Do SDRAMs are meant to be used with fast MCUs?

[Dimitri]

Hi,
I was planning to make a DSP based design. It was  needed to be low-power so I decided to use TMS320C5505.
While examining the c5505 EMIF documents I come across with the following explanation: (you can reach from here pg87:  goo.gl/n1fToS)

This device has limitations to the clock frequency on the EM_SDCLK pin (driven by SDCLK) based on the
CVDD and DVDDEMIF.
• When CVDD = 1.3 V, and DVDDEMIF = 3.3 V, 2.75 V, or 2.5 V, the maximum clock frequency on the
EM_SDCLK pin is limited to 100 MHz (EM_SDCLK ? 100 MHz). Therefore, if SYSCLK ? 100 MHz, the
EM_SDCLK can be configured either as SYSCLK or SYSCLK/2, but if SYSCLK > 100 MHz, the
EM_SDCLK must be configured as SYSCLK/2

So basically -as far as I understand- it says you need to choose an SDRAM that works half of the clock rate of your DSP. I went an searched the SDRAMs in digikey. But all of the devices were working above 100MHz. So they are not suitable for using with that chip. (Unless I somehow could use in lower clockrates.) So I deduced that SDRAM are meant to be used with "not so much low-power" devices running at least couple hundreds of MHz?

But then I look the memory map of this chip, c5505, and half of its adress space is for SDRAM. So I am quite in doubt about my deduction. Does my assumptions are correct? How can I supply the need for extra RAM using this chip?

[dmills]

Don't just look at the first page of the datasheet, look at the actual timing diagrams, you would be surprised just how slowly you can run some of those parts.

All first pages of datasheets are written by the marketing slime (It was a less messy compromise then killing them all and hiding the bodies), and they all go on at length about how fast the things can go, forgetting both how tight that often makes timing and that not everything needs fast.   

Regards, Dan.

[ataradov]

SDRAM can be clocked much slower. There are plenty of MCUs with maximum clock frequency way less than 100 MHz, and they work just fine.

The best way to start is to find some development kit by the manufacturer or third-party.

[daveshah]

SDR SDRAM has no minimum clock frequency in theory (provided you can refresh it fast enough, but that's not a signficant real limit)

Later technologies (DDRx) do have a significant minimum clock, I believe it's 125MHz for DDR2 and 300MHz for DDR3