Wednesday, 31 July 2013

Complex Programmable Logical Devices

I already gave an small introduction about CPLDs in part III of the 3 Ghz counter build.
Cplds and Fpgas are very interesting devices for those who ever wanted to design their own integrated circuit but could not afford a million $ to let your prototype actually build.

A Cpld consists of the following elements:
Programmable And/Or matrices the macro cells
Programmable feedback
Input blocks
Output blocks

They distinguish themselves primarily in speed and the amount of programmable cells and pin count .
Unfortunately if you need many programmable macro cells the designers assumed you probably need many in out pins too. But many in out pins mean very small footprint and very home brew unfriendly.

So the hobby engineer is restricted to low cell count packages and therefore only small designs.

What are the advantages of a Cpld over a micro controller.
A Cpld can work massively parallel a  lightning speed.
The I/Os don't need a higher level control structure they work just by the logic assigned between an input and an output. They are often used for data capturing and collecting then they transfer the data to a high speed ram and then a micro controller  analyzes the data. Fast oscilloscopes would be unthinkable without high speed logic, micro controllers are ridiculously slow compared to Fpgas.

How to get started ?
Unfortunately programming and designing code for Cplds is completely different to program a micro controller and full of pitfalls.So the best way is to start small and use a simple low cell count target chip.
This is where Dangerous prototypes comes in, they designed two breakout boards based on the Coolrunner II series.One has a macro cell count of 72 and the other of 64.So you wont be able to run an Arm chip design on them but you could use them as multiplexer, inverter, ... You could reproduce the basic logic design blocks with them and combinations of them. You also get used to Verilog the programming language.
Later one if you decided you like the environment you could easily transit to the bigger high cell count Cplds.

To get started I would suggest the CoolRunner II XC2C64A CPLD with 64 macro cells and an Altera Usb Blaster programmer clone from Ebay.

As a first step I would work trough the Dangerous Prototypes tutorial section.

Cpld devboard and programmer

Dangerous Prototypes Cpld Board
Cpld Forum

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