Marc Greenberg, Director of Product Marketing in the Cadence SoC Realization Design IP Group, just sent me some slides in connection with the recent introduction of the Cadence design and verification IP portfolio for LPDDR3 low-power SDRAM. I’ve already written a blog about the portfolio introduction (see “Upgrading your mobile or low-power SoC to LPDDR3? Got the right IP? You’ll need it to get maximum performance at minimum power”) but there are slides in Marc’s presentation that detail where LPDDR3 SDRAMs (and Wide I/O memory) fit in the low-power SDRAM universe and that’s information well worth discussing.
First, here’s a graphic showing you the SDRAM universe prior to the introduction of LPDDR2 SDRAM.
As you can see, this is a pretty messy universe. If you don’t need much memory and you don’t need much memory bandwidth, then parallel SRAM (PSRAM in the image) works for you. If you need moderate capacity and don’t need a lot of memory bandwidth, then low-power single-data-rate SDRAM might work. For larger memory capacities you will definitely want some form of DDR SDRAM because that’s how you get the most memory capacity for your money.
You have a lot of DDR choices including LPDDR1, DDR2, and DDR3 SDRAM. The above graphic helps a lot in selecting the right SDRAM technology based on desired system memory capacity and peak memory bandwidth. However, the introduction of LPDDR2 memory simplified this SDRAM universe quite a bit, as the following graphic illustrates:
You can see that if you want low-power SDRAM and you need any sort of capacity at any peak bandwidth up to 16 Gbits/sec, then 16-bit-wide LPDDR2 SDRAM is what you want at the moment. If you need more memory bandwidth, then you switch to (non-low-power) DDR memory or you use 32-bit-wide LPDDR2, as shown in this next image:
With 32-bit LPDDR2 SDRAM, you get all the way to a peak memory bandwidth of 32 Gbits/sec.
But today we’re entering the age of massively powerful mobile multicore processors such as the just introduced Samsung Exynos 5 Mobile Application Processor with two ARM Cortex-A15 CPUs cranking at 2GHz and a WXGA 3D graphics processor that drives a display at 60fps. These are very thirsty creatures and they crave memory bandwidth to process images, video, and audio. And don’t forget responsiveness when it comes to first-person-shooter twitch video games.
Now you’re talking about needing even more memory capacity and a lot more memory bandwidth. Now you’re talking about needing LPDDR3 SDRAM or even Wide I/O SDRAM.
This next graphic shows you where these newest SDRAM variants fit:
Both LPDDR3 (with two 32-bit channels) and Wide I/O memory can take your system beyond 100Gbits/sec in peak memory bandwidth, which is more than three times the bandwidth than shown in the previous image. The two different SDRAM technologies achieve this peak bandwidth in very different ways. As the name implies, Wide I/O SDRAM employs a very wide interface—512 bits—running at a leisurely 200MHz clock. LPDDR3 SDRAM cranks the clock to 800MHz and uses double-data-rate signaling to move 1600 Mbits/sec/pin. Your choice.
Nice to have choices.