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STPC4EDBI Datasheet(PDF) 79 Page - STMicroelectronics

Part No. STPC4EDBI
Description  X86 Core PC Compatible Information Appliance System-on-Chip
Download  93 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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STPC4EDBI Datasheet(HTML) 79 Page - STMicroelectronics

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DESIGN GUIDELINES
Release 1.5 - January 29, 2002
79/93
memory clock input (MCLKI) and any other
component
using
the
memory
clock
are
individually driven from a low skew clock driver
with matched routing lengths. In other words, all
clock line lengths that go from the buffer to the
memory chips (MCLKx) and from the buffer to the
STPC (MCLKI) must be identical.
This is shown in Figure 6-20.
The maximum skew between pins for this part is
250ps. The important factors for the clock buffer
are a consistent drive strength and low skew
between the outputs. The delay through the buffer
is not important so it does not have to be a zero
delay PLL type buffer. The trace lengths from the
clock driver to the DIMM CKn pins should be
matched exactly. Since the propagation speed
can vary between PCB layers, the clocks should
be routed in a consistent way. The routing to the
STPC memory input should be longer by 75 mm to
compensate for the extra clock routing on the
DIMM. Also a 20 pF capacitor should be placed as
near as possible to the clock input of the STPC to
compensate for the DIMM’s higher clock load. The
impedance of the trace used for the clock routing
should be matched to the DIMM clock trace
impedance (60-75 ohms)
.To minimise crosstalk
the clocks should be routed with spacing to
adjacent tracks of at least twice the clock trace
width. For designs which use SDRAMs directly
mounted on the motherboard PCB all the clock
trace lengths should be matched exactly.
Figure 6-19. DIMM placement
DIMM2
DIMM1
STPC
35mm
35mm
15mm
10mm
116mm
SDRAM I/F
Figure 6-20. Clock Routing
MCLKO
DIMM CKn input
STPC MCLKI
DIMM CKn input
DIMM CKn input
Low skew clock driver:
L
L+75mm*
20pF
* No additional 75mm when SDRAM directly soldered on board


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