Xilinx PCI-X v5.1 manual Electrical Compliance

Electrical Compliance

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It is important to note that the frequency of this clock is not guaranteed to be constant. In fact, in a compliant system, the clock may be any frequency, up to and including the maximum allowed frequency, and the frequency may change on a cycle-by-cycle basis. Under certain conditions, the PCI-X core may also apply phase shifts to this clock.

For these reasons, the user application should not use this clock as an input to a DLL or PLL, nor should the user application use this clock in the design of interval timers (for example, DRAM refresh counters).

Electrical Compliance

Virtex-II, Virtex-II Pro, and Virtex-4 devices, as specified in the relevant device data sheet, exhibit a 10 pF pin capacitance. This is compliant with the PCI Local Bus Specification, with one exception. The specification requires an 8 pF pin capacitance for the IDSEL pin, to allow for non-resistive coupling to an AD[xx] pin. In practice, this coupling may be resistive or non-resistive, and is performed on the system board or backplane. For system board or backplane designs, use resistive coupling to avoid non-compliance. For add-in cards, this is not under the control of the designer.

The PCI-X Addendum requires an 8 pF pin capacitance for all pins. Virtex-II, Virtex-II Pro, and Virtex-4 devices do not comply with this requirement.

Although the core interface provides a direct PME# output from a general purpose I/O pin, this output signal has certain limitations. If the FPGA power is removed, the general purpose I/O pin will appear as a low impedance to ground. This appears to the system as an assertion of PME#. For this reason, implementations that use the PME# signal should employ an external buffering scheme that will prevent false assertions of PME# when power is removed from the FPGA device.

For 3.3 volt signaling in Virtex-II Pro and Virtex-4 devices, the VCCO. supply must be reduced to 3.0 volts and derived from a precision regulator. This reduction of the output

driver supply provides robust device protection without sacrificing PCI electrical compliance, even in the extreme case where the 3.3 volt system supply climbs as high as

3.6volts, as allowed by the PCI Local Bus Specification.

Figure 3-1illustrates one possible low-cost solution to generate the required 3.0 volt output driver supply. Xilinx recommends the use of this circuit; however, other approaches using other regulators are possible.

SUPPLY8

5

7

6

INOUT

SHDN# ADJ

GNDBYP

GND GND

LT1763CS8

+3.0V SUPPLY

1

2

4

3

VCCO

38.3, 1%

1.0uF

10uF

26.1, 1%

GND

Figure 3-1:PCI/PCI-X Output Driver VCCO Generation

PCI-X v5.1 165 Getting Started Guide www.xilinx.com 23

UG158 March 24, 2008