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
3. AXI3 supports locked transfers, AXI4 does NOT support locked
4. AXI4 supports QoS, AXI3 does NOT support QoS.
5. AXI4 supports optional 'USER' signals. AxUSER, WUSER, RUSER,
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
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.