一、 基本概念
低频周期性信令（LFPS，Low frequency periodic signaling）
Low frequency periodic signaling (LFPS) is used for side band communication between the two ports across a link that is in a low power link state. It is also used when a link is under training, or when a downstream port issues Warm Reset to reset the link.
For x2 operation, LFPS signals are only transmitted on the Configuration Lane.
SCD1/SCD2
SuperSpeedPlus Capability Declaration (SCD) is a step for a SuperSpeedPlus port, while in the Polling.LFPS substate, to identify itself as SuperSpeedPlus capable by transmitting
Polling.LFPS signals with specific patterns unique to SuperSpeedPlus ports. This section defines SuperSpeedPlus specific patterns in SCD1 and SCD2.
LBPM SuperSpeedPlus LFPS Based PWM Message
LBPM is defined as a low power signaling mechanism for two SuperSpeedPlus ports to communicate with each other based on LFPS signals. The adoption of Pulse Width Modulation (PWM) is to embed the transmitting clock in data and to allow for easy data recovery at the receiver based on LFPS clock defined in Table 6-29. T
二、physical layer
gen1 8b/10b 编码
gen2 128b/132b 编码
gen1 data scrambling
Special Symbols for Framing and Link Management
Training sequence TS1, TS2, TSEQ
三、link layer
3.1 包结构
link layer 包结构，分为 LMP/TP/DP/ITP header packet 和 data packet payload
LMP TP DP ITP header packet
header packet 包括 link control word、packet header
Data Packet Payload
3.2 link commands
Link commands are used for link level data integrity, flow control and link power management.
Link commands are a fixed length of eight symbols and contain repeated symbols to increase the error tolerance. Refer to Section 7.3 for more details. Link command names have the L-preface to differentiate their link level usage and to avoid confusion with packets
| Link Command | b10-9 | b8-7 | b6-4 | b3-0 | |
|---|---|---|---|---|---|
| LGOOD_n | 00 | 00 | 000 | HP Sequence Number 0000:LGOOD_0 … 1111:LGOOD_15 | n: Header Sequence Number. Sent by a port receiving a header packet when all of the following conditions are true: • The header packet has a valid structure and can be recognized by the receiver. • CRC-5 and CRC-16 are valid. • Header Sequence Number in the received header packet matches the expected Rx Header Sequence Number. • An Rx Header Buffer in the receiver is available for storing the received header packet. Mismatch between a Header Sequence Number in the received header packet and the expected Rx Header Sequence Number will result in a port transitioning to Recovery. Received by a port sending a header packet. This is an acknowledgement from a link partnerthat a header packet with the Header Sequence Number of “n” is received properly. Receipt of LGOOD_n mismatching the expected ACK Tx Header Sequence Number will result in a porttransitioning to Recovery. Also sent by a port upon entry to U0 as the Header Sequence Number Advertisement to initialize the ACK Tx HeaderSequence Number of the two ports. |
| LCRD_x LCRD1_x LCRD2_x | 00 | 01 | 000 | Gen 1x1, Gen 1x2, Gen 2x1: b3: Reserved b2: Credit series 0: LCRD_x or LCRD1_x 1: LCRD2_x b1~0: Rx Header Buffer Credit 00: LCRD_A/LCRD1_A/LCRD2_A 01: LCRD_A/LCRD1_A/LCRD2_A 10: LCRD_A/LCRD1_A/LCRD2_A 11: LCRD_A/LCRD1_A/LCRD2_A | used to signal the availability of Rx Header Buffers in terms of Credit. |
| LRTY | 00 | 10 | 000 | reserved | Sent by a port before resending the first header packet in response to receipt of LBAD. |
| LBAD | 00 | 11 | 000 | reserved | Bad header packet. Sent by a port receiving the header packet in response to an invalid header packet. Packet that was received has corrupted CRC-5 and/or CRC-16. Receipt of LBAD will cause a port to resend all header packets after the last header packet that has been acknowledged with LGOOD_n. Refer to Section 7.2.4.1 for details. |
| LGO_Ux | 01 | 00 | 000 | 0001: LGO_U1 0010: LGO_U2 0011: LGO_U3 Others: Reserved | Sent by a port requesting entry to U1. Sent by a port requesting entry to U2 Sent by a downstream port requesting entry to U3. An upstream port shall accept the request. |
| LAU | 01 | 01 | 000 | reserved | Sent by a port accepting the request to enter U1, U2, or U3. |
| LXU | 01 | 10 | 000 | reserved | Sent by a port rejecting the request to enter U1 or U2. |
| LPMA | 01 | 11 | 000 | reserved | Sent by a port upon receiving L AU. Used in conjunction with LGO_Ux and LAU handshake to guarantee both ports are in the same state. |
| LDN | 10- | 00 | 000 | reserved | Downstream port present in U0. Sent by a downstream port every 10 µs when there are no packets or other link commands to be transmitted. Refer to Section 7.5.6.1 for detail |
| LUP | 10 | 01 | 000 | reserved | Device present in U0. Sent by an upstream port every 10 µs when there are no packets or other link commands to be transmitted. Refer to Section 7.5.6.1 for details. |
LCRD 发送规则
初始时根据 buffer 数量发送相应的 LCDR_x
进入 U0 后 → 发送 LGOOD_n → 马上发 LCRD_x。
| Local Rx Header Buffer Credit Count | 要发送的 LCRD 报文 |
|---|---|
| 1 | LCRD_A |
| 2 | LCRD_A, LCRD_B |
| 3 | LCRD_A, LCRD_B, LCRD_C |
| 4 | LCRD_A, LCRD_B, LCRD_C, LCRD_D |
正常工作时
处理完一个 header packet 后 发送一个新的LCRD_x
A B C D 循环发送
3.3 LTSSM
四、protocol layer
Packet Types
- 00000b Link Management Packet (LMP)
subtype
0000b Reserved
0001b Set Link Function
0010b U2 Inactivity Timeout
0011b Vendor Device Test
0100b Port Capability
0101b Port Configuration
0110b Port Configuration Response
0111b Precision Time Management
1000b-1111b Reserved - 00100b Transaction Packet (TP)
subtype
0000b Reserved
0001b ACK
0010b NRDY
0011b ERDY
0100b STATUS
0101b STALL
0110b DEV_NOTIFICATION
0111b PING
1000b PING_RESPONSE
1001b – 1111b Reserved - 01000b Data Packet Header (DP)
- 01100b Isochronous Timestamp Packet (ITP)
