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Chapter: Digital Logic Circuits - Asynchronous Sequential Circuits and Programmable Logic Devices

The PLD Advantage

Fixed logic devices and PLDs both have their advantages. Fixed logic devices, for example, are often more appropriate for large volume applications because they can be mass-produced more economically.

The PLD Advantage

 

Fixed logic devices and PLDs both have their advantages. Fixed logic devices, for example, are often more appropriate for large volume applications because they can be mass-produced more economically. For certain applications where the very highest performance is required, fixed logic devices may also be the best choice.

 

However, programmable logic devices offer a number of important advantages over fixed logic devices, including:

 

·        PLDs offer customers much more flexibility during the design cycle because design iterations are simply a matter of changing the programming file, and the results of design changes can be seen immediately in working parts.

 

·        PLDs do not require long lead times for prototypes or production parts - the PLDs are already on a distributor's shelf and ready for shipment.

 

·        PLDs do not require customers to pay for large NRE costs and purchase expensive mask sets - PLD suppliers incur those costs when they design their programmable devices and are able to amortize those costs over the multi-year lifespan of a given line of PLDs.

 

·        PLDs allow customers to order just the number of parts they need, when they need them, allowing them to control inventory. Customers who use fixed logic devices often end up with excess inventory which must be scrapped, or if demand for their product surges, they may be caught short of parts and face production delays.

 

·        PLDs can be reprogrammed even after a piece of equipment is shipped to a customer. In fact, thanks to programmable logic devices, a number of equipment manufacturers now tout the ability to add new features or upgrade products that already are in the field. To do this, they simply upload a new programming file to the PLD, via the Internet, creating new hardware logic in the system.

 

Over the last few years programmable logic suppliers have made such phenomenal technical advances that PLDs are now seen as the logic solution of choice from many designers. One reasons for this is that PLD suppliers such as Xilinx are "fabless" companies; instead of owning chip manufacturing foundries, Xilinx out sources that job to partners like IBM Microelectronics and UMC, whose chief occupation is making chips. This strategy allows Xilinx to focus on designing new product architectures, software tools, and intellectual property cores while having access to the most advanced semiconductor process technologies. Advanced process technologies help PLDs in a number of key areas: faster performance, integration of more features, reduced power consumption, and lower cost. Today Xilinx is producing programmable logic devices on a state-of-the-art 0.13-micron low-k copper process - one of the best in the industry.

 

Just a few years ago, for example, the largest FPGA was measured in tens of thousands of system gates and operated at 40 MHz. Older FPGAs also were relatively expensive, costing often more than $150 for the most advanced parts at the time. Today, however, FPGAs with advanced features offer millions of gates of logic capacity, operate at 300 MHz, can cost less than $10, and offer a new level of integrated functions such as processors and memory.

 

Just as significant, PLDs now have a growing library of intellectual property (IP) or cores - these are predefined and tested software modules that customer can use to create system functions instantly inside the PLD. Cores include everything from complex digital signal processing algorithms and memory controllers to bus interfaces and full-blown software-based microprocessors. Such cores save customers a lot of time and expense --it would take customers months to create these functions, further delaying a product introduction.

 

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