Abstract: This paper introduces the development of an intelligent pressure transmitter based on the HART protocol and using the XEMICS microprocessor as its core. This intelligent pressure transmitter can be used for real-time field pressure monitoring, featuring temperature and nonlinearity compensation, low power consumption, and compatibility with both digital and analog communication. Keywords: HART protocol, intelligent pressure transmitter, digital communication, low power consumption. Early control systems were mainly analog instrument control systems, transmitting 1-5V or 4-20mA analog signals between devices. The signal accuracy was low, and the transmission was susceptible to interference. With the development of electronic and computer technologies, especially the advent of fieldbus, new fully digital intelligent instruments have gradually replaced traditional analog instruments, and their performance is constantly evolving towards higher accuracy, higher reliability, and higher environmental adaptability. The adoption of digital intelligent instruments is an inevitable trend. However, due to the widespread use of analog field instruments and the limitation of preserving original investment, the transition from analog instruments to fully digital intelligent instruments will still take a long time. During this period, developing an intelligent instrument compatible with both analog and digital signals will have significant practical importance. The intelligent pressure transmitter introduced in this paper was developed against this background. 1. Introduction to HART Protocol HART (Highway Addressable Remote Transducer) is an open communication protocol for high-speed channels of addressable remote sensors. It is a type of fieldbus and a transitional protocol. Its key feature is the ability to achieve digital signal communication on existing analog signal transmission lines, providing a risk-free solution for improving communication between instruments. It has strong market competitiveness during the transition from analog to digital systems. The HART protocol uses FSK frequency shift keying signals based on the Bell202 standard, superimposing a 0.5mA audio digital signal onto a low-frequency 4-20mA analog signal for bidirectional digital communication, with a data transmission rate of 1.2Mbps. The HART protocol references the ISO/OSI Open Systems Interconnection model, adopting its simplified three-layer structure: the physical layer, the data link layer, and the application layer. 1.1 Physical Layer The physical layer specifies the signal transmission method and medium. To enable simultaneous analog and digital communication without interference, the HART protocol uses Frequency Shift Keying (FSK) technology, superimposing an audio digital signal onto a 4-20mA analog signal. The frequency signal adopts the Bell 202 international standard, with 1200Hz representing logic "1" and 2200Hz representing logic "0". The signal amplitude is 0.5mA, as shown in Figure 1. The transmission baud rate of the digital signal is set to 1200bps. The choice of communication medium depends on the transmission distance. Typically, when using twisted-pair coaxial cable as the transmission medium, the maximum transmission distance can reach 1500m, and the total line impedance should be between 230 and 1100Ω. 1.2 Data Link Layer The data link layer specifies the format of the HART frame as shown in Figure 2, realizing the functions of establishing, maintaining, and terminating link communication. The HART protocol uses an automatic repeat request mechanism based on redundant error detection code information to eliminate data communication errors caused by line noise or other interference, achieving error-free transmission of communication data. For field instruments to execute HART commands, the operands must conform to the specified size. Each independent character includes a start bit, eight data bits, a parity bit, and a stop bit. Because the presence and length of data are not constant, the length of HART data also varies, with the longest HART data containing 33 bytes. 1.3 Application Layer The application layer is the HART command set used to implement HART instructions. Commands are divided into three categories: general commands, ordinary commands, and special commands. General commands are universal and applicable to all smart devices conforming to the HART protocol (regardless of the company's product). Examples include reading manufacturer and product model information, reading process variables and their units, and reading current percentage output. Ordinary commands are applicable to most smart devices, but each product can choose to use them according to its own needs. They are used for common operations, such as setting ranges, setting process variable units, and writing damping time constants. Special commands are designed for the specific characteristics of specific devices to achieve special functions not included in the first two types of commands but required by the device itself. In HART protocol communication, the main variables and control information are transmitted in the form of 4–20mA. When needed, additional measurement, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol. 2 Hardware Design of a Smart Pressure Transmitter Based on HART Protocol This paper presents a smart transmitter developed based on the HART protocol for semiconductor pressure sensors. The hardware of this transmitter mainly includes the following modules: MCU module, LCD display module, HART communication module, sensor module, and power supply module. The system hardware block diagram is shown in Figure 3. The power supply voltage of the smart pressure transmitter is selected as 3.3V, provided by the MAX6129AEUK33-T regulated power supply module. The sensor module mainly includes a sensor bridge circuit, as shown in Figure 4. The temperature coefficient of resistor R20 should not exceed 50PPM/℃. The MCU module mainly consists of a microprocessor XE8A8LC05A and a non-volatile memory EEPROM93AA76C. The XE8A8LC05A integrates a 12-bit A/D converter and a 10-bit D/A converter. The pressure and temperature signals output from the sensor module are shaped and bandpass filtered before being connected to two channels of the microprocessor's built-in A/D converter. The sensor bridge power supply voltage is used as the reference voltage for the A/D converter to eliminate errors caused by bridge instability. After A/D conversion, the CPU performs linearization, range switching, and damping operations. Finally, the microprocessor's built-in D/A converter converts the digital signal into a current signal for output. EEPROM stores all configuration, characterization, and digital fine-tuning data. The HART communication module is the hardware implementation of the HART protocol physical layer. It uses the HT2012 microintegrated circuit as the HART modem. The HT2012 operates at 460.8kHz and is powered by an independent low-power oscillator, the HT7210. HART signals received from the HART bus are amplified, filtered, and compared before being sent to the HT2012, where they are demodulated into logic 1 or logic 0 digital signals and transmitted to the microprocessor. Similarly, the digital signal sent by the microprocessor is modulated by a modem into a corresponding 1200Hz or 2200Hz FSK frequency shift keying signal, which is then superimposed on the loop and sent to the HART bus. HART communication is in half-duplex mode. The LCD display module is driven and controlled by the HT1620 chip to display relevant data. Due to the use of a capacitor-type bias voltage charge pump, the HT1620 has a very low operating current, meeting the low power consumption requirements of this system. This solution effectively solves this problem by extensively using low-power components. The digital circuit operates at 3.3V. Under these conditions, the XE88LC05A, with its 12-bit ADC and 10-bit DAC operating simultaneously and the CPU processing two million instructions per second, draws only 670μA. The typical operating current of the HT2012 is 40μA, the 93AA76C's read current is 500μA, the HT7210's operating current at 1MHz is only 130μA, and the HT1620's operating current is less than 3μA. Furthermore, the current from the shaping circuit, bandpass filter, and other analog circuitry is no more than 1.2mA. Therefore, the total operating current of the entire circuit is no more than 2.1mA, far less than 3.4mA. This means the transmitter can provide a maximum current of 1.3mA to the sensor module, which is sufficient for many sensors.