Altera PCI Compiler - Manual (Page 85)

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.