When one memory (DDR4) device (1× 16-bit) is used,
consider point-to-point topology.
Summary of point-to-point topology implementation:
- External VTT terminations for address and control
signals are optional (not required).
- For differential clock DDR0_CK0, DDR0_CK0_n, AC
differential termination 2× R in series (value = Zo − Single-ended
impedance) and a filter capacitor 0.01μF or value recommended by the memory
manufacturer connected to the center of two resistors and DDR PHY IO supply
VDDS_DDR is recommended.
- VREFCA (VDDS_DDR/2) is the reference voltage used
for control, command, and address inputs to the memory (DDR4) devices. VREFCA is derived
from VDDS_DDR using a resistor divider (two resistors (recommended resistor value is
1kΩ, 1%) connected to VDDS_DDR and VSS) with filter capacitor (recommended value is
0.1μF) connected in parallel to both the resistors. An additional decoupling capacitor
is connected to the VREFCA pin (close to memory (DDR4) device).
Alternatively, VTT terminations on the address and
control signal for one memory (DDR4) device and Sink or Source DDR Termination
Regulator to generate the VTT supply can be used.
When two memory (DDR4) devices (2× 8-bit) are
used, the recommendation is to follow the Fly-by topology.
Summary of Fly-by topology implementation:
- External terminations (VTT) for address, control,
and clock signals are recommended.
- Sink or Source DDR Termination Regulator is recommended to generate the VTT supply.
- The Sink or Source DDR Termination Regulator generates the reference voltage VREFCA (VDDS_DDR/2).
- Add decoupling capacitors for the reference voltage.