MORE ABOUT FPGA

Modern circuits make ever more extensive use of FPGAs. Special software is required when designing with these devices. One of the companies that produces such software is Altium. In order to allow their software to be evaluated, they offer a cheap evaluation kit. Once the evaluation period has expired, the kit is still very useful as a development platform for your own designs!

In September 2002 we published a JTAG interface that was compatible with the programming software from Altera. Unfortunately, the software from Xilinx didn’t work in combination with this interface.The interface published here is compatible with the software from Xilinx, so you can use it to program their range of CPLDs and FPGAs.The circuit is very simple and consists of just two ICs and a handful of discrete components.

In sharp contrast with plummeting prices of FPGA chips, suitable development systems for these interesting devices are costly and hard to find. However, that situation seems to have changed for the better with the arrival of a low-cost kit from Australia-based BurchEd.The FPGA (Field Programmable Gate Array) is a programmable logic building block for use in complex digital circuits. Using either a hardware descriptive language or a graphics editor, users of FPGAs are able to design logic circuits that can be downloaded into an FPGA chip. The result (hopefully) is the FPGA acting as the desired logic configuration in all its complexity. As may be expected, FPGAs contain a massive amount of universal elements consisting of gates and/or flip-flops, depending on the FPGA chip you have selected for the job. These elements may be interconnected to form configurations, hence the ability of an FPGA to perform countless digital functions designed by you, the user.

For some time now, all sorts of designs using FPGAs, CPLDs and other programmable logic devices can be found on the Internet. The complexity of these designs is often limited to the level of simple adders, BCD-to-7-segment decoders and similar standard circuits, but there are also other circuits that can be downloaded from the Internet free of charge. Presently, a large group of people is working on implementing wellknown processors in FPGAs.

The testing of large logic ICs such as FPGAs, CPLDs, ASICs, etc. is very difficult and time consuming when conventional test probes are used. The measurement of internal signals that are not brought out to an IC pin is impossible in this way. A number of companies have therefore joined forces to find a common solution to these problems. The result of this is the JTAG interface (IEEE 1149.1).In the industry (and also increasingly in the electronics hobby world) more and more use is made of larger, complex ICs. The advantages of these ICs are clear: the PCB can be smaller, the current consumption is less, parts of the design can be easily re-used, etc. Unfortunately this development also has a few disadvantages. The testing of these ICs hasn’t become any easier. In the case of SMD ICs the test probe has to be applied precisely in order to measure the signal at a specific pin and for BGA (ball grid array) ICs on a multi-layer PCB this becomes all but impossible.

So far in this series of articles, we have looked at two areas of FPGA usage. Having looked at the design methodologies available to us, and the usage of FPGA design software, we are now in a position to consider the hardware implications of FPGA usage.

Having looked last month at the ways in which we can design FPGAs, and coming down on the side of VHDL as a design method, we need to see how we can take our VHDL and turn it into a format we can user to program a physical device. This involves a number of steps from entering the design VHDL, turning it into a format our place and route software can understand, and then turn that information into a bitstream which can be used to program the device.

In professional electronics today, some of the greatest advances have been made in the field of programmable devices, especially those designed to implement logic functions. From using boards full of discrete logic devices (typically 74 series devices), now many complex functions are often implemented in a single programmable device. This is achieved by the use of devices with an ever-growing number of ‘gates’ and pins. Today, these devices are almost always Field Programmable Gate Arrays (FPGAs).