Widening The Channels

By Ed Sperling
Wide I/O—both as a specific memory standard and as a generic approach for on-chip networking—has been looked at for the past couple of chip generations as a way of improving SoC performance. Increasingly, it also is being used as a key strategy for reducing energy consumption.

Wide I/O refers to a number of different approaches in on-chip networking, ranging from through-silicon vias in 3D stacks to interposers in 2.5D stacking. It also refers to a standard for memory communication being developed by JEDEC, as well as more dedicated channels for signals. In all cases, the added benefit is a reduction in power needed to drive a signal.

The tradeoff typically is between serial I/O and wide I/O. Serial I/O is simpler to design and works over longer distances, but it is far less power efficient. Wide I/O, in contrast, is higher bandwidth with big power savings—Samsung, for example, estimates its new 1Gbit mobile DRAM based on a 50nm process consumes 87% less power—but the technology is also more complicated to use. And in most cases, it’s also more costly.

Eliminating complexity while adding more
The concept of bigger pipes has always been a last resort for chip architects. It’s well known that shortening the distance a signal travels and reducing the resistance can drive down the amount of power needed for a signal. Reducing the overhead of serialization and deserialization can cut the power even further. But ironically, it has taken an explosion in SoC complexity for chip architects to seriously consider simplifying signal paths.

“We always go through this pendulum swing of what’s the optimal physical implementation vs. what’s the simplest way to do it even if it costs more silicon,” said Steve Roddy, vice president of marketing and business development at Tensilica. “So you can do things with 128 wires using serialized I/O, or you can do it with a lot fewer using wide I/O. The serialized I/O requires deserialization, which costs power. With wide I/O, which could simply be a lot of wires connected to the next block, you can lower the frequency and widen the channel.”

In a 2.5D stack, that extra silicon is easier to justify because it doesn’t add significantly to the overall footprint. In a system-in-package or package-on-package it may involve an interposer, which is another piece of silicon. It also can involve a through-silicon via in a 3D stack, which is wide enough to avoid any congestion.

“With a TSV you don’t need a standard I/O, which includes the I/O circuitry, patch and bond wire,” said Tom Quan, deputy director of design methodology and service marketing at TSMC. “So you get rid of all the I/O circuitry, and you have the same area, power and current. That results in a tremendous power savings. You also get a big boost in timing. And if you use an interposer, that’s silicon so it has the same resistance and capacitance of a standard IC. You can simulate them both together and get a predictable result.”

Eliminating bottlenecks
There are many good reasons for using wider pipes. One is that multicore and multiprocessor implementations generally are inefficient. The whole idea behind these implementations was that software would be able to run across multiple cores and multiple processors. That didn’t work out as planned, due to the inability to parallelize many applications, but cores were still designed to share the same memory.

That’s inefficient from a performance and a power perspective. Cores that are not in use should be turned off or powered way down. Moreover, when they need to connect to memory it should be along a clear path with as little congestion as possible and over the shortest distance possible.

“For some years to come we’re going to be seeing systems in package with interposers as the ideal solution,” said Joe Sawicki, vice president and general manager of Mentor Graphics’ Design-To-Silicon Division. “That will involve a lot faster interconnects, mostly to memory, and potentially to homogeneous logic. One of our customers was developing a digital chip and needed Bluetooth. They did it in a digital IC and they also did it in a SiP. The SiP destroyed the SoC in performance and power.”

But the question also is at what cost. While 2.5D approaches are relatively straightforward, the interposer does add some cost and the TSV can add even more.

“We are pursuing full 3D and so are most of the people in the phone business, primarily because of the form factor and cost,” said Riko Radojcic, director of engineering at Qualcomm. If you think about an interposer, you’re adding another die to the cost. Conceptually an interposer is an elegant solution and it works fine for someone who sells a product for $100. If you throw in a $1 interposer it’s no big deal. But if you’re making a $5 die and you throw in an interposer, it is a big deal.”

The same is true of through-silicon vias, although the ultimate advantages of this approach are expected to become more significant over time.

“TSV is expensive but is a good way of meeting the form factor,” said Navraj Nandra, senior director of marketing for Synopsys’ DesignWare Analog and MSIP Solutions Group. “You need to optimize for both low power and low cost packages. It’s like buying a $50k hybrid car that gives you 32mpg compared to a $22k 1.2L, 3-cylinder petrol engine car that gives you 50mpg. Everyone is excited about the hybrid car.”

Optimizing the signals
Behind the hubbub about the I/O technology is another often overlooked piece of the equation. The move to multiple processors and multiple cores was done largely as a knee-jerk response to the end of classical scaling at 90nm. What has happened since then is a much more measured response to how to use these cores more effectively, which requires much more granularity in the design process. Not all cores need to be on an ARM or MIPS processor, for example, and not all of them need to be in one place on an SoC—or even on the same die of a SiP or 3D stack.

In addition, not all of those cores or processors need to be the same size or run the same software.

“In addition to wide I/O there are dedicated point-to-point connections to relieve the system congestion,” said Tensilica’s Roddy. “Those can include general purpose memory and processor. When the system architect knows beforehand what’s going to be in the system they can add those connections up front. So you may have a video decoder and buffer and an audio decoder using separate memories, and those may change depending on whether they end up in a cell phone or a set-top box. But there are some things you don’t know at design time and you need the ability to generate system-specific interconnects, which is what’s being sold by companies like Arteris and Sonics.”

And finally, there is a simple mathematic principle behind the push to reduce power.

“The longer a signal has to travel, the more power it takes,” said Qi Wang, technical marketing group marketing director for Cadence Solutions Marketing. “A lot of issues in design come down to power. If you put the memory outside the chip, that takes power. If you want to speed up performance, that takes power.”

Bigger pipes over shorter distances can help solve that problem, and it’s a solution that is beginning to garner much more attention these days.