AXI3 Vs AXI4 The difference between AXI3 and AXI4
Connect @ Aviral Mittal


1. AXI3 supports burst lengths up to 16 beats only. While AXI4 supports burst lengths of up to 256 beats.
2. AXI3 supports write interleaving. AXI4 does NOT support write interleaving
3. AXI3 supports locked transfers, AXI4 does NOT support locked transfers
4. AXI4 supports QoS, AXI3 does NOT support QoS.
5. AXI4 supports optional 'USER' signals. AxUSER, WUSER, RUSER, BUSER.
  These can be used as sideband signals to pass user defined data from end2end.
  The protocol does not define the use of these signals, so the users have to be careful to use it in a 'local' env,
  without the expectation that any standard IP would a). Support them, b). Support them for the same intended purpose.
6. AXI4 supports Regions. AxREGION signals. These are region identifier signals sent as AWREGION or ARREGION.
These can be max 4 bits wide identifying up to max 16 different regions. These can be used by a single slave to have
multiple decode regions.
7. You can also see at times that AXI4 Slave may not have WID signal. The WID always has to match corresponding AWID
and in absence of write-interleaving support in AXI4, the information on WID becomes redundant.
Hence you may see AXI4 Slaves and even Masters for that matter without the WID signal.
In fact in AXI4 and for that matter in AXI5, the WID signals cam be completely ignored.

I have seen many IP providers e.g. Synopsys supporting burst lengths up to 256 beats in AXI3
I have also seen many IP providers e.g. Synopsys NOT supporting write interleaving in AXI3.

Looks like the industry norm is to use AXI3 with burst lenghts up to 256 beats without support for write Interleaving.

Can I connect AXI3 Master to AXI4 Slave?
Yes, you can, as long as you can specify the write interleaving depth of the AXI3 Master to be 1, it will interact
with the AXI4 just fine.
Can I connect AXI4 Master to AXI3 Slave?
Yes, you can. But keep in mind that AXI 4 Master may or may not have WID signal. If WID signal is not present in AXI4 Master, the Slave AXI WID signals can be connected to AXI4 Master's AWID signal.

AMBA Home:
AHB Info