DDR2 + CycloneIII data rate - question for understanding

Altera_Forum · ‎09-01-2010

Hello,

I'm using Cyclone III Dev. Kit and I've got a short question for understanding regarding the Cyclone III DDR2 interface speed:

On page 8 in "1. Cyclone III Device Family Overview" is said:

"... DDR2 SDRAM memory interfaces support data

rates up to 400 Mbps for Cyclone III devices and 333 Mbps for Cyclone III LS devices. ..."

So does it mean that I can't read data from DDR2-RAM with more than 400Mbps?

So I also used the VIP-Suite. There is a Frame Buffer, which puts video data to DDR2-RAM using the DDR2 High Performance Controller. So if I have a video stream 1080p60 that means:

1920px * 1080px * 24bit (color depth) * 60Hz = 2986Mbps

And the system works. So how could it be? Because 2986Mbps is much faster than 400Mbps... And what is the maximum data rate I can use Cyclone III + DDR2 High Performance Controller?

Thanks for really every hint!

Best regards,

tonib

Altera_Forum · ‎09-01-2010

Actually the data sheet should better say : 400 'Mega transfers per seconds'. As the SDRAM device is 16 bits wide (I guess) it has a peak transfer-rate of 400 * 16 = 6400 Mbps, well above what you need.

Altera_Forum · ‎09-01-2010

Hey josyb,

thanks for your explanation! :-)

Altera_Forum · ‎09-02-2010

Hi,

The CIII dev kit has a total of 72 bits wide DDR2 memory (split between the top and bottom banks as 32 bits and 48 bits), so if you use both banks simultaneously (and I am not sure whether you can do this with a single HPC DDR2 controller) then you have 72*400 = 28gbps bandwidth.

Remember that with the frame buffer you have a reading and writing port, so to sustain 1080p60 through the frame buffer you need twice the bandwidth of the video stream. 1920 * 1080 * 60 * 24 * 2 = about 6gbps. Which is still much less than what you have available, so it seems easy.

You also need to have a system clock in excess of 125MHz to be able to sustain the data transfers of the Avalon ST interfaces (1920*1080*60=124.4M). If you run the DDR2 at 200MHz, then this would be s logical choice of system clock, but in my experience, on a CIII, this is not that easy to achieve and for any realistic system (not just a demo), you end up with a system clock of 90, maybe 100MHz. Although it is possible to get a 1080p60 stream through a CIII, it is not easy and you would probably need a fast speed grade device (also note that 200MHz DDR2 is only possible on the fastest speed grade device. On the normal speed grade -8 device, you are limited to 166MHz).

Regards,

Niki