SPRAD66A February   2023  – December 2023 AM62A3 , AM62A3-Q1 , AM62A7 , AM62A7-Q1 , AM62P , AM62P-Q1

 

  1.   1
  2.    AM62Ax/AM62Px LPDDR4 Board Design and Layout Guidelines
  3.   Trademarks
  4. 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
    5. 1.5 Velocity Compensation
  5. 2LPDDR4 Board Design and Layout Guidance
    1. 2.1  LPDDR4 Introduction
    2. 2.2  LPDDR4 Device Implementations Supported
    3. 2.3  LPDDR4 Interface Schematics
    4. 2.4  Compatible JEDEC LPDDR4 Devices
    5. 2.5  Placement
    6. 2.6  LPDDR4 Keepout Region
    7. 2.7  Net Classes
    8. 2.8  LPDDR4 Signal Termination
    9. 2.9  LPDDR4 VREF Routing
    10. 2.10 LPDDR4 VTT
    11. 2.11 CK and ADDR_CTRL Topologies
    12. 2.12 Data Group Topologies
    13. 2.13 CK0 and ADDR_CTRL Routing Specification
    14. 2.14 Data Group Routing Specification
    15. 2.15 Channel, Byte, and Bit Swapping
    16. 2.16 Data Bus Inversion
  6. 3LPDDR4 Board Design Simulations
    1. 3.1 Board Model Extraction
    2. 3.2 Board-Model Validation
    3. 3.3 S-Parameter Inspection
    4. 3.4 Time Domain Reflectometry (TDR) Analysis
    5. 3.5 System Level Simulation
      1. 3.5.1 Simulation Setup
      2. 3.5.2 Simulation Parameters
      3. 3.5.3 Simulation Targets
        1. 3.5.3.1 Eye Quality
        2. 3.5.3.2 Delay Report
        3. 3.5.3.3 Mask Report
    6. 3.6 Design Example
      1. 3.6.1 Stack-Up
      2. 3.6.2 Routing
      3. 3.6.3 Model Verification
      4. 3.6.4 Simulation Results
  7. 4Appendix: SOC Package Delays
  8. 5References
  9. 6Revision History

3.6.2 Routing

The below examples from a J7 design show the LPDDR4 Clock and CA routing on an example 10-layer PCB design. The clock is routed differentially with target impedance of 70 Ω. For the T-branch to match the impedance of the trace, the impedance needs to be doubled. This can create challenges, as the higher impedances can be difficult to achieve in some PCB stackups. The CA signals are routed targeting 35 Ω, with the T-branch at two times the source impedance.

GUID-CEBB3BA2-AF75-451D-A67A-39DA78C5F44D-low.png Figure 3-7 Example LPDDR4 Clock and CA Routing

On the same 10 layer reference design, the data groups are routed on layers 2 and 4. The upper layers are used due to the minimum via travel, which minimized the via inductance and via-to-via coupling. Because the data signals are point-to-point, T-branch routing is not required.

GUID-A0DD1D59-0D31-472C-BA88-6C5BF86E5A90-low.png Figure 3-8 Example LPDDR4 Data Byte(s) and DQS(s) Routing
Table 3-8 LPDDR4 Performance Impact on Routing Layer (Read at pad)
Routing Layer Via Type Back Drilling EW Margin (ps) EH Margin (mV)
L1, L12 PTH No -7.10 56.72
L1, L12 PTH Yes -4.86 55.71
L1, L3 PTH No 5.70 40.29
L1, L3 PTH Yes 8.37 34.54
Table 3-9 LPDDR4 Performance Impact on Routing Layer (Write at pad)
Routing Layer Via Type Back Drilling EW Margin (ps) EH Margin (mV)
L1, L12 PTH No 17.42 39.22
L1, L12 PTH Yes 20.04 41.93
L1, L3 PTH No 27.66 41.37
L1, L3 PTH Yes 27.76 48.63