Revision 1.31 – February 16, 2012
440EPx – PPC440EPx Embedded Processor
Data Sheet
Board Layout Restrictions
The paths (traces) for the data and the associated data strobe signal should be routed with the same length
between the PPC440EPx and the SDRAM devices, allowing the rising and falling edges of the strobe to arrive at
the capture logic at the same time the data is in transition. All of the following timing assumes a trace velocity of
167ps/in.
Board designs must meet the following criteria:
• Skew on the signals in any byte lane should not exceed 50ps (0.3 in).
• Data subgroup trace lengths must be no more than 5in. (800ps) and have a difference of no more than 2.5in.
(400ps).
• Byte lane subgroup trace length must be no less than 1.25in. (209ps).
For example, traces that average 3.00in. in length and 167ps/in., and meet the maximum 50ps skew requirement,
would have a maximum length difference of 0.3in. So, they would be between 2.85in. and 3.15in. in length.
If the above timing recommendations are followed, the package wire bond lengths can be ignored.
Clocking
Clocking skew to all DRAMs must be minimized. The maximum allowed is considered to be 10ps. Because of the
stringent requirements on DDR device clock inputs, it is expected that board designers will use some type of
external PLL suitable to redrive the clock to the DDR SDRAMs. In such a system, the PLL acts like a zero-delay
insertion buffer.
When using unbuffered DIMMS, the loading on the address bus will be considerably greater than the clock (up to
18 loads for double-sided DIMMs). In this case, it is strongly suggested that a delay of 500ps in the clock path so
that the Address/Command setup time at the DIMMs can be met. This delay is sufficient to meet the setup time,
without having to change the programmable delay (internal to the PPC440EPx) between the DQS/DQ/DM and the
clock (assuming nominal settings as specified in the PPC440EPx Users Manual). While the clock is now 500ps
later than the nominal DQS arrival time, this still falls well within the window allowed by the JEDEC spec for T
DQSS
(± 0.25 cycle, or 1.5ns at 166MHz). In the case where it is not possible to anticipate which kind of DIMMs may be
employed in a system, it is always safe to use this 500ps clock delay, since registered DIMMs (the least heavily
loaded) will have more than enough margin (almost 1/2 cycle) to accommodate the slight decrease in address hold
time.
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