Issue link: https://iconnect007.uberflip.com/i/1491842
12 DESIGN007 MAGAZINE I FEBRUARY 2023 superimpose them, and you end up getting a square wave. Well, how square does that square wave need to be? is is the part that throws a lot of designers off. When we talk about rise time, we're really talking about the time it takes that square wave, the digital signal, to change from a logic 0 to a logic 1. As the die shrinks, that time also shrinks. About 20 years ago, we were having rise times and fall times in the multiples of nanoseconds—five to 10 nanoseconds. It took that signal five to 10 nanoseconds to change from a logic 0 to a logic 1. I was just looking at one FPGA with rise times as fast as 0.25 nano- second, and that's at 16 nanometers. My friends who work in next-generation sili- con at some of the big telecom companies are working in 5, 3, and 2 nanometer, and going sub 100 picosecond. Instead of 0.25 nanosec- ond, it's 0.1 nanosecond and 0.05 nanosecond rise times. ey're such incredibly fast rise times that the number of harmonics we need to create a vertical square edge that transitions from A0 to A1 that fast means that the frequen- cies involved in that superposition in that Fou- rier series are up into the multiple gigahertz of frequency content. at means that you're in the RF frequency range. Shaughnessy: Even if you're not an RF guy, there you are, dealing with RF. Moyer: You're in the same frequencies as an RF type of design, where you have to deal with skin effect and lossy transmission line models. Very-high-frequency designs that used to be the arena only for the RF guys are now affect- ing digital and analog, though not so much for analog. But digital engineers who are dealing with these fast rise times now have to consider all of this high-frequency RF content. I think a lot of engineers don't realize that with the Fourier series, that conversion, that content needs to be there to create a square wave of that fast of an edge. Basically, it says that frequency content exists. But I don't care what the clock frequency is. e frequency content exists in your square wave because the silicon has now shrunk, which means the DI/DT of that edge rate now contains this fre- quency content, whether you wanted it or not, and that is the single biggest issue that design- ers need to understand and fully conceptual- ize. In short, frequency content exists simply because the silicon has shrunk. Shaughnessy: ey're accidental RF engineers at this point. Moyer: Pretty much. Just 20 to 30 years ago, maybe you had a frequency of 100 megahertz and you could still get away with pretty easy design rules. e other big problem that hap- pens because of these edge rates is what we call the transitional electrical length or the distance that the signal will travel down the transmis- sion line while it's actively changing from A0 to A1. at distance has also shrunk, and this is the other problem with digital design. Historically we didn't have much problem routing digital signals, as long as the length of the trace that your signal is propagating down Kris Moyer