Advanced eXtensible Interface

The Advanced eXtensible Interface (AXI), part of the ARM Advanced Microcontroller Bus Architecture 3 (AXI3) and 4 (AXI4) specifications,[1] is a parallel high-performance, synchronous, high-frequency, multi-master, multi-slave communication interface, mainly designed for on-chip communication.

AXI has been introduced in 2003 with the AMBA3 specification. In 2010, a new revision of AMBA, AMBA4, defined the AXI4, AXI4-Lite and AXI4-Stream protocol. AXI is royalty-free and its specification is freely available from ARM.

AXI offers a wide spectrum of features, including:

  • separate address/control and data phases
  • support for unaligned data accesses
  • burst-based transfers, with a single transmission of the starting address
  • separate and independent read and write channels
  • support for outstanding transactions
  • support for out-of-order transaction completion
  • support for atomic operations.

AMBA AXI specifies many optional signals, which can be optionally included depending on the specific requirements of the design,[2] making AXI a versatile bus for numerous applications.

While the communication over an AXI bus is between a single master and a single slave, the specification includes detailed description and signals to include N:M interconnects, able to extend the bus to topologies with more masters and slaves.[3]

AMBA AXI4, AXI4-Lite and AXI4-Stream have been adopted by Xilinx and many of its partners as main communication buses in their products.[4][5]

Handshake

AXI defines a basic handshake mechanism, composed by an xVALID and xREADY signal.[6] The xVALID signal is driven by the source to inform the destination entity that the payload on the channel is valid and can be read from that clock cycle onwards. Similarly, the xREADY signal is driven by the receiving entity to notify that it is available to receive data.

When both the xVALID and xREADY signals are high in the same clock cycle, the data payload is considered "transferred" and the source can either provide a new data payload, by keeping high xVALID, or terminate the transmission, by de-asserting xVALID. An individual data transfer, so a clock cycle when both xVALID and xREADY are high, is called "beat".

Two main rules are defined for the control of these signals:

  • A source must not wait for a high xREADY to assert xVALID.
  • Once asserted, a source must keep a high xVALID until a handshake occurs.

Thanks to this handshake mechanism, both the source and the destination can control the flow of data, throttling the speed if needed.

Channels

In the AXI specification, five channels are described:[7]

  • Read Address channel (AR)
  • Read Data channel (R)
  • Write Address channel (AW)
  • Write Data channel (W)
  • Write Response channel (B)

Other than some basic ordering rules,[8] each channel is independent from each other and have its own couple of xVALID/xREADY handshake signals.[9]

AXI Read Address and Read Data channels.
AXI Write Address, Write Data and Write Response channels.

AXI

Signals[10]

Signals of the Read and Write Address channels
Signal descriptionWrite Address channelRead Address channel
Address ID, to identify multiple streams over a single channelAWIDARID
Address of the first beat of the burstAWADDRARADDR
Number of beats inside the burstAWLEN[nb 1]ARLEN[nb 1]
Size of each beatAWSIZEARSIZE
Type of the burstAWBURSTARBURST
Lock type, to provide atomic operationsAWLOCK[nb 1]ARLOCK[nb 1]
Memory type, how the transaction has to progress through the systemAWCACHEARCACHE
Protection type: privilege, security level and data/instruction accessAWPROTARPROT
Quality of Service of the transactionAWQOS[nb 2]ARQOS[nb 2]
Region identifier, to access multiple logical interfaces from a single physical oneAWREGION[nb 2]ARREGION[nb 2]
User-defined dataAWUSER[nb 2]ARUSER[nb 2]
xVALID handshake signalAWVALIDARVALID
xREADY handshake signalAWREADYARREADY
Signals of the Read and Write Data channels
Signal descriptionWrite Data channelRead Data channel
Data ID, to identify multiple streams over a single channelWID[nb 3]RID
Read/Write dataWDATARDATA
Read response, to specify the status of the current RDATA signalRRESP
Byte strobe, to indicate which bytes of the WDATA signal are validWSTRB
Last beat identifierWLASTRLAST
User-defined dataWUSER[nb 2]RUSER[nb 2]
xVALID handshake signalWVALIDRVALID
xREADY handshake signalWREADYRREADY
Signals of the Write Response channel
Signal descriptionWrite Response channel
Write response ID, to identify multiple streams over a single channelBID
Write response, to specify the status of the burstBRESP
User-defined dataBUSER[nb 2]
xVALID handshake signalBVALID
xREADY handshake signalBREADY
  1. Different behavior between AXI3 and AXI4
  2. Available only with AXI4
  3. Available only with AXI3

Bursts

AXI is a burst-based protocol,[11] meaning that there may be multiple data transfers (or beats) for a single request. This makes it useful in the cases where it is necessary to transfer large amount of data from or to a specific pattern of addresses. In AXI, bursts can be of three types, selected by the signals ARBURST (for reads) or AWBURST (for writes):[12]

  • FIXED
  • INCR
  • WRAP

In FIXED bursts, each beat within the transfer has the same address. This is useful for repeated access at the same memory location, such as when reading or writing a FIFO.

In INCR bursts, on the other hand, each beat has an address equal to the previous one plus the transfer size. This burst type is commonly used to read or write consequential memory areas.

WRAP bursts are similar to the INCR ones, as each transfer has an address equal to the previous one plus the transfer size. However, with WRAP bursts, if the address of the current beat reaches the "Higher Address boundary", it is reset to the "Wrap boundary":

with

Transactions

Reads

To start a read transaction, the master has to provide on the Read address channel:

  • the start address on ARADDR
  • the burst type, either FIXED, INCR or WRAP, on ARBURST (if present)
  • the burst length on ARLEN (if present).

Additionally, the other auxiliary signals, if present, are used to define more specific transfers.

After the usual ARVALID/ARREADY handshake, the slave has to provide on the Read data channel:

  • the data corresponding to the specified address(es) on RDATA
  • the status of each beat on RRESP

plus any other optional signals. Each beat of the slave response is done with a RVALID/RREADY handshake and, on the last transfer, the slave has to assert RLAST to inform that no more beats will follow without a new read request.

Writes

To start a write operation, the master has to provide both the address information and the data ones.

The address information are provided over the Write address channel, in a similar manner as a read operation:

  • the start address has to be provided on AWADDR
  • the burst type, either FIXED, INCR or WRAP, on AWBURST (if present)
  • the burst length on AWLEN (if present)

and, if present, all the optional signals.

A master has also to provide the data related to the specified address(es) on the Write data channel:

  • the data on WDATA
  • the "strobe" bits on WSTRB (if present), which conditionally mark the individual WDATA bytes as "valid" or "invalid"

Like in the read path, on the last data word, WLAST has to be asserted by the master.

After the completion of both the transactions, the slave has to send back to the master the status of the write over the Write response channel, by returning the result over the BRESP signal.

AXI4-Lite

AXI4-Lite is a subset of the AXI4 protocol, providing a register-like structure with reduced features and complexity.[14] Notable differences are:

  • all bursts are composed by 1 beat only
  • all data accesses use the full data bus width, which can be either 32 or 64 bits

AXI4-Lite removes part of the AXI4 signals but follows the AXI4 specification for the remaining ones. Being a subset of AXI4, AXI4-Lite transactions are fully compatible with AXI4 devices, permitting the interoperability between AXI4-Lite masters and AXI4 slaves without additional conversion logic.[15]

Signals[16]

Write address channelWrite data channelWrite response channelRead address channelRead data channel
AWVALIDWVALIDBVALIDARVALIDRVALID
AWREADYWREADYBREADYARREADYRREADY
AWADDRWDATABRESPARADDRRDATA
AWPROTWSTRBARPROTRRESP

See also

References

  1. "AMBA | Documentation". Arm Holdings.
  2. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 109–118. Retrieved 5 July 2019.
  3. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 23–24. Retrieved 5 July 2019.
  4. "AMBA AXI4 Interface Protocol". www.xilinx.com. Xilinx Inc.
  5. "AXI4 IP". www.xilinx.com. Xilinx Inc.
  6. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 37–38. Retrieved 5 July 2019.
  7. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 22–23. Retrieved 5 July 2019.
  8. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 40. Retrieved 5 July 2019.
  9. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 38. Retrieved 5 July 2019.
  10. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 28–34. Retrieved 5 July 2019.
  11. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 22. Retrieved 5 July 2019.
  12. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 45–47. Retrieved 5 July 2019.
  13. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 44. Retrieved 5 July 2019.
  14. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. pp. 121–128. Retrieved 5 July 2019.
  15. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 124. Retrieved 5 July 2019.
  16. Arm Holdings. "AMBA AXI and ACE Protocol Specification" (PDF). developer.arm.com. p. 122. Retrieved 5 July 2019.
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