VIA Technologies Apollo Pro133A Design Manual page 49

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AGP Clock Trace Length Calculation
Pre-route AGP clock traces from the pin GCLKO of the VT82C694X to the AGP slot as short as possible. Then the trace length
for the signal GCLK should be the GCLKO trace length plus 3 inches.
Clock Trace
GCLKOUT (NB à AGP Slot)
GCLKIN (NB à NB)
Note: Here, the 3" represents the estimated trace length added into GCLKI for AGP clock alignment.
PCI Clock Trace Length Calculation
Pre-route PCI clock traces from the system clock synthesizer to the VT82C694X (NPCLK) and VT82C686A (SPCLK) as short as
possible. Then pre-route PCI clock traces PCLK0~PCLK4 from the system clock synthesizer to all PCI slots as short as possible.
The length of these clocks will be based on the longest one (L
A calculation example is shown below.
Clock Trace
Clock chip à VT82C694X (NB)
Clock chip à VT82C686A (SB)
Clock chip à PCI1
Clock chip à PCI2
Clock chip à PCI3
Clock chip à PCI4
Clock chip à PCI5
Note: Here, the 3" represents the estimated trace length added into NPCLKI and SPCLK for PCI clock alignment.
Notes for the length calculation of all clock traces:
1. Shortest length means the minimum routable trace length between both clock ends.
2. Desired length means the real length of the clock traces on PCB layout.
3. Allowable difference means the maximum length difference between clock traces of the same type.
4. Allowable range means the acceptable clock length range for the specific clock.
5. The location of the system clock chip can affect the length of all clock traces. To optimize the clock alignment, place the clock
chip at an appropriate location.
6. In addition, the trace impedance of all clock traces should be in the range of 40 ohms and 55 ohms.
Preliminary Revision 0.5, November 19, 1999
Design Guide - VT82C694X Apollo Pro133 with VT82C686A
Shortest Desired
Length Length
L
GOUT
L L
GIN GOUT
). Usually PCI5 is the farthest PCI slot from the North Bridge chip.
5
Shortest Desired
Length Length
L
L
NB
L L
SB
L
1
L
2
L
3
L
4
L ( > the others)
5
39
Allowable
Difference
L -
GOUT
+ 3" 0.5"
Allowable
Difference
1"
+ 3"
5
+ 3" 1"
5
L 1"
5
1"
L
5
1"
L
5
1"
L
5
L -
5
Motherboard Design Guidelines
Allowable
Range
1"~9"
4"~12"
Allowable
Range
4"~15"
4"~15"
1"~12"
1"~12"
1"~12"
1"~12"
1"~12"