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Chapter 8: Reset and Clocks
8–5
Clocks
June 2012
Altera Corporation
Stratix V Hard IP for PCI Express
As
illustrates, the RX transceiver reset includes the following steps:
1. After
npor
is deasserted, the core deasserts the
npor_serdes
input to the TX
transceiver.
2. The SERDES reset controller waits for
pll_locked
to be stable for a minimum of
127 cycles before deasserting
tx_digitalreset.
Clocks
The Hard IP contains a clock domain crossing (CDC) synchronizer at the interface
between the PHY/MAC and the DLL layers which allows the Data Link and
Transaction Layers to run at frequencies independent of the PHY/MAC and provides
more flexibility for the user clock interface. Depending on parameters you specify, the
core selects the appropriate
coreclkout_hip
as listed in
Table 8–1 on page 8–1
. You
can use these parameters to enhance performance by running at a higher frequency
for latency optimization or at a lower frequency to save power.
In accordance with the
PCI Express Base Specification 2.1
, you must provide a 100 MHz
reference clock that is connected directly to the transceiver. As a convenience, you
may also use a 125 MHz input reference clock as input to the TX PLL.
Stratix V Hard IP for PCI Express Clock Domains
illustrates the clock domains when using coreclkout_hip to drive the
Application Layer and the
pld_clk
of the Stratix V Hard IP for PCI Express IP Core.
As
indicates, the IP core includes three clock domains.
Figure 8–5. Clock Domains and Clock Generation for the Application Layer
Note to
(1) The Example Design connects
coreclkout_hip
to the
pld_clk
. However, this connection is not mandatory.
100 MHz
(or 125 MHz)
refclk
Hard IP for PCI Express
PHY/MAC
Clock
Domain
Crossing
(CDC)
Data Link
and
Transaction
Layers
TX PLL
PCS
Transceiver
250 or 500 MHz
p_clk
coreclkout_hip
Application
Layer
pld_clk
(62.5, 125
or 250 MHz)
serdes_pll_locked
pld_core_ready
(1)