DDR3 Routing Guidelines for Trace Management and Data Grouping

November 7, 2019 Cadence PCB Solutions

DDR3 circuit board

 

I was branded a nerd when I was in university. Besides classes, I’d be holed up in my room playing FIFA and Warcraft for the rest of the day. That was until the newer releases required higher RAM capacity, which my aged-old computer lacked.

That was the start of a period where gaming requirements kept increasing drastically. As a student, I didn’t have the budget to upgrade the hardware, be it even a single piece of RAM. Today, computing memory has evolved to the DDR technology, and thankfully, I could afford to keep pace with the latest computing requirements for gaming. 

While plugging a DDR3 SDRAM into the motherboard is a piece of cake, designing a PCB that involves one offers great challenges. 

DDR3 Basics 

DDR stands for Double-Data Rate, and DDR3 SDRAM was first introduced in 2007 to replace its predecessor, DDR2. DDR3 memory chips remain relevant today, despite the introduction of DDR4 in 2014.

A DDR3 SDRAM supports data bus frequency of up to 1066MHz. However, data transfer between the memory chip and a microcontroller happens at twice the clock rate. For each clock pulse, two bits of data are transferred in a single data signal pin. 

Data exchange between a microprocessor and the DDR3 SDRAM is done over an interface that consists of an address bus, data bus, data strobe, data mask, and the clock signal. The data bus is 64-bits wide, and there are 15 address pins for accessing the entire memory cells. 

Challenges With DDR3 Routing

A DDR3 SDRAM on a DIMM package has 240 pins while the microprocessor that connects to the memory chips has larger pin counts. Depending on the module of the DDR3 SDRAM, the data rate ranges from 400 MHz to 1066 Mhz. Even at its lowest frequency, you’ll be dealing with high-speed signals on a limited amount of PCB space.

While the DIMM package has the pin spread out in a horizontal line, microprocessors often have their pinouts arranged in Pin Grid Array (PGA) or Land Grid Array(LGA) form factor. Chips with grid arrays pinouts are more challenging to route as there is lesser space on the PCB to maneuver.

 

Microprocessor with an LGA package assembled

DDR3 Routing is made harder with the microprocessor LGA/PGA package.

 

There are 64 data connections where bits are transferred at twice the clocking frequency. In such a limited space and high-speed signals, the top concern is crosstalk which will affect the signal integrity. Also, the trace length of the data, address, clock, and control signals are also crucial to prevent issues with propagation delay. 

Routing Guidelines for DDR3

DDR3 routing isn’t for the faint-hearted as you’ll be dealing with multiple high-speed traces on a crowded PCB. Here are some tips that will help you out.

Establish Data Grouping

On the DIMM DDR3 SDRAM, there are individual modules that are connected by the data strobes, often referred to as lanes. Each lane corresponds to 8 bits of the data bus. You’ll want to group each data strobe with the corresponding data mask and data signals.  

 

Circuit board with potential for signal fluctuation from PCI slots

Cross-coupling and propagation delay may affect DDR3 routing.

 

Route Data Signals First

Routing priority should be given to data signals. You’ll want to start by routing the grouped data signals from the microprocessor to the DDR3 RAM before moving on to address, and other control signals. The clock signal is routed last.

However, it is important to remember that the data, controls, address are referenced to the clock, and it’s important to prevent disparity in the length of these traces. 

Use Minimal Or Equal Amount Of Vias

It’s important to ensure that signal traces for a specific lane share similar impedance characteristics. Therefore, it’s best to avoid using vias. When vias are used, ensure that all traces on the same group have an equal count. 

Route Signal Traces Adjacent To Ground

All of the data, address, control, and clock signals must be kept as short as possible. Besides that, they must be routed adjacent to a ground plane to ensure a clean and short return path. This prevents high-speed noise coupling to other traces on the PCB.

Even when you’ve carefully routed the connections for a DDR3 SDRAM, it’s best to simulate the layout on the PCB analysis software. OrCAD PCB Designer provides the tools needed in simulating and analyzing the constraints in DDR3 design with a smart layout function and strong DRC mechanisms. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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