DM385, DM388
www.ti.com
SPRS821D –MARCH 2013–REVISED DECEMBER 2013
8.13.3.16 Routing Specification
8.13.3.16.1 CK and ADDR_CTRL Routing Specification
Skew within the CK and ADDR_CTRL net classes directly reduces setup and hold margin and, thus, this
skew must be controlled. The only way to practically match lengths on a PCB is to lengthen the shorter
traces up to the length of the longest net in the net class and its associated clock. A metric to establish
this maximum length is Manhattan distance. The Manhattan distance between two points on a PCB is the
length between the points when connecting them only with horizontal or vertical segments. A reasonable
trace route length is to within a percentage of its Manhattan distance. CACLM is defined as Clock Address
Control Longest Manhattan distance.
Given the clock and address pin locations on the processor and the DDR3 memories, the maximum
possible Manhattan distance can be determined given the placement. Figure 8-80 and Figure 8-81 show
this distance for four loads and two loads, respectively. It is from this distance that the specifications on
the lengths of the transmission lines for the address bus are determined. CACLM is determined similarly
for other address bus configurations; that is, it is based on the longest net of the CK/ADDR_CTRL net
class. For CK and ADDR_CTRL routing, these specifications are contained in Table 8-70.
A8(A)
CACLMY
CACLMX
A8(A)
A8(A)
A8(A)
A8(A)
Rtt
A2
A3
A4
A3
AT
Vtt
=
A. It is very likely that the longest CK/ADDR_CTRL Manhattan distance will be for Address Input 8 (A8) on the DDR3
memories. CACLM is based on the longest Manhattan distance due to the device placement. Verify the net class that
satisfies this criteria and use as the baseline for CK/ADDR_CTRL skew matching and length control.
The length of shorter CK/ADDR_CTRL stubs as well as the length of the terminator stub are not included in this
length calculation. Non-included lengths are grayed out in the figure.
Assuming A8 is the longest, CALM = CACLMY + CACLMX + 300 mils.
The extra 300 mils allows for routing down lower than the DDR3 memories and returning up to reach A8.
Figure 8-80. CACLM for Four Address Loads on One Side of PCB
Copyright © 2013, Texas Instruments Incorporated
Peripheral Information and Timings
247
Submit Documentation Feedback
Product Folder Links: DM385 DM388
TI [ TEXAS INSTRUMENTS ]
