To ensure good signaling performance,
the following general board design guidelines must be followed:
- Always follow TI's example layouts/EVM designs as close as possible. If concepts
or routing strategies are not understood, questions can be posted on E2E.
- All signals need ground reference (strongly suggest on both
sides). Maintain the common ground/reference for all signals and for all
bypass/decoupling capacitors.
- Avoid crossing plane splits in the signal reference
planes.
- Use the widest trace that is practical between decoupling
capacitors and memory modules.
- Minimize inter-symbol interference (ISI) by keeping impedances
matched. This is especially true for the 'T-branch' signals where trace widths
are adjusted to match trace impedance.
- Minimize crosstalk by isolating sensitive signals, such as
strobes and clocks, and by using a proper PCB stack-up. Add additional spacing
for clock and strobe traces to also help minimize crosstalk.
- Avoid return path discontinuities by adding vias or capacitors
whenever signals change layers and reference planes.
- Minimize reference voltage noise through proper isolation and
proper use of decoupling capacitors on the reference input pins on the
SDRAMs.
- Keep the signal routing stub lengths as short as possible.
- Route all signals as strip-line. Avoid micro-strip traces
except for BGA break-out areas.
- Via-to-via coupling can be significant part of PCB-level
crosstalk. Dimension and pitch of vias is important. For high speed interfaces,
consider GND shielding vias. This via coupling is one factor for recommending
data signals be routed on layers closest to processor.
- Via stubs affect signal integrity. Via back-drilling can improve
signal integrity, and may be required in some instances.
For more information, see the High-Speed Interface Layout Guidelines Application Report. It
provides additional general guidance for successful routing of high-speed
signals.