Author Topic: What's the point in DDR?  (Read 951 times)

0 Members and 1 Guest are viewing this topic.

Offline Boscoe

  • Regular Contributor
  • *
  • Posts: 140
What's the point in DDR?
« on: May 14, 2018, 08:50:24 pm »
I've always found the objective of lowering the clock speed while maintaining the data rate strange. Who cares for having just one of your 40 signals of your RAM going at half the bandwidth as the rest? The 8/16/32/64 bits of data is still running at twice the clock rate. It doesn't seem to have any significant benefit to me and only makes clocking more of a hassle in logic.

What am I missing?
 

Online langwadt

  • Frequent Contributor
  • **
  • Posts: 776
  • Country: dk
Re: What's the point in DDR?
« Reply #1 on: May 14, 2018, 08:54:01 pm »
I've always found the objective of lowering the clock speed while maintaining the data rate strange. Who cares for having just one of your 40 signals of your RAM going at half the bandwidth as the rest? The 8/16/32/64 bits of data is still running at twice the clock rate. It doesn't seem to have any significant benefit to me and only makes clocking more of a hassle in logic.

What am I missing?

with ddr clock and data is at the same rate
 

Offline Boscoe

  • Regular Contributor
  • *
  • Posts: 140
Re: What's the point in DDR?
« Reply #2 on: May 14, 2018, 09:02:53 pm »
That's true, I hadn't thought it through properly. Still, is it worth it?
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 11158
  • Country: gb
    • Mike's Electric Stuff
Re: What's the point in DDR?
« Reply #3 on: May 14, 2018, 09:24:52 pm »
Transmission line effects. the rising edge and falling edges both need to propagate and do any ringing, settling down etc. before the next transition. With DDR you send data on each transition, so for a given settling time, you get twice as much data
 
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline cyr

  • Regular Contributor
  • *
  • Posts: 247
  • Country: se
Re: What's the point in DDR?
« Reply #4 on: May 14, 2018, 09:26:46 pm »
For DDR RAM, only the data and associated strobe signals run at DDR.

Think of it as doubling the transfer rate without having to increase the speed of the clock or the address/control lines (which typically have more load because they go to all RAM chips).

 

Offline NorthGuy

  • Frequent Contributor
  • **
  • Posts: 913
  • Country: ca
Re: What's the point in DDR?
« Reply #5 on: May 14, 2018, 11:01:58 pm »
That's true, I hadn't thought it through properly. Still, is it worth it?

There's maximum pulse width you can handle. Say, it is 1ns. This dictates the maximum clock rate - 500MHz for 1 ns pulses. If you use SDR, you use 2 ns pulses for data and the best you can do is 500Mb/s. But with DDR you use 1 ns pulses, which gives you 1Gs/s. Is it worth it?
 

Offline Berni

  • Super Contributor
  • ***
  • Posts: 1264
  • Country: si
Re: What's the point in DDR?
« Reply #6 on: May 14, 2018, 11:10:47 pm »
There is also the benefit of the driving logic inside the chips. Typically DDR is implemented by simply copy pasting the input logic, but inverting the clock on one of them. That way the logic inside also runs at the same clock speed as before yet it can get twice as much data trough the same pins as one half is handling the rising edge and other the falling edge data.

Similar is done on the transmit side. There can be two copies of the data pin driving logic while the clock line drives a MUX that ping pongs between the two, showing the output of one on the high state of the clock and the output of the other on the low state.
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 7118
  • Country: us
  • DavidH
Re: What's the point in DDR?
« Reply #7 on: May 15, 2018, 12:13:31 am »
Think of it instead as using both edges of a single clock signal instead of using a 180 degree biphase clock which would require two clock lines.
 

Online langwadt

  • Frequent Contributor
  • **
  • Posts: 776
  • Country: dk
Re: What's the point in DDR?
« Reply #8 on: May 15, 2018, 02:53:27 am »
Think of it instead as using both edges of a single clock signal instead of using a 180 degree biphase clock which would require two clock lines.

it does anyway since the clk is differential
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 7118
  • Country: us
  • DavidH
Re: What's the point in DDR?
« Reply #9 on: May 15, 2018, 04:20:19 am »
Think of it instead as using both edges of a single clock signal instead of using a 180 degree biphase clock which would require two clock lines.

it does anyway since the clk is differential

But that is not how the differential clock lines are being used.  Internally after a differential receiver which rejects common mode noise, either positive and negative edge flip-flops or latches are used or the clock signal is converted to a biphase clock.  Toggling one of the external differential clock lines does not toggle half of the flip-flops or latches; externally it is just one differential clock and not two single ended biphase clocks.
 

Offline NorthGuy

  • Frequent Contributor
  • **
  • Posts: 913
  • Country: ca
Re: What's the point in DDR?
« Reply #10 on: May 15, 2018, 04:44:19 am »
it does anyway since the clk is differential

A differential line uses two wires, but it is still one line.
 

Offline Berni

  • Super Contributor
  • ***
  • Posts: 1264
  • Country: si
Re: What's the point in DDR?
« Reply #11 on: May 15, 2018, 05:27:54 pm »
The reason behind differential pairs is mostly to get around the pesky parasitic effects of PCBs

Toggling a diff pair doesn't send any common mode current into the ground while keeping all currents that are part of the signal tightly against each other to couple them. This makes the pair capable of carrying very fast signals over a long distance without degrading them. But the fact that they reject common mode interference on the receiving end is also a very useful thing because the ground plane in the PCB might ring about at 100mV or more in these very high speed digital systems.
 

Online langwadt

  • Frequent Contributor
  • **
  • Posts: 776
  • Country: dk
Re: What's the point in DDR?
« Reply #12 on: May 15, 2018, 05:32:42 pm »
Think of it instead as using both edges of a single clock signal instead of using a 180 degree biphase clock which would require two clock lines.

it does anyway since the clk is differential

But that is not how the differential clock lines are being used.  Internally after a differential receiver which rejects common mode noise, either positive and negative edge flip-flops or latches are used or the clock signal is converted to a biphase clock.  Toggling one of the external differential clock lines does not toggle half of the flip-flops or latches; externally it is just one differential clock and not two single ended biphase clocks.

I know how it works, the point was that one differential and two single ended clocks require the same number of wires
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf