AM3874 datasheet(298/361 Pages) TI

298 / 361 page

AM3874, AM3871

SPRS695C – SEPTEMBER 2011 – REVISED DECEMBER 2013

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8.13.4.2.4.10 VTT

Like VREF, the nominal value of the VTT supply is half the DDR3 supply voltage. Unlike VREF, VTT is

expected to source and sink current, specifically the termination current for the ADDR_CTRL net class

Thevinen terminators. VTT is needed at the end of the address bus and it should be routed as a power

sub-plane. VTT should be bypassed near the terminator resistors.

8.13.4.2.4.11 CK and ADDR_CTRL Topologies and Routing Definition

The CK and ADDR_CTRL net classes are routed similarly and are length matched to minimize skew

between them. CK is a bit more complicated because it runs at a higher transition rate and is differential.

The following subsections show the topology and routing for various DDR3 configurations for CK and

ADDR_CTRL. Only the components shown in the topologies are allowed. Items such as test points and

additional terminations are specifically disallowed. The figures in the following subsections define the

terms for the routing specification detailed in Table 8-75.

Care should be taken to minimize layer transitions during routing. If a layer transition is necessary, it is

better to transition to a layer using the same reference plane. If this cannot be accommodated, ensure

there are nearby ground vias to allow the return currents to transition between reference planes if both

reference planes are ground or DVDD_DDR. Ensure there are nearby bypass capacitors to allow the

return currents to transition between reference planes if one of the reference planes is ground. The goal is

to minimize the size of the return current loops.

8.13.4.2.4.11.1 Four DDR3 Devices

Four DDR3 devices are supported on the DDR EMIF consisting of four x8 DDR3 devices arranged as one

bank (CS). These four devices may be mounted on a single side of the PCB, or may be mirrored in two

pairs to save board space at a cost of increased routing complexity and parts on the backside of the PCB.

8.13.4.2.4.11.2 CK and ADDR_CTRL Topologies, Four DDR3 Devices

Figure 8-58 shows the topology of the CK net classes and Figure 8-59 shows the topology for the

corresponding ADDR_CTRL net classes.

Figure 8-58. CK Topology for Four x8 DDR3 Devices

Figure 8-59. ADDR_CTRL Topology for Four x8 DDR3 Devices

8.13.4.2.4.11.3 CK and ADDR_CTRL Routing, Four DDR3 Devices

Figure 8-60 shows the CK routing for four DDR3 devices placed on the same side of the PCB. Figure 8-61

shows the corresponding ADDR_CTRL routing.

298

Peripheral Information and Timings

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