Intel IXP45X, IXP46X manual 6.3.1Trace Length Limits, PCI Devices, Driver, PCI Clock Topology

PCI Interface Design Considerations—Intel®IXP45X and Intel® IXP46X Product Line of

Network Processors

Figure 27. PCI Clock Topology

				PCI Devices
	A	Rs	B
33/66 MHz	Clock
	Driver
	A	Rs	B
				Intel® IXP46X
	A		B	Product Line
		Rs		Network
				Processor
				B4114-02

Table 26.	PCI Clock Routing Guidelines
	Parameter	Routing Guidelines
	Signal Group	PCI Clock
	Topology	Point-to-Point
	Reference Plane	Ground
	Characteristic Trace Impedance	55 Ω ±10%
	Nominal Trace Width	5 mils
	Nominal Trace Separation	10 mils
	Spacing to Other Groups	20 mils
	Trace length A	Maximum 300 mils
		There is no limit as long as the trace
	Trace length B	length is maintained for each clock and
		that maximum clock skew is not violated.
	Resistor Rs	22 Ω ±10%
	Maximum VIAS	6

6.3.1Trace Length Limits

Maximum trace lengths can be calculated for specific speeds at which the bus is intended to run. Typically, PCI boards with devices that can support up to 66 MHz are designed to function at up to 66 MHz, even if the design is originally intended to run at 33 MHz. This way, if design requirements change to 66 MHz, then timing is met at the higher frequency. In this case, the only additional requirement is to change the clock speed and the expansion bus initial strapping at the EX_ADDR[4] signal. If you are designing your board for 66 MHz and intend it to operate at 33 MHz, ensure that timing equations in Section 6.2 are met at 33 MHz and 66 MHz.

	Intel® IXP45X and Intel® IXP46X Product Line of Network Processors
February 2007	HDD
Document Number: 305261, Revision: 004	73