With the advent of low-power CPLDs, low-power electronic product designers now have new options for implementing many of the functions traditionally performed by microcontrollers. This white paper discusses when it is advantageous to use a CPLD instead of a microcontroller, and when it makes sense to use a CPLD as a companion to a microcontroller.

Introduction

Tell a group of portable electronics designers that there is a low-power digital device that allows them to use a software program to reconfigure the operation of the hardware, and nine out of ten will assume that it is some form of a microcontroller. This is understandable. With a vast array of features and packages, abundance of software development tools, and huge library of application code, the ubiquitous microcontroller has found a home in nearly every type of portable application. However, with the arrival of low-power CPLDs, designers now have new options for implementing many of the functions traditionally performed by microcontrollers.

The examples given in this white paper are grouped into three categories, based on their function and level of complexity. The first category is I/O management, focusing on pin-level applications. The second category is port management, with the emphasis on the various interfaces between devices. The third category is system management, for applications that use a pin or port to control system-level functions.

Designers who are new to programmable logic will find many aspects of CPLD design to be similar to designing with traditional microcontrollers. A simplified description of CPLD design flow is as follows:

1. The design is written in a high-level language, such as Verilog or VHDL, using a software development tool.
2. The design is simulated for functional correctness.
3. The design is “fit” to a particular CPLD, providing the physical aspects such as resource utilization and timing paths.
4. The design is simulated for timing correctness. 
5. The design is programmed into the physical device. 

One major difference is the availability of sophisticated in-circuit emulators for microcontroller check out. However, once the nuances of the programmable technology are understood, microcontroller designers do well with CPLD design.

Examples of CPLDs Replacing Microcontrollers

This section describes some applications where a CPLD can cost-effectively replace a microcontroller.

I/O Management

When considering whether to use a CPLD or a microcontroller for I/O management, the quantity and the type of I/Os needed are critical considerations. Microcontrollers enjoy a reputation as small and inexpensive, and certainly, there are many small, inexpensive microcontrollers from which a designer can choose. However, if an application requires a large quantity of general-purpose I/Os, often a CPLD can be cost-competitive with a microcontroller. Small, inexpensive microcontrollers are usually limited to a serial port and, at most, a few general-purpose I/O pins. Designers discover that microcontrollers with dozens of I/Os are no longer so small and inexpensive. CPLDs, on the other hand, tend to be I/O intensive; it is not uncommon for a small form factor CPLD to have well over 50 I/Os. For example, the Altera® MAX® IIZ EPM240Z CPLD in a 5-mm-by-5-mm package has 80 I/Os.

Author: Rafael Camarota, Non-Volatile Product Line Manager, Low-Cost Products

Mr. Camarota joined Altera in 2002, where he is responsible for all CPLDs, and configuration products. He has more than 25 years of semiconductor experience and in the PLD industry. He holds a BSEE from Carnegie-Mellon University, and has over 25 patents relating to programmable logic circuits.