STPC12GDYC datasheet(99/111 Pages) STMICROELECTRONICS

99 / 111 page

DESIGN GUIDELINES

Issue 1.0 - July 24, 2002

99/111

6.4.4.3. Board Layout Issues

The physical layout of the motherboard PCB

assumed in this presentation is as shown in Figure

6-29. For the PCI interface, the most critical signal

is the clock. Any skew between the clocks at the

PCI components and the STPC will impact the

timing budget. In order to get well matched clocks

at all components it is recommended that all the

PCI clocks are individually driven from a serial

resistance with matched routing lengths. In other

words, all clock line lengths that go from the

resistor to the PCI chips (PCICLKx) must be

identical.

The figure below is for PCI devices soldered on-

board. In the case of a PCI slot, the wire length

must be shortened by 2.5" to compensate the

clock layout on the PCI board. The maximum

clock skew between all devices is 2ns according

to PCI specifications.

The Figure 6-30 describes a typical clock delay

implementation. The exact timing constraints are

listed in the PCI section of the Electrical

Specifications Chapter.

Figure 6-29. Typical PCI clock routing

Length(PCICLKI) = Length(PCICLKx) with x = {A,B,C}

Note: The value of 22 Ohms corresponds to tracks with Z0 = 70 Ohms.

PCICLKI

PCICLKO

PCICLKA

PCICLKB

PCICLKC

Device A

Device B

Device C

Figure 6-30. Clocks relationships

PCICLKO

PCICLKI

HCLK

PCICLKx

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