What I'm doing this summer - part 2

All right. Now that you've taken a quick view of reconfigurable hardware, we can look at the FPAA in particular. How is it different from an FPGA? Well the main difference is how information is encoded.

Instead of passing information as digital ones and zeroes, analog processing allows you to take advantage of the full available voltage range and even curents to carry information. This means you can do great things like carry 5-9 bits of information on a single wire. So what's the catch? Well analog signals aren't as pretty as digital signals. If you only have to differentiate between 0 and 1, you're going to have a much easier time at it than if you have to be able to differentiate 2.4 and 2.5. And in some cases it gets much worse than that - if you're using a voltage as to control a transistor's gate, minute changes are vastly amplified. This means that you're accutely vulnerable to many things that digital systems barely have to consider.

Are your lines near each other? They'll have parasitic capacitances that'll couple them. Did you route it using a global line instead of 2 nearest-neighbor lines? You can expect different performance and in some cases even complete loss of functionality since the parasitic capacitance from routing is extremely significant. Are you in place that has a temperature? If so, you'll get thermal noise that'll mean that all of your signals have some randomness associated with them. These problems, especially the last one, are not easy to solve.

So why bother? Well, like I mentioned, you can carry more data on a single wire. Why does this matter? Well you can do things like run two relevant signals into a multiplier that uses only a handful of transistors and get out a direct answer. This is far faster and less power-hungry than the digital method of splitting your signal up to 1 bit per wire, running it into a many-stage multiplier that is far slower and takes hundreds of times more transistors and eats through way more power. Sound awesome? That's because analog is awesome - just difficult.

So how does an FPAA work? Let me start by describing the structure. There are two main parts to the structure, the routing grid and the CABs (Computational Analog Blocks). If we look at a single column, there are lines that run the length of the chip (global verticals), lines that run from one CABto a neighboring CAB (nearest neighbor lines), and lines that only run within a CAB (local lines). Each horizontal line is attached to something in the CAB. Thus, to connect components, you just have to attach two horizontal lines to the same vertical line. This is done by turning on switches. Additionally, there are horizontal lines that can handle inter-column connections in a similar manner.

The actual switches used in the FPAA are (who'd guess it) analog switches. They are actually capable of being partly on. This allows the connections to take part in calculations if you're clever enough to work them into your design. The way these switches work is by using floating gate transistors. Transistors can pass an amount of current that's controlled by their gate voltage - building up charge on a floating gate allows you to set a voltage and then simply leave it alone without having to constantly source the appropriate bias voltage.

So what's in the CABs? Well this actually varies. Some of the common things in CABs are nMOS and pMOS transistors, capacitors, OTAs (Operational Transconductance Amplifiers), Gilbert Multipliers and current mirrors. What are all of these? I'll give some explanation in my next post.