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Design of AD574 Controller with CPLD
**CPLD**
CPLD, or Complex Programmable Logic Device, is a type of digital integrated circuit that evolved from earlier programmable logic devices like PAL and GAL. It offers a larger scale and more complex architecture, making it part of the large-scale integrated circuits (LSI) family. Users can design their own logic functions based on specific requirements. The basic design process involves using an integrated development platform, where schematic diagrams or hardware description languages (HDLs) are used to generate target files. These files are then programmed into the CPLD via a download cable, enabling in-system programming to implement the desired digital system.
CPLDs typically consist of a central programmable interconnect matrix unit surrounded by programmable logic macro cells (MCs). These MCs have a complex internal structure with flexible I/O connections, allowing users to create custom circuits for various applications. Since the internal connections are made through fixed-length metal lines, the timing behavior of the logic circuits is predictable, avoiding the timing uncertainties associated with segmented interconnect structures.
**Development History and Application Fields**
The first programmable logic device, known as PLD, emerged in the 1970s. It featured a programmable logic macro cell structure, offering greater flexibility compared to traditional digital circuits. However, its limited complexity restricted it to smaller-scale designs. To overcome this limitation, CPLDs were introduced in the mid-1980s. Today, CPLDs are widely used in fields such as networking, instrumentation, automotive electronics, CNC systems, and aerospace control equipment.
**Device Characteristics**
CPLDs offer numerous advantages, including flexible programming, high integration, short design cycles, broad application scope, advanced development tools, low cost, and ease of use for designers. They do not require extensive hardware knowledge, and they allow for quick prototyping and mass production (typically under 10,000 units). Their versatility makes them suitable for replacing small-scale general-purpose digital ICs. As a result, CPLDs have become essential components in modern electronic systems, and mastering their design is crucial for electronics engineers.
**How to Use**
A CPLD allows users to build custom logic functions. The standard design method involves using an integrated development environment, such as Altera Max+Plus II, to draw schematics or write HDL code (like VHDL or Verilog). After compilation, simulation is performed to verify the logic output. Then, input/output pins are assigned, and the final code is generated and downloaded to the chip via a programming cable.
For example, in a simple answering device project, the CPLD controls LEDs, switches, and a buzzer. Once the design is verified, it can be replicated for mass production. If changes are needed, such as adding a traffic light function, the design can be modified by editing the schematic or HDL code and repeating the same process. This makes CPLD-based design highly reconfigurable and efficient.
**AD574**
The AD574 is a high-speed, 12-bit successive approximation analog-to-digital converter (ADC) developed by Analog Devices. It has a conversion time of 35 microseconds and a typical conversion error of 0.05%. Known for its wide application and moderate price, the AD574 features a three-state output that can interface directly with microprocessors. It is compatible with both CMOS and TTL levels, eliminating the need for additional interface circuits. Its internal reference voltage and clock make it easy to use without external components.
**Design of AD574 Controller with CPLD**
In automotive electronic systems that process sensor signals via a PCI bus, an ADC module is essential. This design uses the AD574 to convert analog signals from the signal conditioning board into digital data, which is then read by the MCU through the PCI bus. By using Verilog HDL on a CPLD, the AD574 is controlled efficiently, reducing the MCU's workload and improving system performance and stability.
**1. AD574 Operation**
**1.1 AD574 Structure and Features**
The AD574 is a high-speed, 12-bit successive approximation ADC with an internal sample-and-hold circuit. It uses a hybrid bipolar design, requiring minimal external components. It offers low power consumption, high precision, and automatic zero calibration. The chip supports four input voltage ranges: ±5V, ±10V, 0–10V, and 0–20V, and operates on dual power supplies (±12V or ±15V for analog, ±5V for digital).
**1.2 AD574 Pin Functions and Timing Control**
The AD574 requires several control signals: CS (chip select), R/C (read/convert), CE (enable), AO (address), and STS (status). When CE=1 and CS=0, the ADC starts. R/C=0 initiates conversion, while R/C=1 reads the data. The 12/8 pin determines whether the output is 12-bit or 8-bit, and AO selects the data format.
**1.3 AD574 and PCI Interface Timing**
In the system, the AD574 connects to the PCI9052 chip for data transmission. CPLD controls the timing of the AD574 based on signals from the PCI9052, such as ADS#, BLAST, LRDYi, and LCLK. These signals manage the data transfer and conversion process.
**2. CPLD Design and Implementation**
Using Verilog HDL, the AD574 sampling control logic was implemented. A counter was added to ensure proper timing for accurate conversion. Quartus II software was used for synthesis, routing, and simulation. ModelSim was used for waveform verification, confirming that the design met all timing and accuracy requirements.
**3. Conclusion**
By leveraging CPLD technology, the complexity of hardware design is reduced, and the system becomes more flexible. CPLDs allow for easy modification, upgrades, and maintenance without altering the physical hardware. This approach improves data processing speed and reduces the MCU’s workload, making it an ideal solution for real-time and embedded systems.