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CPLD VS FPGA: Understanding the Better Option for Your Digital System

Thanks to technological advancements, we can appreciate the introduction of programmable logic devices (PLDs). Some examples of PLDs include CPLD vs FPGA

Often, beginners and students find it hard to distinguish between the two PLDs, CPLD and FPGA. They may also not know the best option for their digital systems. 

Well, worry not because, in today’s article, we will compare CPLD VS FPGA devices. Additionally, we will discuss where each PLD is applicable and its architectures. 

  1. What is CPLD?

CPLD (Complex Programmable Logic Device) is an IC that assists in implementing digital systems. Furthermore, it has a set of programmable functional blocks, each with macrocells (CPLD building blocks). 

Then, a reconfigurable GIM (Global Interconnection Matrix) connects the outputs and inputs of the functional blocks (similar to a logic gates array). Thus, you can’t change the contacts amidst the blocks.

To understand the architecture and features of CPLD, we will use a practical example, i.e., Xilinx CoolRunner-II.  

Features in Xilinx CoolRunner-II CPLDs

  • Despite the signal number limitation in each functional block, you can still program the AIM to interconnect the signal as you please. 
  • It has macrocells with a PLA array of a sum of products and one flip-flop that you can use to build sequential or combinational logic. 
  • In addition to each functional block, there are 16 macrocells (MC1-MC16) that can access 16 signals from I/O blocks.
  • Furthermore, the function blocks receive inputs of 40 lines of signals from the Advanced Interconnect Matrix (AIM). Conversely, AIM gets 16 lines of output from every function block. 
  • Lastly, Xilinx consists of a few function blocks with numbers ranging from 2-to-32. The labels go from ‘Function block 1’ to ‘Function Block n.’

Note 

An In-System Programming (ISP) electrically programs the logic function of CPLD in all macrocells. Afterward, the flash-based connections keep on being persistent for eons. Therefore, every time you power up your CPLD, it instantaneously starts working because an ISP has burned/flashed its circuitry into them. 

What is FPGA?

FPGA (Field Programmable Gate Array) is an IC that a designer or customer has to configure before using. Moreover, the semiconductor device uses programmable interconnects to connect FPGA’s matrix of configurable logic blocks. 

Xilinx XC4005XL FPGA Integrated Circuit

(Xilinx XC4005XL FPGA Integrated Circuit)

Components of FPGA 

They include;

  • IO blocks; It is applicable in making chip connections.
  • Programmable routing (interconnects); permits implementation routing functions.
  • Programmable logic blocks; permit the implementation of logic functions.

Generally, FPGA manufacturers rarely release exact architecture. Nonetheless, we will use a model to help you understand. 

  • First, several CLBs (Configurable Logic Blocks) are implanted in programmable interconnects. Besides being able to implement complex logic functions, the CLBs are more complex than CLPDs. 
  • Then, a D-type Flip-Flop (DFF), Multiplexers (mux), and Look-Up Tables (LUT) primarily make up a CLB as illustrated below;
  • FPGAs can also have millions of flip-flops and CLBs in one device. 

Applications

FPGA hardware can perform functions like

  • PCIe,
  • MMCMs and PLLs,
  • External Memory Controllers,
  • Digital Signal Processing (DSP) blocks,
  • High-speed Serial Transceivers, and
  • RAM. 

Working 

  • Since FPGA is LUT-based, on powering it up, it’ll always be blank. 
  • Thus, it’ll use a Configuration Circuit to read the configuration data from an external Read-only memory before configuring the LUT according to your design. 
  • Once the configuration is complete, the FPGA will start working even though it may have some delays. 

CPLD VS FPGA comparison summary

The following table summarizes the main differences between CPLD and FPGA.

Where to use CPLD or FPGA?

Often, different product designs require either a CPLD or FPGA. For example, FPGAs are recommendable for most applications, whereas CPLDs favor instant-on circuits or designs requiring similar functionality or simple glue logic. 

Other times, manufacturers may combine FPGA and CPLD in a design. In such circuit designs, CPLDs control the boot and reset sequence of a complete board, boot FPGA, and perform a simple glue logic. 

Therefore, depending on its application, you may use both integrated circuits in a specific design.

How to choose CPLD or FPGA?

Generally, choosing either an FPGA or CPLD mainly revolves around your design’s application. The explanations below should get you started.

Thermal management and signal integrity

New CPLDs have less power consumption, thus making them more energy efficient in thermal management and battery-powered devices. 

Contrarily, FPGAs need thermal management like an evaporative heat exchanger and heat sink because they switch at high frequencies. 

FPGA VS CPLD Integrated circuits

The differences in ICs dwell on their characteristic features summarized in the comparison table. For instance, FPGAs can have about 100,000 logic blocks, whereas CPLDs have only 1000. Therefore, FPGAs work well in complex applications and computations, while CPLDs favor simple applications. 

Conclusion

In summary, CPLDs and FPGAs are crucial integrated circuits in digital systems. Further, they both have unique features making them effective in specific applications. The article has outlined all that you need to know concerning the two.

For more information or queries, however, you can contact us

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Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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