Abstract: CS5534 is a low-noise 24-bit A/D converter launched by CIRRUS LOGIC, USA. This paper introduces the chip's structure, functions, working mode and other characteristics, and applies it to the design of a high-precision analog acquisition card; at the same time, the paper gives the acquisition card's hardware circuit design and the corresponding C51 software design.
Keywords: A/D converter, signal acquisition card, hardware circuit, software design
1 CS5534 chip introduction
In the intelligent instruments and detection equipment composed of single-chip microcomputers, the input signal changes are very sensitive. The selection of A/D converters will have a great impact on the performance of these devices. Traditional 12-bit parallel interface A/D converters (such as AD574) usually cannot meet the measurement accuracy requirements of the above instruments. Therefore, this paper selects the new high-precision A/D conversion chip CS5534 [1] to design a high-precision analog quantity card that can be widely used in the field of industrial control. The CS5534 chip uses charge balance technology and extremely low-noise programmable gain chopper-stabilized measurement amplifier, and the output resolution can reach 24 bits. The chip uses a serial interface to communicate with the central processor, and the interface circuit is relatively simple. In addition, the CS5534 has a complete self-calibration system inside, which can perform self-calibration and system calibration to eliminate the zero point error and drift error of the A/D itself.
1.1 Internal logic structure diagram of CS5534
The internal logic structure of CS5534 is shown below. CS5534 has four channels of analog input, and the built-in multiplexer is used to select which channel's physical signal to convert. After passing through a programmable gain amplifier, the signal enters the fourth-order regulator, and finally is filtered by a programmable digital filter to obtain the A/D conversion result. In addition, CS5534 has a serial interface that can easily communicate with the 51 single-chip microcomputer, and the communication mode is SPI bus mode.
Figure 1 CS5534 internal logic structure diagram
1.2 CS5534 internal register resources
CS5534 has an embedded online controller, which contains many user-accessible registers. These registers are used to save the results of gain calibration and offset calibration, configure the chip's operating mode, save conversion instructions and conversion results, etc.
Specifically, it includes 1 command register, 1 configuration register, 1 conversion data storage register (read-only), 4 channel setting registers and gain registers and offset registers of each physical channel. Except for the command register which is 8 bits, the other registers are 32-bit registers. The following is an introduction to each register:
1.2.1 Command Register
The command register is used to store various instructions issued by the 51 microcontroller to the A/D conversion chip. These instructions are mainly divided into two categories, which are distinguished by their highest bit (D7) being 0 or 1. When D7=0, the microcontroller performs read and write operations on the internal registers of the CS5534; when D7=1, it indicates that the A/D conversion or calibration is started.
1.2.2 Configuration Registers
The configuration register has 32 bits, but only 11 bits are used to set some operating parameters of CS5534, such as operating power, system reset, input short circuit, reference voltage selection and filter rate. CS5534 has four working modes, namely normal mode, low power mode, backup mode and sleep mode. These working modes are controlled by D21 (LMP), D31 and D30. When the system is reset, D29 (RS) is set to 1. After the reset is completed, D28 (RV) will automatically set to 1 to indicate that the reset is valid. In addition, the value of D25 (VRS) needs to be determined by the size of the reference voltage, and the value of D19 (FRS) will affect the filter rate.
1.2.3 Channel Setting Register
CS5534 has four channel setting registers, each of which consists of two programmable 16-bit setting registers. The channel setting register can be used to specify which physical channel to convert, at what gain, rate and polarity. When the crystal frequency is 4.9152MHZ, the gain range is 1X, 2X, 4X, 8X, 16, X, 32X, 64X, and the conversion rate can be 7.5HZ, 15HZ, 30HZ, 60HZ, 120HZ, 480HZ, 960HZ, 1.92HZ, 3.84HZ.
1.2.4 Conversion Data Register
The A/D converter stores the converted result in this register. This register is a read-only register, and the upper 24 bits are data bits. D3 is the overflow flag, and D1 and D0 are the channel flags.
1.2.5 Gain and Offset Registers
Each physical channel contains a 32-bit gain register and offset register to store the calibration results. The range of the gain register is 0~64-2-24 . After reset, each bit of these registers is 0.
1.3 CS5534 A/D conversion method
CS5534 provides two conversion modes: single-step conversion and continuous conversion. For the former conversion mode, after the user executes a single-step conversion command and obtains the result of this conversion, the conversion ends. For the latter conversion mode, after the user executes a continuous conversion command, the converter will continue to convert data unless the CPU issues an instruction to stop the conversion.
2 Hardware Design of High-Precision Analog Acquisition Card Based on CS5534
In industrial control sites, it is often necessary to collect various analog signals in real time with high precision. For this purpose, the CS5534 chip is used as an A/D converter and together with the 51 single-chip microcomputer, an analog signal acquisition card is formed. The signal acquisition card circuit mainly consists of two parts: the analog input part collects the analog input signals of the industrial site; the communication interface part is used to communicate with the host computer. The communication interface uses MAX483 [2] as the communication transceiver, and PC410 as the input and output optoelectronic isolator. The following mainly introduces the circuit of the analog input part of the acquisition card, and its circuit diagram is as follows:
Figure 2 Interface circuit between CS5534 and CPU
In the figure, CS5534 has a serial interface, which can easily communicate with the CPU. The interface has four control lines, CS, SDI, SDO, and SCLK. CS is the chip enable terminal, which is valid at low level. SDI is the serial data input, which sends data to the converter. SDO is the serial data output, which outputs data from the converter. SCLK is the serial clock control, which controls the shift of the A/D serial port data. The SPI bus is used to communicate with the CPU, the interface circuit is simple, and the CPU's I/O port resources are saved.
3. C51 Software Design of Acquisition Card
3.1 System Initialization
After the system is powered on, the serial port of the A/D converter needs to be initialized so that the serial port is in the state of receiving commands. The initialization process is that the CPU sends at least 15 SYNC1 (0FFH) and then sends 1 SYNC0 (0FEH) to end the initialization state.
Since CS5534 has wide dynamic characteristics, programmable output rate, flexible power supply mode and simple three-wire serial output mode, it has a simple interface with the microcontroller and can be widely used in some high-resolution measurement occasions. Using CS5534 as a signal acquisition card for A/D converter, various analog signals in industrial sites can be collected with high precision and the data can be sent to the host computer in real time. This acquisition card can be used in some control systems with high precision requirements (DCS, CNC machine tools, etc.), has good versatility and has good application and promotion value.
[1] 16-BIT and 24-BIT ADCS with Ultra Low Noise PGIA, Cirrus Logic, inc. 2004
[2] Li Guangdi. Fundamentals of Single Chip Microcomputers [M]. Beijing Aeronautics and Astronautics Press. 2001
[3] He Limin. MCS-51 series single chip microcomputer application system design. System configuration and interface technology [M] Beijing University of Aeronautics and Astronautics Press. 1990.
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