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15–6
Chapter 15: Debugging
Link Hangs in L0 Due To Deassertion of tx_st_ready
Stratix V Hard IP for PCI Express
June 2012
Altera Corporation
f
For more information about SignalTap, refer to the
SignalTap II Embedded Logic Analyzer
chapter in volume 3 of the
Quartus II Handbook
.
Check PIPE Interface
Because the LTSSM signals reflect the behavior of one side of the PCI Express link,
you may find it difficult to determine the root cause of the link issue solely by
monitoring these signals. Monitoring the PIPE interface signals in addition to the
ltssmstate
bus provides greater visibility.
The PIPE interface is specified by Intel. This interface defines the MAC/PCS
functional partitioning and defines the interface signals for these two sublayers. Using
the SignalTap logic analyzer to monitor the PIPE interface signals provides more
information about the devices that form the link.
During link training and initialization, different pre-defined Physical Layer Packets
(PLPs), known as ordered sets are exchanged between the two devices on all lanes. All
of these ordered sets have special symbols (K codes) that carry important information
to allow two connected devices to exchange capabilities, such as link width, link data
rate, lane reversal, lane-to-lane de-skew, and so on. You can track the ordered sets in
the link initialization and training on both sides of the link to help you diagnose link
issues. You can use SignalTap logic analyzer to determine the behavior.
lists the PIPE interface signals for a two-lane simulation that you can
monitor on the
test_out
bus.
Table 15–3. PIPE Interface Signals (Part 1 of 3)
Signal Name
Lane 0
Lane 1
Description
reserved[57:0]
[159:102]
[319:262]
—
lanereversalenable
[101]
[261]
When asserted, enables lanes reversal. The following
encodings are defined:
■
0: Lanes not reversed
■
1: Lanes reversed
eidleinfersel[2:0]
[100:98]
[260]
Electrical idle entry inference mechanism selection. The
following encodings are defined:
■
3'b0xx: Electrical Idle Inference not required in current
LTSSM state
■
3'b100: Absence of COM/SKP Ordered Set the in 128
us window for Gen1 or Gen2
■
3'b101: Absence of TS1/TS2 Ordered Set in a 1280 UI
interval for Gen1 or Gen2
■
3'b110: Absence of Electrical Idle Exit in 2000 UI
interval for Gen1 and 16000 UI interval for Gen2
■
3'b111: Absence of Electrical idle exit in 128 us
window for Gen1
txdeemph
[97]
[257]
Transmit de-emphasis selection. The Stratix V Hard IP for
PCI Express sets the value for this signal based on the
indication received from the other end of the link during
the Training Sequences (TS).
txmargin[2:0]
[96:94]
[256:254]
Transmit V
OD
margin selection.