2 DDR4 Board Design and Layout Guidance
2.1 DDR4 Introduction
DDR4 board designs are similar to DDR3 board designs. Fly-by routing is required just as it is with DDR3, and
thus leveling is required. To achieve higher data rates with DDR4, there are several enhancements added to the
interface specification that must be accommodated by both the SDRAM and the processor’s interface (PHY).
The enhancements that affect the board interconnect and layout are listed below:
• Addition of ACT_n pin – This pin provides signaling to allow the pins previously called Command pins
(RAS_n, CAS_n and WE_n) to be used as additional address pins. These pins behave as row address pins
when ACT_n is low and as command pins when ACT_n is high. This is valid only when CS_n is low.
• Removal of one BA (Bank Address) pin and addition of 2 BG (Bank Group) pins – This adds flexibility with
accesses similar to DDR3, but with 16 banks bundled in four bank groups of four banks each. This results in
additional timing parameters, because adjacent accesses within a bank group are faster than adjacent
accesses to another bank group. Successive accesses to locations within a single bank are the fastest
option.
• Addition of PAR (Parity) and ALERT_n pins (use is optional) – The PAR pin supplies parity monitoring for the
command and address pins using even parity from the controller to the SDRAM. ALERT_n is the indicator
(open-drain output) from the SDRAMs that indicate when a parity error has been detected.
• Change to POD termination – Pseudo-Open Drain (POD) output buffers are implemented rather than
traditional SSTL push-pull outputs. This allows the data bit termination, ODT, to go to the I/O power rail,
VDDQ, rather than to the mid-level voltage, VTT. Power consumption may be reduced, because only driving
a bit low draws current.
• Addition of DBI – Data bus invert (DBI) is a feature that allows the data bus to be inverted whenever more
than half of the bits are zero. This feature may reduce active power and enhance the data signal integrity
when coupled with POD termination.
• Addition of a VPP power input – The VPP power supply (2.5 V) provides power to the internal word line logic.
This voltage increase allows the SDRAM to reduce overall power consumption.
• Separation of data VREF from address/control VREF – The data reference voltage, VREFDQ, is now
internally generated both within the SDRAM and within the PHY. It can be programmed to various levels to
provide the optimum sampling threshold. The optimum threshold varies based on the ODT impedance
chosen, the drive strength, and the PCB track impedance. The address/control reference voltage, VREFCA,
is a mid-level reference voltage, the same as it is on DDR3.
Note
These features may not be supported on all devices. Refer to the device-specific documentation for
supported features.
2.2 DDR4 Device Implementations Supported
There are several possible combinations of SDRAM devices supported by the DDR4 EMIF. Table 2-1 lists the
supported device combinations. The SDRAMs used in each combination must be identical: that is, they must
have the same part number.
Table 2-1. Supported DDR4 SDRAM Combinations
Number of DDR4 SDRAMs DDR4 SDRAM Width (bits) DDR4 EMIF Width (bits)
1 16 16
2 8 16
DDR4 Board Design and Layout Guidance www.ti.com
6 AM64x DDR Board Design and Layout Guidelines SPRACU1 – OCTOBER 2020
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