Design Your Own IC: Prototyping with CPLDs （part 1）

Published:2011/7/22 2:14:00 Author:Amy From:SeekIC

by Paul Goossens

Designing a circuit always involves several somewhat disagreeable tasks, such as soldering, looking up the pinouts of ICs and laying out a maze of tracks on a printed circuit board. All of this is now no longer necessary, since you can program your own digital circuit in a CPLD.

Experimenter’s board features
-CPLD version: EPM7128
-JTAG interface
-all I/O lines available via box headers
-supplementary I/O can be disabled using jumpers
-7-segment display
-8LEDs
-8 switches with debouncing circuitry
-on-board supply voltage regulator
-Eurocard format
-extensive development environment available free of charge from Altera

The CPLD experimenter’s board pre­sented here makes an excellent start­ing point for sitting down at the key­board and directly designing your own circuit.

For most people, experimenting with digital logic brings to mind an image of an experimenter’s board stuffed full of ICs interconnected by countless wires. Anyone who has ever worked in this manner knows that such an arrangement usually takes many hours to construct, not to mention the modifications that have to be made if the circuit does not work properly right off the bat.

A CPLD can be used to replace such a mforest’ of digital logic ICs. CPLD ICs contain a large number of digital building blocks that can be intercon­nected in a wide variety of manners simply by programming. Such an IC can actually be considered to be the modern version of an experimenter’s board full of ICs.

Instead of soldering all sorts of little wires in place, designers and experi­menters can now conveniently design circuits while seated in front of a PC, and then program their latest designs into the IC in a trice. This clearly saves a lot of time, and it has the advantage that it isn’t neces­sary to document modifications after­wards on paper or in some other man­ner. This is because the circuit is drawn (or described, as discussed later on) on the computer, after which the PC computes how the design must ultimately be realised in the IC.


CPLD organisation

Our experimenter’s board is based on the Altera MAX7000 family of CPLDs. The organisation of these CPLDs is shown in Figure 1. At the top of the drawing you can see the four special inputs for the CPLD. These signals are connected to the macrocells in the IC in a manner that allows them to opti­mally perform their intended functions (CLK, OE, and CLEAR). The inputs and outputs of the various macrocells are connected to each other as desired via the Programmable Inter­connect Array (PIA). There are also inputs and outputs that are connected to the various I/O Control Blocks. Their job is to connect the internal signals to the I/O pins of the IC.




Four Logic Array Blocks (LABs) are also shown in Figure 1. These LABs contain the actual programmable logic. Each LAB consists of 16 macrocells. Various CPLD versions are available in the MAX7000 family. Tne type number of the IC indicates how many macro-cells are present in the IC.

Macrocells

The actual programmable logic is located in the macrocells. The structure of a macrocell is shown in Fig­ure 2. Each macrocell has its own Logic Array, which consists of 36 inputs connected using a program­mable AND function. Four of these AND functions are combined into a single signal in the Product Term Select Matrix. The capabilities of the IC can be increased by further increasing the number of product terms. If you want to know more about this, we recommend reading the data sheet for these CPLDs.





The signal generated by the combina­tion of AND and OR operations then arrives at the register and its surround­ing logic. This portion can be used as a D-type, T-type, JK or SR flip-flop, or it can be completely bypassed if no regis­ter is needed.

Fortunately, as designers we do not have to be overly concerned about exactly how our designs can be imple­mented in this manner. This is where the (free) software comes to our aid. But before we delve into the software, let’s first take a closer look at the experimenter’s board.

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