ddr with multiple users

Weng,

It occured to me that your circuit was identical to the ring buffer one. N users each had a fifo going to the DDR. Then there was one stream coming out of the DDR, so it's (N+1) interfaces. But then you said each user needs its own fifo so it can store + forward data from the DDR. So you've got 2N interfaces effectively. The new fifos are just moved into the user's realm and not part of the DDR controller.

My point is the same circuitry exists in both cases. You've just exercised some creative accounting :).

-Dave

HI David, I am really surprised to what you recognized.

They are totally different.

This is ring topology:

A --> B --> C --> D ^ -----------------------|

This is my topoloty:

A --> E --> | --> A B --> E --> | --> B C --> E --> | --> C D --> E --> | --> D

Weng

Real-world ring-buses:

- IBM Power4 Multi-Chip-Module core-to-core interconnect

- IBM Power4 MCM-to-MCM interconnect

- IBM Power4 system-to-system interconnect

- ATI X1600/X1800 memory ring-bus

IBM made lots of noise about its ring bus architecture a few years ago but I am pretty sure I read about something similar many years earlier. I am guessing Power5 must be using much of the same even though IBM did not make as much noise about it.

Right, ok I didn't understand what ring topology was, sorry. Snip out my reference to ring topology then but my observation still goes. E is the DDR, right? Anyway you don't show the fifos associated with ABCD on the right side.

-Dave

Hi David, Yes, every component has a read fifo internally. I don't have to show them in the interface. They are not the part of interface.

1. We are talking about how to implement a multiple DDR controller interface;
2. We are talking about its performance efficiency;
3. We are talking about the minimum number of wires of the DDR controller interface;

An individual read fifo for each component will give designers a freedon or a buffer to isolate many differences among DDR and compoments.

For example, they may use different clocks, different data rate and so on.

Weng

Hi Daniel,

1. IBM uses ring buses in its core-to-core interconnect. It is a good option there, because time latency among cores are not urgent. That saves a fast switch design among multiple CPUs. You may imagine if there are 16 cores, how difficult it is to design a fastest switch among the 16 cores!!! And IBM has good experiences with ring net system.

1. I guess you cannot list a 2nd example to use ring buses in a CPU among its tens of registers. Why? Internally nodoby could afford the clock latency among any registers.

2. You option to use ring buses in your application is justified: you are not concerned about the latency.

Thank you for ring bus information.

Weng