The three most common AXI4-Stream signals are TVALID, TREADY and TDATA. Of all the AXI4-Stream signals, only TVALID is denoted as mandatory, all other signals are optional. All information-carrying signals propagate in the same direction as TVALID; only TREADY propagates in the opposite direction.
Since AXI4-Stream is a point-to-point interface, the concept of master and slave interface is pertinent to describe the direction of data flow. A master produces data and a slave consumes data.
AXI4-Stream signals are named in the following manner:
m denotes the Role (master),
axis the ClassName (AXI4-Stream) and
s denotes the Role (slave),
control the BusName which distinguishes between multiple
instances of the same class on a particular IP, and
Notes on TREADY/TVALID Handshaking
The TREADY/TVALID handshake is a fundamental concept in AXI to control how data is exchanged between the master and slave allowing for bidirectional flow control. TDATA, and all the other AXI4-Stream signals (TSTRB, TUSER, TLAST, TID, and TDEST) are all qualified by the TREADY/TVALID handshake. The master indicates a valid beat of data by the assertion of TVALID and must hold the data beat until TREADY is asserted. TVALID once asserted cannot be de-asserted until TREADY is asserted in response (this behavior is referred to as a “sticky” TVALID). AXI also adds the rule that TREADY can depend on TVALID, but the assertion of TVALID cannot depend on TREADY. This rule prevents circular timing loops. The timing diagram below provides an example of the TREADY/TVALID handshake.
Handshaking Key Points
- A transfer on any given channel occurs when both TREADY and TVALID are high in the same cycle.
- TVALID once asserted, may only be de-asserted after a transfer has completed (TREADY is sampled high). Transfers may not be retracted or aborted.
- Once TVALID is asserted, no other signals in the same channel (except TREADY) may change value until the transfer completes (the cycle after TREADY is asserted).
- TREADY may be asserted before, during or after the cycle in which TVALID is asserted.
- The assertion of TVALID may not be dependent on the value of TREADY. But the assertion of TREADY may be dependent on the value of TVALID.
- There must be no combinatorial paths between input and output signals on both
master and slave interfaces:
- Applied to AXI4-Stream IP, this means that the TREADY slave output cannot be combinatorially generated from the TVALID slave input. A slave that can immediately accept data qualified by TVALID, should pre-assert its TREADY signal until data is received. Alternatively TREADY can be registered and driven the cycle following TVALID assertion.
- The default design convention is that a slave should drive TREADY independently or pre-assert TREADY to minimize latency.
- Note that combinatorial paths between input and output signals are permitted across separate AXI4-Stream channels. It is however a recommendation that multiple channels belonging to the same interface (related group of channels that operate together) should not have any combinatorial paths between input and output signals.
- For any given channel, all signals propagate from the source (typically master) to the destination (typically slave) except for TREADY. Any other information-carrying or control signals that need to propagate in the opposite direction must either be part of a separate channel ("back-channel" with separate TREADY/TVALID handshake) or be an out-of-band signal (no handshake). TREADY should not be used as a mechanism to transfer opposite direction information from a slave to a master.
- AXI4-Stream allows TREADY to be omitted which defaults its value to 1. This may limit interoperability with IP that generates TREADY. It is possible to connect an AXI4-Stream master with only forward flow control (TVALID only).