DDR3/4 Design & Layout Services





DDR (Double Rate Rate) memory configurations are constantly evolving. With each iteration of the DDR specification, chip providers are finding faster ways to move data to and from memory. Their approach is twofold – to increase the speed on the memory component and then to find configurations which increase the speed between PCB components.

The function of memory is to provide a stored data location to the component processing the information. Although microprocessors are an excellent example of a component that makes extensive use of external memory, other devices such as DSPs (Digital Signal Processors) and FPGAs (Field Programmable Gate Array) also use external memory.

Regardless of which component is using the memory, the routes between the processing component and the memory(ies) need to be properly designed. The two concerns are propagation times and signal integrity. The signal cannot arrive late in a non-readable condition.


I came to Nine Dot Connects with an extremely complicated PCB layout request, including multiple FPGAs with DDR4 interfaces and a large number of transceiver connections at 10+ Gbps. Virginia was the engineer assigned to the project, and she was very detail-oriented and did a great job with the many high-speed interfaces. Communication was always quick, and she was able to accommodate some schematic changes as the layout went on. In addition, she was able to leverage the FPGA experience of another engineer at Nine Dot Connects to help with the package skews of the FPGA. Despite the difficulty of the project, I am confident in the final layout.
Nima Badizadegan, EE - Calliphony


Memory configurations have changed through the years to optimize the throughput of the data to and from the memory(ies). DDR (Double Data Rate) takes advantage of both the rising and falling edges in a clock cycle.


In the past, memories only operated on the rising edge of the clock. Now that both edges are employed, any setup for the next signal, whether it be inbound or outbound, needs to be ready when the next edge occurs.

DDR3 is the currently the industry standard. However, DDR4 is making its way into the industry, especially in gaming computers. Regardless of DDR version, PCB layout must be able to handle the strict timing requirements while maintaining the integrity of the signal. This is not an easy task.

The following is an outline of the strategy Nine Dot Connects uses to design a DDR board:



Initial design (Schematic)



  • DDR3 Memory device selection
  • Termination schemes
  • Routing topology (fly-by vs. T-branch)
  • Differential pair declaration
  • Power and decoupling considerations

Processor Component consideration



  • Memory controller interface concerns
  • Pin usage (if FPGA)
  • Controller pin package delay calculations
  • Bit swapping of data for optimal routing paths
  • Reference voltages
  • Impedance control

Layer stackup



  • Number and type of layers
  • Use of microstrip vs. stripline
  • Symmetric vs. asymmetric striplines
  • PCB material and dielectric constants

Design Rules



  • Proper dimensions for correct impedance control
  • Size, type and use of vias
  • Custom rules for different layers
  • Signal group classes, (and use of Altium "xSignal" setup, if applicable)

High-speed routing



  • Calculate signal timing using "delay tuning" vs. just trace length tuning
  • Routing for best signal integrity
  • Pin swapping (if FPGA)
  • Proper use of serpentine traces
  • Return current paths
  • Correct usage of reference planes

How can Nine Dot Connects help you with your DDR3 design? We can assist with any or all the steps of DDR3 design and layout, doing as much or as little as you require. Everything from simple design reviews to full-on turnkey layouts, even if it is just the DDR3 portion of your project.

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The following is an overview of what one needs to do when designing DDR3 onto a PCB:



Schematics



DDR starts with the initial design prior to layout. Memory components need to be selected, as each memory manufacturer has their own requirements and recommendations. The routing topology (fly-by vs, T-branch) must be considered when drawing out the circuit in the schematics. Each topology type has its advantages and disadvantages.


Example of a T-Branch


Example of a Fly-By Route


These details will determine the component placement on the PCB. The schematic will generate the netlist for the PCB layout designer to follow during the trace routing between these components.

FPGAs



In many cases, DDR memory will be interfaced with an FPGA. Because they are configurable, the circuit board designer must balance the internal logic utilization aspect of the FPGA with the signals assigned to each of its pins. There can be hundreds of pins on one FPGA.


Example of a 1299 Pin BGA


What may look good on the schematics and work internally in the FPGA could be a routing nightmare in the PCB layout. Because FPGAs are configurable, pin swapping is permitted to alleviate the routes from crisscrossing or that are too far distant from each other. However, this can impact FPGA internal timing.

This is an iterative process of swapping pins, performing impact checks on the FPGA and modifying the schematic. Other aspects of the FPGA, such as slew rates (the speed in which the signal goes from logic ‘1’ to logic ‘0’ and vice versa), voltage levels (the voltage level required for a logic ‘1’ signal, and propagation delays (a.k.a. timing delays) that can be configured for each pin, must also be given consideration.



Layout



The layout of the board is a major factor in the design. The number of layers (a.k.a. the stack up) will determine the impedances for each of the layers. Impedances must be matched during the routing of the DDR memory signals to avoid signal integrity issues. The number of signal layers are not the only consideration. Planes of copper must also be considered to assist the signal propagation through the trace and to minimize electromagnetic field interferences between signal paths. The nonconductive material (a.k.a. dielectric) that separates the copper layers is critical in the impedance calculation as well.


iCD Stackup Planner can provide impedance information based on the stack up information


To assist with the impedance measurements, the designer works closely with the board fabricator. Based on the criteria described above, the fabricator can provide the preliminary impedance numbers to assist the designer during the routing process. Additionally, the iCD planner is used to allow the designer to further calculate impedances based on the layers used, the desires trace widths and the spacing between traces.


Match length tuning and understanding the propagation time per layer is imperative


Design Rules



The layout of the board is constrained using rules. These rules prevent the layout designer from placing routes that would cause signal integrity issues. They also dictate constraints such as trace width and spacing, component clearances, and differential pair spacings, which are derived from the stack up of the physical board, as described above.

Placement of Components



Component placement is critical to minimize congestion points between the DDR memories and the processing component. Topography selection during schematic entry will dictate a good portion of the placement.

Routing



The objective is to ensure the routes match timing requirements. This also requires an understanding how different layers impact the signal propagation speed. Propagation time for one layer is not the same for another. The designer must translate the propagation time into a wire length.

Since high speed signals are involved, one cannot neglect the field energy produced when a signal goes from logic’1’ to ‘0’ or vice versa. It must be ‘channeled’ appropriately, otherwise it will radiate as noise, bleed onto other routes, and harm the signal itself. Voltage planes must be correctly referenced and return paths must be absent of any copper voids.


Serpentines are employed to assist with match length tuning and propagation delay


In Summary



This process should not be rushed. Each decision made on the board must be made with careful consideration. The components need to be placed in an optimal configuration. Each route must be fine tuned to meet the timing requirements. In high speed design, proper methods must be employed to mitigate noise and maintain signal integrity. In this brief strategy discussion, we purposely neglected mentioning other important items such as via type and size, drill pairs, and decoupling requirements, to name a few.

In order to showcase the necessary steps involved in properly designing with DDR3 memories, we conducted two part webinar series. You can find details on this series here.

DDR3 Webinar Series



How can Nine Dot Connects help you with your DDR3 design? We can assist with any or all the steps of DDR3 design and layout, doing as much or as little as you require. Everything from simple design reviews to full-on turnkey layouts, even if it is just the DDR3 portion of your project.


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