Most microcontroller designs require a certain amount of "glue logic", which is used to connect the microcontroller to peripheral components. This logic can consist of logic gates, buffers, etc. In the past discrete gates consisting of multiple chips would be used on a board. Today, FPGA or CPLD devices are used to replace all of this digital logic into a single complex programmable logic device.
There are several advantages to using CPLDs instead of multiple small scale logic chips:
CPLDs are very affordable these days. For example, the Xilinx XC9536XL CPLD can be had at under two dollars in moderate quantities.
Some of the larger FPGAs from Xilinx allow an amazing amount of integration. For example, you can integrate a PCI interface with a virtual microcontroller core, all on a single FPGA.
Before the advent of hardware definition languages such as VHDL, engineers would use schematic based entry for FPGAs, or a myriad of cryptic languages for small programmable logic chips. The main problem with schematic entry of logic design is that it is not portable among venders, and is time consuming to enter.
Hardware definition languages such as VHDL or verilog make digital designs completely portable across chip vendors, and allow you to even create your own custom ASIC if you so desire.
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