RK3288 Hardware Design Guide
Make sure that a complete layer must be applied to reference for all DDR signals,
in order to get the minimum return path impedance, and get the continuity of
impedance.
Prohibit all DDR signals bridge on different power layers.
Prohibit other signal traces through the DDR area.
Place a decoupling capacitor close to each VCC_DDR PIN of both RK3288 and DDR
chip, and vias should be placed close to pins to reduce inductance.
Recommendations for VREF
CPU and DDR3 VREF should be located separately, which will achieve partial
voltage from VDDQ, VREF should be placed close to CPU, trace length is the
shorter the better, and the trace should be routed separated from any other traces,
ensure no interferences (especially from adjacent layers), VDDQ has a better
following performance, make sure VREF voltage value will change according to
noise and temperature with VDDQ;
VREF only need to be provided with an extremely small current (input current is
almost 3mA), 1nF bypass capacitors should be added close to each pin of VREF
(capacitor quantity for each trace should be less than 5, to reduce influence of
power following feature), width of trace is suggested to be less than 10mils.
Recommendations for impedance
The characteristic width of single signal is 5 mils and the impedance should be
limited within 50~75 ohm. Meanwhile variational impedance via different layers
should be less than 10ohm.
The impedance of differential pair should be limited within 100~130 ohm
，
Meanwhile variational impedance via different layers should be less than 20ohm.
The range of dielectric constant of PCB filling material is from 4.0 to 4.5, the value
changes with frequency and temperature. FR-4 is a typical dielectric material, the
average dielectric constant of which is 4.2 at 100MHz. It is recommended to use
FR - 4 as filling material of PCB.
Integrality of copper pour
Making sure the integrality of copper pour for GND, which will enhance
performance and compatibility for DDR chip. The good design with integrality of
copper pour is shown as Fig 8-9:
Содержание RK32 Series
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