SPRACU1A October   2020  – June 2021 AM2431 , AM2432 , AM2434 , AM6411 , AM6412 , AM6421 , AM6441 , AM6442

 

  1.   Trademarks
  2. 1Overview
    1. 1.1 Board Designs Supported
    2. 1.2 General Board Layout Guidelines
    3. 1.3 PCB Stack-Up
    4. 1.4 Bypass Capacitors
      1. 1.4.1 Bulk Bypass Capacitors
      2. 1.4.2 High-Speed Bypass Capacitors
      3. 1.4.3 Return Current Bypass Capacitors
    5. 1.5 Velocity Compensation
  3. 2DDR4 Board Design and Layout Guidance
    1. 2.1  DDR4 Introduction
    2. 2.2  DDR4 Device Implementations Supported
    3. 2.3  DDR4 Interface Schematics
      1. 2.3.1 DDR4 Implementation Using 16-Bit SDRAM Devices
      2. 2.3.2 DDR4 Implementation Using 8-Bit SDRAM Devices
    4. 2.4  Compatible JEDEC DDR4 Devices
    5. 2.5  Placement
    6. 2.6  DDR4 Keepout Region
    7. 2.7  VPP
    8. 2.8  Net Classes
    9. 2.9  DDR4 Signal Termination
    10. 2.10 VREF Routing
    11. 2.11 VTT
    12. 2.12 POD Interconnect
    13. 2.13 CK and ADDR_CTRL Topologies and Routing Guidance
    14. 2.14 Data Group Topologies and Routing Guidance
    15. 2.15 CK and ADDR_CTRL Routing Specification
      1. 2.15.1 CACLM - Clock Address Control Longest Manhattan Distance
      2. 2.15.2 CK and ADDR_CTRL Routing Limits
    16. 2.16 Data Group Routing Specification
      1. 2.16.1 DQLM - DQ Longest Manhattan Distance
      2. 2.16.2 Data Group Routing Limits
    17. 2.17 Bit Swapping
      1. 2.17.1 Data Bit Swapping
      2. 2.17.2 Address and Control Bit Swapping
  4. 3LPDDR4 Board Design and Layout Guidance
    1. 3.1  LPDDR4 Introduction
    2. 3.2  LPDDR4 Device Implementations Supported
    3. 3.3  LPDDR4 Interface Schematics
    4. 3.4  Compatible JEDEC LPDDR4 Devices
    5. 3.5  Placement
    6. 3.6  LPDDR4 Keepout Region
    7. 3.7  Net Classes
    8. 3.8  LPDDR4 Signal Termination
    9. 3.9  LPDDR4 VREF Routing
    10. 3.10 LPDDR4 VTT
    11. 3.11 CK and ADDR_CTRL Topologies
    12. 3.12 Data Group Topologies
    13. 3.13 CK and ADDR_CTRL Routing Specification
    14. 3.14 Data Group Routing Specification
    15. 3.15 Channel, Byte, and Bit Swapping
  5. 4Revision History

1.3 PCB Stack-Up

The minimum stack-up for routing the DDR interface is a six-layer stack up. However, this can only be accomplished on a board with routing room with large keep-out areas. Additional layers are required if:

  • The PCB layout area for the DDR Interface is restricted, which limits the area available to spread out the signals to minimize crosstalk.
  • Other circuitry must exist in the same area, but on layers isolated from the DDR routing.
  • Additional planes layers are needed to enhance the power supply routing or to improve EMI shielding.

Board designs that are relatively dense require 10 or more layers to properly allow the DDR routing to be implemented such that all rules are met.

DDR signals with the highest frequency content (such as data or clock) must be routed adjacent to a solid VSS reference plane. Signals with lower frequency content (such as address) can be routed adjacent to either a solid VSS or a solid VDDS_DDR reference plane. If a VDDS_DDR reference plane is used, bypass capacitors must be implemented near both ends of every route to provide a low-inductance, AC path to ground for these routes. Similarly, when multiple VSS reference planes exist in the DDR routing area, stitching vias must be implemented nearby wherever vias transfer signals to a different VSS reference plane. This is required to maintain a low-inductance return current path.

It is strongly recommended all DDR signals be routed as strip-line. Some PCB stack-ups implement signal routing on two adjacent layers. This is acceptable only as long as the routing on these layers is perpendicular and does not allow for broad-side coupling. Severe crosstalk occurs on any trace routed parallel to another trace on an adjacent layer, even for a short distance. Also, DDR signal routing on two adjacent layers is only allowed when implementing offset stripline routing, where the distance between the adjacent routing layers is more than 3x the distance from the traces to their adjacent reference plane.

Table 1-1 PCB Stack-up Specifications
Number Parameter MIN TYP MAX UNIT
PS1 PCB routing plus plane layers 6
PS2 Signal routing layers 3
PS3 Full VSS reference layers under DDR routing region (1) 1
PS4 Full VDDS_DDR power reference layers under the DDR routing region (1) 1
PS5 Number of reference plane cuts allowed within DDR routing region (2) 0
PS6 Number of layers between DDR routing layer and reference plane (3) 0
PS7 PCB routing feature size 4 Mils
PS8 PCB trace width, w 4 Mils
PS9 Single-ended impedance

40

 Ω

PS10

 Differential impedance

80

 Ω
PS11 Impedance control (4) Z-10% Z Z+10% Ω
(1) Ground reference layers are preferred over power reference layers. Return signal vias need to be near layer transitions. When using power reference layers, include bypass caps to accommodate reference layer return current, as the trace routes switch routing layers.
(2) No traces should cross reference plane cuts within the DDR routing region. High-speed signal traces crossing reference plane cuts create large return current paths, which can lead to excessive crosstalk and EMI radiation. Beware of reference plane voids caused by via antipads, as these also cause discontinuities in the return current path.
(3) Reference planes are to be directly adjacent to the signal layer, to minimize the size of the return current loop.
(4) Z is the nominal singled-ended impedance selected for the PCB specified by PS9 and PS10.