August 15, 2005
Rambus XDR2
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Jack Horgan - Contributing Editor

by Jack Horgan - Contributing Editor
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GDDR is primarily supplied by Samsung. They have about 85% of the GDDR market. There are efforts by Hynix and Infineon at this time to produce GDDR3. These are still very small players, about 5% of the market each. Micron is also trying to come up with some GDDR offering.

Other than NVIDIA and ATI who are some of the possible end users of XDR2?

Basically anybody who produces graphics chips. NVIDIA and ATI make up a huge percentage of the market. Their combined total dwarfs any other market shares. But we clearly have partnership with companies like S3 and SGI and want to continue those relationships. We feel that XDR2 could bring value to their products as well. Those are the ones off the top of my head but anyone who plays in the graphics space.

This product is initially targeted at graphics industry although there are some applications in the consumer electronics and networking space …

This product is initially target at graphics similar to the way that XDR1 was. The reason XDR1 was primarily adopted for the cell processor is because it offered the highest possible performance that you could get. It resonated well with Sony because they were targeting the cell processor for Play Station 3. Since then more vendors in the consumer electronics space specifically DTV and microdisplays have seen the value of using XDR because they can actually reduce system cost and component count. XDR2 we imagine will follow suit and really provided this unprecedented performance that would allow a graphics vendor or game console maker to differentiate themselves from their competition.
Then as memory technology progresses, it would become something that is more mainstream, something that could penetrate into the consumer electronic and networking markets. At least for XDR1 we are looking to get into main memory as well for applications like servers and PCs.

What market share does XDR1 command?

This is a unique technology. There are currently no products shipping in volume with it. The cell processor is the first announced product that has it and that is coming to bear currently. Sony is releasing the first cell based product in the spring of 2006. IDC estimates that by 2009 over 800 million units of XDR1 will have shipped primarily driven by game console and consumer electronics. We have a number of customers in the CE space as well but at this point all of them are still confidential.

Some of the technologies in XDR2 such as differential signaling are contained in other Rambus' products.

Differential signaling was essentially invented for XDR. It is also used in FlexIO processor interconnection which is a high speed logic to logic interconnect. It came from the same base core technology development as XDR. It was the incarnation of XDR that help interconnect two chips, kind of like a front side bus would if you look at an Intel Northbridge connect to an Intel CPU. Something like that would be a good application for FlexIO interconnect. The cell processor also integrates FlexIO to enable multiple cell processors to talk to each other as well as peripheral chips like graphics synthesizer and Southbridge.

Rambus is the only vendor using differential signaling.

XDR is the only memory technology currently using differential signaling for its data. There are some generations of DDR that use differential signaling for their strobes. Currently DDR still uses single ended data.

What are the advantages of the Rambus approach?

If you get to higher data rates like 3 GHz or 8 GHz differential becomes your best viable option. The primary advantage of differential signaling is that it is cleaner from a signal integrity standpoint. We are able to reach orders of magnitude higher bandwidths using orders of magnitude lower swing. Using 200 milivolt differential swing as opposed to DDR which is more than one volt. Another key benefit of having differential signaling is the fact that it is very immune to crosstalk and it is well suited to emi (electromagnetic interference) sensitive applications. That's another key point that resonates well with consumer electronic vendors because any technology that requires them to
add more emi shielding adds cost to their overall product line. By the mere fact that it is 200 milivolts, it's also much lower power from the interface standpoint. On the silicon side the power distribution becomes easier because the way differential signaling drivers and receivers work. It is not like you are constantly switching on and off massive amount of current from the power supply. It is basically a constant current drop from the driver and receivers. So it is easier to design.

You are optimistic about XDR2's future based on the parallel with XDR1 in that applications other than those you are initially targeting will come on board.

Absolutely. It is not that we are not targeting those applications at the onset, we just see the initial applications, the initial adopters to be in the graphics arena similar to XDR1. The value doesn't currently exist for consumer electronics and networking but just from an historical perspective we would expect graphics vendors to pick up the technology first.

Does incorporating this technology present any challenges to EDA tools?

We do a lot of work up front to ensure that the development of the technology is as easy as possible. Historically what we have done especially with XDR1 and DRAM is to have design the interface for our customers. We can do that in any design flow whether it is COT, ASIC, ASAP or you name it. A lot of that is essentially dealt with up front by our engineers. What this does for the end customer is that it reduces the amount of risk that they take on. They don't need to worry about designing a difficult XDR2 interface, if their core competency is graphics COPU or networking packet buffers. As the other memory technologies increase in speed it is difficult for them to simply integrate a
memory controller.

Any challenges on the manufacturing side?

We are still evaluating it from a technology standpoint. Currently there is nothing we have to publicly announce about the different technologies or the manufacturing differences between XDR1 and XDR2. Over the next year or two it will be something we are looking at internally.

I thought it would be helpful to give a few details about micro-threading, a DRAM core innovation developed to increase memory system efficiency and to enable DRAMs to provide more usable data bandwidth to requesting memory controllers, while minimizing power consumption. See figure below comparing traditional memory core to micro-threaded core.

Most DRAM cores divide their memory storage into discrete banks that can be accessed concurrently. Banks are typically split across both rows of the DRAM die. Since DRAM pins are also split across the two halves, each half-bank delivers its data to the pins that correspond to its half. Each time a row within a bank is accessed, the DRAM core dedicates resources on both sides of the DRAM.

A typical DRAM core component has eight independent banks. One bank consists of an “A” half connected to “A” data pins and a “B” half connected to “B” data pins. The two bank halves operate in parallel in response to row and column commands. A row command selects a single row within each bank half, and two column commands select two column locations within each row half. Each group of four bank halves (a “quadrant”) has its own set of column and row decoder circuits. However, these resources are operated in parallel, with each transaction utilizing two diagonal quadrants and not using the other two quadrants.

After a row command is received, the selected row is accessed (sensed and latched). Before another bank can perform a row access, a time tRR must elapse. This time interval represents the time the bank's row circuitry is occupied. After a column command is received, the selected column is accessed. In the case of a read command, the data at the memory location is driven onto the data pins and in the case of a write command the data on the data pins is stored in the location. Before the bank can perform a column access, a time tCC must elapse. This time interval represents the time the bank's column circuitry is occupied. 16 bits are transported
on each link during a column access. With 16 data links, the column granularity is 32 bytes. The row granularity is 64 bytes.

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-- Jack Horgan, Contributing Editor.

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