitter has two buttons. These buttons are used to wake the transmitter, set the status ID, generate a test audio, or begin transmitting the current temperature and transmitter on/off status. When SW1 is pressed, the transmitter code is set to automatically change the transmitter's status ID. Simultaneously, because each MC74 has a permanent I2C address, which depends on the sensor's serial number, the transmitter code uses the I2C signal to automatically determine the I2C address on the current onboard MC74. 4.2 Data Transmission Section of the Transmitter The information packet transmitted by the transmitter contains a 32-bit header, a 4-bit synchronization format, and 32 bits of data. The data field includes the status ID (8 bits), temperature (8 bits), sensor type (4 bits), information type (2 bits), button status (2 bits), and verification (8 bits). Each time a transmission event is invoked, this entire set of information is sent three times consecutively. Below is a code snippet of the data transmission from the MC74 sensor's transmitter: [align=center] [/align] 5 Conclusion The MC74 is well-suited for low-cost and miniaturized applications, such as thermal protection for computer hard drives or other PC peripherals. It is also suitable for less demanding temperature measurement and control systems. In the aforementioned embedded system PIC12C509AG of wireless digital temperature sensors, if an Internet wireless network communication protocol can be embedded, they can form a wireless sensor network.