Frequently Asked Questions

The following FAQ list was generated using standard responses provided to PCI-SIG members by Technical Support and PCI-SIG Administration. For questions relative to the PCI Specifications, please reference the specifications themselves as the authoritative text. 

PCI Express

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What is ExpressCard technology?

ExpressCard technology is the name of a standard introduced by PCMCIA in Spring 2003. The ExpressCard standard promises to deliver thinner, faster and lighter modular expansion to desktop and notebook computer users. Consumers will be able to add hardware capabilities such as memory, wired and wireless communications and security features by simply inserting a module into their systems. All ExpressCard slots will accommodate modules designed to use either Universal Serial Bus (USB) 2.0, or the emerging PCI Express standards. Over time ExpressCard technology is expected to become the preferred solution for hot-pluggable internal I/O expansion for desktop and mobile computers.

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What is the PCI Express x16 Graphics 150W-ATX 1.0 Specification?

PCI Express x16 Graphics 150W-ATX 1.0 is a supplementary specification to the PCI Express architecture specification specifically for delivery for higher performance capabilities for the next generation of platforms with leading-edge graphics and multimedia applications. The specification considers power and thermal issues in these platforms and provides a solution space for designers of these systems.

PCI Express - 2.0

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Section 4.2.6.10.1 - The Loopback slave should wait until Symbol lock is archived after link speed change during Loopback.Entry substate. However, the base spec does not appear to define whether symbol lock should be archieved on some Lanes or all Lanes.

The Loopback slave transitions to Loopback.Active immediately after exiting Electrical Idle following the link speed change. It attempts to acquire symbol lock on all of the lanes that were active when it entered Loopback.Entry.

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SECTION 4.2.6.2.1 -- During Polling.Active, should Device A transmit TS1s on 4 lanes while Device B transmits TS1s on 8 lanes? Or, TS1s must be transmitted in both directions on the identical number of lanes?

Since device B has transmitters on only 4 lanes, it cannot transmit TS1s on more than 4 lanes. Device A will transmit TS1s on only the lanes where it detected receivers (and that is a maximum of 4 lanes).

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What were the initial target applications for PCIe 2.0?

The same set of core applications, high-performance graphics, enterprise-class storage and high-speed networking that benefited from the introduction of PCIe 1.0 architecture have led the charge for adoption of PCIe 2.0.

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SECTION 7.7 - Is a PCI Express Root Complex required to support MSI?

All PCI Express device Functions (including root ports) that are capable of generating interrupts must implement MSI or MSI-X or both.

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What prompted the need for another generation of PCI Express (PCIe)?

The PCIe 1.1 specification was developed to meet the needs of most I/O platforms. However a few applications, such as graphics, continue to require more bandwidth in order to enrich user experiences. PCI-SIG also saw the opportunity to add new functional enhancements (listed below), as well as incorporate all edits it had received to the PCIe 1.1 spec (via ECNs). In response to these needs, PCI-SIG developed PCI Express 2.0 (PCIe 2.0). It provides faster signaling, which doubles the bit rate from 2.5GT/s to 5GT/s.

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SECTION 4.2.6.4.4 - Referring to section 4.2.6.4.4 (Recovery.Idle), our EP is implemented such that it will send Idle data once entry into recovery.idle. If Hot Reset bit is asserted in two consecutive received TS1 ordered set, then we will move to HotReset state. Will the RC respond to the idle data that the EP sends out and falsely trigger into L0 state even though RC is directed to enter into HotReset?

For this case, the LTSSM of the Downstream Port above the Endpoint is already in the Hot Reset state, since that is how it transmitted TS1 Ordered Sets with the Hot Reset bit asserted.

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Section 6.18 - If a Switch supports the LTR feature, which of its ports must support LTR?

If a Switch supports the LTR feature, it must support the feature on its Upstream Port and all Downstream Ports.

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Section 5.3.2.3 - Is the following error scenario valid? - RC sends PME_Turn_off message to EP - EP doesn't respond with ACK due to delay - EP responds with PME_TO_Ack message - EP sends PM_Enter_L23 and not sending ACK. Can the EP do without ACK?

The Endpoint is required to send an Ack for the PME_Turn_Off message. There is no valid reason for an extended delay of the Ack.

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SECTION 4.2.6.2.1 -- Device A has transmitters on 8 lanes. Device B has transmitters on 4 lanes. Both devices are connected via a link. During Receiver Detection sequence in Detect.Active: Device A detects that Device B has drivers on 4 lanes, and Device B detects that Device A has drivers on 8 lanes.

PCIe Link is symmetric - so each component has the same number of Transmitters as Receivers. Since device B has transmitters on only 4 lanes, it also has receivers on 4 Lanes. Hence it would not be capable of detecting receivers on 8 lanes of device A.

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Section 4.2.4.3 - What is the purpose of the "inferred" electrical idle?

The purpose of the "inferred" electrical idle is to permit a method of detecting an electrical idle that does not use an analog circuit. Using an analog circuit can be difficult at 5.0 GT/s and the inferred method is an alternate (permitted) method.

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What other features are introduced in the PCIe 2.0 specification?

The most predominant feature in PCIe 2.0 is 5GT/s speed, which includes new mechanisms for software control of link speed, reporting of speed and width changes, and control of loopback. Other new features include:

  • PCI compatibility using the established PCI software programming models, thus facilitating a smooth transition to new hardware while allowing software to evolve to take advantage of PCI Express features
  • Enhanced Completion Timeout Control, which includes required and optional aspects, reduces false timeouts and increases the ability to 'tune' the timeouts
  • Function Level Reset and Access Control Services, giving enhanced robustness and support of certain IOV features (optional)
  • Slot Power Limit Changes to allow for higher powered slots, which support the newer, high-performance graphics cards; this new feature works in tandem with the 300W Card Electro-mechanical specification
  • Speed Signaling Controls to enable software to determine whether a device can operate at a specific signaling rate, which can be used to reduce power consumption, as well as provide gross level I/O to memory

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SECTION 4.2.6.4.4 - Is the following lane setting valid: executing a downconfiguration from x4 to x2, with lane0=ACTIVE, lane1=INACTIVE, lane2=ACTIVE, lane3=INACTIVE?

The active lanes must be consecutively numbered lanes starting with lane 0. Your example would configure as a x1 link.

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How can I get a copy of the PCI Express (PCIe) 2.0 specification?

Members may access specifications online on our Specifications web page or non-members may purchase specifications (order form is available on our Ordering Information web page).

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SECTION 6.1.4 - This question relates to MSI. More specifically this question also relates to the Conventional PCI 3.0 spec (on page 237) for MSI where it states that - The Multiple Message Enable field (bits 6-4 of the Message Control register) defines the number of low order message data bits the function is permitted to modify to generate its system software allocated vectors. Does this mean that the binary value of the LSBs of the message data specifies the vector number?

Yes (up to a total of 5 bits). Also to avoid confusion for the function, software sets each of the low order message data bits to 0, that correspond to the low order message data bits the function is permitted to modify to generate its system software allocated vectors.

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Section 4.2.6.1.1 - According to Section 4.2.6.1.1 in PCIe Base Specification 2.0, "The next state is Detect.Active after a 12 ms timeout or if Electrical Idle is broken on any Lane". Does this mean next state is Detect.Active only when electrical idle is broken?

It means the next state is Detect.Active after a 12 ms timeout, or the next state is Detect.Active (prior to the end of the 12 ms timer) if Electrical Idle is broken on any Lane.

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Section 4.2.6.4.3 - An Endpoint is in Recovery.RcvrCfg state and has received the 8 required consecutive TS2's. But before it is able to complete sending 16 TS2s, the downstream port sends EIEOS and then starts sending TS1s. At this point, should the Endpoint move to Recovery.Idle after sending 16 TS2s? Or is it required to reset its RX counter and start counting TS1s and try to go to Configuration?

Transition to Recovery.Idle after sending the 16 TS2s since the requirements for that transition are met.

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Section 4.2.4.1 - What does Link Upconfigure mean? What is it used for?

Link Upconfigure means the device is capable of increasing the link width. When Upconfigure is supported by both devices on a link, the link width may be reduced to conserve power. When link use is going to increase, the devices will increase the link width to support the needed high data rate preferred by the device.

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SECTION 7.8.6 -- Relative to Bits 3:0 in Section 7.8.6 - Link Capabilities Register, Supported Link Speeds. Is it OK for my device to support 0010b" and only support 5GT/s (and not support 2.5GT/s)?"

A device that supports 5GT/s must also be able to support and operate at 2.5GT/s.

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Is PCIe 2.0 backward compatible with PCIe 1.1 and 1.0?

Yes. The PCIe Base 2.0 specification supports both the 2.5GT/s and 5GT/s signaling technologies. A device designed to the PCIe Base 2.0 specification may support 2.5GT/s, 5GT/s or both. However, a device designed to operate specifically at 5GT/s must also support 2.5GT/s signaling. The PCIe Base specification covers chip-to-chip topologies on the system board. For I/O extensibility across PCIe connectors, the Card Electromechanical (CEM) and ExpressModule? specifications will also need to be updated, but this work will not impact mechanical compatibility of the slots, cards or modules.

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SECTION 2.3.1 - What is the correct behavior if a read or write exceeds a bar limit? For example, let's say a BAR is 128 bytes, and the Read or write request to the address space mapped by the BAR is for a size that is larger than 128 bytes. In this case what is the correct response from the device?

It should be handled as an unsupported request.

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SECTION 4.2.8 - In the PCIe Base Spec 2.0, Section 4.2.8, page 239, under Key below the table it states - D Delay Symbol K28.5 (with appropriate disparity) What exactly does the term 'appropriate disparity' mean in the above lines from Spec?

Appropriate disparity means that the D symbol must have the correct disparity for the specified sequence of symbols.

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Section 4.2.6.10.1 - I have a question about LTSSM in Loopback state. When the LTSSM is in Loopeback.Entry(p.233L24), Loopback master will send TS1 with Compliance Receive bit (Symbol 5 bit 4)=0b and Loopback bit=1b and wait to receive identical TS1 with Loopback bit asserted less than 100 ms. In this time, both sides of link are probably in 5GT/s. Then if Loopback slave cannot do Symbol lock, how long does Loopback slave need to wait, and what is the next substate?

The slave stays in Loopback.Active indefinitely until it receives an EIOS (or detects or infers an Electrical Idle). There is not timeout.

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Section 2.2.62. - How does a CPU know a device exists and where the position of the device is?

Configuration softrware reads configuration space address 00h (using different bus, device and function numbers). When it gets a response, it knows a device exists at that ID.

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Where can interested parties get more information?

PCI-SIG is the sole source for PCIe specifications. In addition, both the PCI-SIG and its members provide a plethora of technical and marketing collateral in support of the PCIe architecture. Please visit www.pcisig.com for additional information.

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SECTION 6.2.3.2.3 -- If a device encounters more than one error, will it log all the errors or the most significant error only (according to the precedence list).

It is recommended that only the highest precedence error associated with a single TLP be reported. However, it is recognized that reasonable implementations may not be able to support the recommended precedence order, which is why this is recommended rather than required behavior.

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Are both 2.5GT/s and 5GT/s signaling rates supported in the PCIe 2.0 specification?

The PCIe Base 2.0 specification supports both 2.5GT/s and 5GT/s signaling rates, in order to retain backward compatibility with existing PCIe 1.0 and 1.1 systems. Aside from the faster bit rate, there are a number of improvements in this specification that allow greater flexibility and reliability in designing PCIe links. For example, the interconnect can be dynamically managed for platform power and performance considerations through software controls. Another significant RAS feature is the inclusion of new controls to allow a PCIe link to continue to function even when some lanes become non-operational.

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SECTION 7.5.1.1 - We implement Memory Space Enable and IO Space Enable bit in our Endpoint. If the Endpoint receives a Memory Write TLP when Memory Space Enable bit is not set. How should the Endpoint handle this TLP? Also, if the Endpoint receives a Memory Write TLP and its data payload exceeds Max_Payload_Size when Memory Space Enable bit is not set. How should the Endpoint handle this TLP in each case?

For the first case, the Endpoint must handle the Request as an Unsupported Request. For the second case, it is recommended that the Endpoint handle the Request as a Malformed TLP, but the Endpoint is permitted to handle the Request as an Unsupported Request.

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SECTION 7.5.3.6 ̐ Can you please clarify the behavior of a Switch Downstream Port when the Secondary Bus Reset bit is Set in its Bridge Control register? It is our understanding that a Secondary Bus Reset will not affect anything in the Downstream Port where it is Set, only in components Downstream (i.e. components on or below the secondary bus of that virtual Bridge). Should the primary side of the virtual Bridge reset or preserve its Requester ID after the Secondary Bus Reset bit is Set?

When software sets the Secondary Bus Reset bit in a Switch Downstream Port, the Downstream Port must not reset any of its own configuration settings, and it must transition the Link below it to the Hot Reset state, assuming the Link is not down. The description of the Secondary Bus Reset bit in Section 7.5.3.6 states "Port configuration registers must not be changed, except as required to update Port status."

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Section 2.9.1 - For a PCIe 2.0 Switch, when upstream port goes to DL_down, it is stated in pg. 131 line 11 that the config registers will be reset, also line 15 says propagate reset to all other ports (which I interpret as all downstream ports, am I right?) But on line 11 of pg. 130, it says downstream port registers are not affected except status update, do these contradict?

Yes, when a link reports DL_Down the upsteam port on the switch (and all other downstream devices) are reset.

The section 2.9.1 text covers two contexts. The context of a Downstream Port in DL_Down and the context of an Upstream Port in DL_Down. Care must be taken to apply the requirements in this section to the correct context.

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Section 2.2.4.1 - In the PCIe spec 2.0 page 57, there is a sentence "For Memory Read Requests and Memory Write Requests, the Address Type field is encoded as shown in Table 2-5, with full descriptions contained in the Address Translation Services Specification, Revision 1.0." If the value of AT field is invalid, what will PCIe do? Will it report an error, and if so, what error will be reported?

Endpoints that do not support Address Translation Services set the AT field to 00b on transmitted TLPs and ignore the AT field on received TLPs.

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SECTION 4.2.6.6.2.2 -- I have an LTSSM L0s question. Let's say we have an EP that has both its RX and TX in L0s - specifically Rx_L0s.Idle and Tx_L0s.Idle. Also assume the EP receives and EI exit, and then the receiver transitions from RX_L0s.Idle to Rx_L0s.FTS. - What should Tx_L0s.Idle transition to, or should it stay in the same state?

The transmitter stays in TX_L0s.Idle.

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What test tools and other infrastructure are available to support the development of PCIe 2.0 products?

The established PCIe ecosystem delivers both pre-silicon and post-silicon tools to assist design engineers with implementing PCIe 2.0 products. In addition, PCI-SIG provides updated hardware test fixtures and test software upgrades to facilitate compliance verification at its Compliance Workshops.

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SECTION 4.2.6.2.1 -- This is in reference to the Polling.Active state as described in section 4.2.6.2.1 - "Next state is Polling.Configuration after at least 1024 TS1 Ordered Sets were transmitted, and all Lanes that detected a Receiver during Detect receive eight consecutive TS1 or TS2 Ordered Sets or their complement with both of the following conditions." We have a question relative to the statement eight consecutive TS1 or TS2 Ordered Sets". Our understanding is that it means 8 consecutive TS1 or 8 consecutive TS2. It doesn't mean a mixture of TS1 and TS2. "

The transition to Polling.Configuration follows either 8 consecutive TS1s, or 8 consecutive TS2s on all lanes that detected a receiver in Detect. Note that the intent of the spec also is to allow the 8 to be any mixture of 8 consecutive TS1s or TS2s for this particular case (not necessarily for other LTSSM transitions, however). Note also that the PCIe 2.0 Errata item A42 (Polling.Active Substate) modifies this section (see Errata item A42 at www.pcisig.com/specifications/pciexpress/base2/).

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SECTION 4.2.6.5 - In Base Spec 2.1 on page 246 line 10, it states that - "If directed" is defined as both ends of the Link having agreed to enter L1 etc. and then refers to Section 4.3.2.1, but there is no such section in the spec. Is there a section in the spec that provides more detail on this?

The reference in the spec should be to Section 5.3.2.1, which provides more detail (note that this reference will be fixed through upcoming errata to the 2.1 spec).

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What are the benefits of PCIe 2.0? What business opportunities does it bring to the market?

While doubling the bit rate satisfies high-bandwidth applications, faster signaling has the advantage of allowing various interconnect links to save cost by adopting a narrow configuration. For example, a PCI Express 1.1 x8 link (8 lanes) yields a total aggregate bandwidth of 4Gbps, which is the same bandwidth obtained from a PCI Express 2.0 x4 link (4 lanes) that adopts the 5GT/s signaling technology. This can result in significant savings in platform implementation cost while achieving the same performance level. Backward compatibility is retained as 2.5 GT/s adapters can plug into 5.0 GT/s slots and will run at the slower rate. Conversely, PCIe 2.0 adapters running at 5.0 GT/s can plug into existing PCIe slots and run at the slower rate of 2.5 GT/s.

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SECTION 4.2.6.3.5.2 - Based on the PCIe 2.0 spec, Line 13 page 212: - The next state is Configuration.Idle immediately after all Lanes that are transmitting TS2 Ordered Sets receive eight consecutive TS2 Ordered Sets with matching Lane and Link numbers (non-PAD) and identical data rate identifiers (including identical Link Upconfigure Capability (Symbol 4 bit 6)), and 16 consecutive TS2 Ordered Sets are sent after receiving one TS2 Ordered Sets. Does the received eight consecutive TS2 Ordered Sets with identical data rate identifiers (including identical Link Upconfigure Capability (Symbol 4 bit 6)) need to match the transmitted TS2 Ordered Sets if the next state is Configuration.Idle?

The received Link number must match the transmitted Link number. The received Lane number must match the transmitted Lane number. The received data rate identifier must be the same on all received lanes (but is not required to be the same as the transmitted data rate identifier). The received Link Upconfigure Capability bit must be the same on all received lanes (but is not required to be the same as the transmitted Link Upconfigure Capability bit).

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Section 2.7.2.2 - In PCIe 2.0 Spec P.128, a Poisoned I/O or Memory Write Request, or a Message with data (except for vendor-defined 25 Messages), that addresses a control register or control structure in the Completer must be handled as an Unsupported Request (UR) by the Completer. The completer receiving this kind of TLP needs to report error as UR or Poison TLP Received?

The intent is for this error case to be handled as a Poisoned TLP Received error. Errata is being developed against the 2.1 Base spec to clarify this. Due to ambiguous language in earlier versions of the spec, a component will be permitted to handle this error as an Unsupported Request, but this will be strongly discouraged.

PCI Express - 3.0

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What is scrambling? How does scrambling impact the PCIe 3.0 architecture?

Scrambling is a technique where a known binary polynomial is applied to a data stream in a feedback topology. Because the scrambling polynomial is known, the data can be recovered by running it through a feedback topology using the inverse polynomial. Scrambling affects the PCIe architecture at two levels: the PHY layer and the protocol layer immediately above the PHY. At the PHY layer, scrambling introduces more DC wander than an encoding scheme such as 8b/10b; therefore, the Rx circuit must either tolerate the DC wander as margin degradation or implement a DC wander correction capability. Scrambling does not guarantee a transition density over a small number of unit intervals, only over a large number. The Rx clock data recovery circuitry must be designed to remain locked to the relative position of the last data edge in the absence of subsequent edges. At the protocol layer, an encoding scheme such as 8b/10b provides out-of-band control characters that are used to identify the start and end of packets. Without an encoding scheme (i.e. scrambling only) no such characters exist, so an alternative means of delineating the start and end of packets is required. Usually this takes the form of packet length counters in the Tx and Rx and the use of escape sequences. The choice for the scrambling polynomial is currently under study.

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Section 4.2.6.4.1 - While in the LTSSM Recovery.RcvrLock state, if a Port receives TS Ordered Sets with a Link or Lane number that does not match those being transmitted on at least one Lane, but receives TS Ordered Sets with Link and Lane numbers that match those being transmitted and the speed_change bit is equal to 1b on at least one other Lane, should the Port transition to the LTSSM Recovery.RcvrCfg state or the LTSSM Detect state after a 24 ms timeout?

The Port should transition to the LTSSM Recovery.RcvrCfg state.

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Do PCIe 3.0 specifications only deliver a signaling rate increase?

The PCIe 3.0 specifications comprise the Base and the Card Electro-mechanical (CEM) specifications. There may be updates to other form factor specifications as the need arises. Within the Base specification, which defines a chip-to-chip interface, updates have been made to the electrical section to comprehend 8GT/s signaling. As the technology definition progresses through PCI-SIG specification development process, additional ECN and errata will be incorporated with each review cycle. For example, the current PCIe protocol extensions that address interconnect latency and other platform resource usage considerations have been rolled into the PCIe 3.0 specification revisions. The final PCIe 3.0 specification consolidates all ECN and errata published since the release of the PCIe 2.1 specification, as well as interim errata.

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Section 4.2.3 - Section 4.2.3 states, "After entering L0, irrespective of the current Link speed, neither component must transmit any DLLP if the equalization procedure must be performed, and until the equalization procedure completes." Does that result in the following sequence: 1. Negotiate a Link and enter L0. Do not allow DLLP transmission while in L0. 2. Change the data rate to 8.0 GT/s and execute the equalization procedure. 3. Enter L0. Allow DLLP transmission.

Yes, that is the expected sequence when the autonomous equalization mechanism is executed. Note that Section 4.2.3 also describes other equalization mechanisms.

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Section 4.2.6.4.3 - While down configured and a rate change request occurs, do the unused lanes also participate in the rate change?

The transmitter of the unused lanes remains in Electrical Idle during the speed change.

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Section 4.2.6.2.3 - Can you please clarify the below statement quoted from section "4.2.6.2.3. Polling.Configuration" of PCI Express 3.0 specification: Receiver must invert polarity if necessary (see Section 4.2.4.4). Does this imply the polarity inversion can only be initiated by receiver in Polling.Configuration state or can the inversion happen in Polling.Active state as well?

When polarity needs to be inverted, it must be done before exiting Poilling.Configuration, which permits it to be done in Polling.Active.

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Section 5.3.1.4.1 - A Root Port is connected to a multifunction Endpoint. The Root Port is ECRC capable. The multifunction Endpoint only has 1 function that is ECRC capable, the others are not. Software enables ECRC checking and generation in the Root Port and also enables ECRC checking and generation in the 1 Endpoint function that supports it. Given that one function is enabled for ECRC check, is the EP required to check the TD bit & ECRC on all TLPs that target any of the endpoint's functions regardless of whether the receiving function is ECRC capable?

Per Section 2.7.1, the device is required to check ECRC for all TLPs where it is the ultimate PCI Express Receiver. Note that per Section 6.2.4, an ECRC Error is not Function-specific, so it must be logged in all Functions of that device.

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Does PCIe 3.0 enable greater power delivery to cards?

The PCIe Card Electromechanical (CEM) 3.0 specification consolidates all previous form factor power delivery specifications, including the 150W and the 300W specifications.

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Section 7.11.7 - Software has enabled Virtual Channel VC1 and currently UpdateFC DLLPs for VC1 are being transmitted on the link. Now, software disables VC1. So my question is, should UpdateFC DLLPs for VC1 be transmitted on the link?

When VC Enable for VC1 is set to 0b, the device must stop transmitting UpdateFC DLLPs for VC1.

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Section 4.2.6.4 - What is the specification condition on transmitting TS1 Ordered sets while in Recover.RcvrLock state?

While the LTSSM is in Recovery.RcvrLock the Transmitter must send TS1 Ordered Sets on all configured lanes continuously with the following exceptions:
1.      At data rates above 2.5 GT/s send an EIEOS every 32 TS1 ordered Sets (4.2.4.2)

  •        EIEOS guarantees that electrical Idle exit will be detected by the link partner

2.      At all data rates send SKPOS according to 4.2.7.3

  •        for clock compensation

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What is 8b/10b encoding?

8b/10b encoding is a byte-oriented coding scheme that maps each byte of data into a 10-bit symbol. It guarantees a deterministic DC wander and a minimum edge density over a per-bit time continuum. These two characteristics permit AC coupling and a relaxed clock data recovery implementation. Since each byte of data is encoded as a 10-bit quantity, this encoding scheme guarantees that in a multi-lane system, there are no bubbles introduced in the lane striping process.

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