What are the most important benefits of DDR?

[glb]

I don't quite get the big deal with DDR... meaning is there a big savings by having the clock run at the data rate as opposed to 2x?

Say the data is toggling at 50 Mhz, DDR would use a 50 Mhz clock, but a single data rate would use 100 Mhz clock. No big deal.

But say the data rate is high, say 500 Mhz, then DDR would let you avoid routing a 1 Ghz clock, as a 500 Mhz clock would do it... is that the key benefit?

 

[bert_kak]

Re: DDR??

I don't quite get the big deal with DDR... meaning is there a big savings by having the clock run at the data rate as opposed to 2x?

Say the data is toggling at 50 Mhz, DDR would use a 50 Mhz clock, but a single data rate would use 100 Mhz clock. No big deal.

But say the data rate is high, say 500 Mhz, then DDR would let you avoid routing a 1 Ghz clock, as a 500 Mhz clock would do it... is that the key benefit?

Using double data rate (DDR) means that you double the bandwidth with the same frequency thus making the signal integrity requirement on the board lower than making the clock doubled.

Added after 1 hours 44 minutes:

 

[glb]

Re: DDR??

Right, so if the single-rate clock is manageable, then there's not much benefit going with double rate?

 

[bert_kak]

Re: DDR??

The lowest clock frequency you can avail in DDR is 100Mhz which means that it has a transfer data rate of 200Mbps which is greater than the maximum clock offered in SDRAM of 166Mhz and has a transfer data rate of 166Mbps. But if you're only looking theoritacally SDRAM of 200Mhz will have the same effect as DDR of 100Mhz in terms of speed. But DDR uses lower supply voltage than SDRAM, which leads to less heat dissipation and improved power management.

 

[glb]

Re: DDR??

Thanks. I am talking theoretical - and not about RAM per se