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Altera Corporation User Guide Version 11.1 3–11

October 2011

Functional Description

PCI Bus Signals

The following PCI signals are used by the pci_mt64, pci_mt32,

pci_t64, and pci_t32 functions:

■ Input—Standard input-only signal

■ Output—Standard output-only signal

■ Bidirectional—Tri-state input/output signal

■ Sustained tri-state (STS)—Signal that is driven by one agent at a time

(e.g., device or host operating on the PCI bus). An agent that drives

a sustained tri-state pin low must actively drive it high for one clock

cycle before tri-stating it. Another agent cannot drive a sustained

tri-state signal any sooner than one clock cycle after it is released by

the previous agent.

■ Open-drain—Signal that is shared by multiple devices as a wire-OR.

The signaling agent asserts the open-drain signal, and a weak pull-

up resistor deasserts the open-drain signal. The pull-up resistor may

require two or three PCI bus clock cycles to restore the open-drain

signal to its inactive state.

1 All of the PCI MegaCore function’s logic is clocked by the PCI

clock (clk). If you are interfacing to logic that has a different

clock, you must design appropriate clock domain crossing logic.

Table 3–2 summarizes the PCI bus signals that provide the interface

between the PCI MegaCore functions and the PCI bus.

Table 3–2. PCI Interface Signals (Part 1 of 4)

Name Type Polarity Description

clk

Input –

Clock. The

clk input provides the reference signal for all other

PCI interface signals, except

rstn and intan.

rstn

Input Low

Reset. The

rstn input initializes the PCI interface circuitry and

can be asserted asynchronously to the PCI bus

clk edge.

When active, the PCI output signals are tri-stated and the open-

drain signals, such as

serrn, float.

gntn

Input Low

Grant. The

gntn input indicates to the PCI bus master device

that it has control of the PCI bus. Every master device has a pair

of arbitration signals (

gntn and reqn) that connect directly to

the arbiter.

l_dis_64_extn

Input Low Disable 64-bit extension signals. When you assign an APEX

device in a 32-bit PCI bus and drive this signal low, it disables

the PCI 64-bit extension signals. The extension signals include

ad_63..32_, cben_7..4_, req64n, ack64n, and

par64.

reqn

Output Low

Request. The

reqn output indicates to the arbiter that the PCI

bus master wants to gain control of the PCI bus to perform a

transaction.

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