Hi,
Was going through AXI spec.
As per AXI spec:
"AXI3 supports burst lengths of 1 to 16 transfers, for all burst types."
"AXI4 extends burst length support for the INCR burst type to 1 to 256 transfers. Support for all other burst types in
AXI4 remains at 1 to 16 transfers."
Also, as per AXI spec:
"for wrapping bursts, the burst length must be 2, 4, 8, or 16"
These are two contradicting lines for WRAP burst type. Can anyone confirm what's allowed specifically for AXI3 & AXI4?
Thanks
Utkarsh
Actually my question is related to supported burst length for WRAP transfers.
As per AXI3 spec,
Q1. Is this true? or does it support all transfer lengths? Confusion is because of this line in AXI spec: "AXI3 supports burst lengths of 1 to 16 transfers, for all burst types." which indicates WRAP should support any transfer length between 1 to 16.
Q2. Is it same for both AXI4 & AXI3?
Regards
the below is from A3.4.1 Address structure of AMBA® AXI™ and ACE™ ProtocolSpecification AXI3™, AXI4™, and AXI4-Lite™ ACE and ACE-Lite™ ARM IHI 0022E (ID022613)".Please read the following descriptions and the questions would be solved.
Burst lengthThe burst length is specified by:• ARLEN[7:0], for read transfers• AWLEN[7:0], for write transfers.In this specification, AxLEN indicates ARLEN or AWLEN.AXI3 supports burst lengths of 1 to 16 transfers, for all burst types.AXI4 extends burst length support for the INCR burst type to 1 to 256 transfers. Support for all other burst types inAXI4 remains at 1 to 16 transfers.The burst length for AXI3 is defined as,Burst_Length = AxLEN[3:0] + 1The burst length for AXI4 is defined as,Burst_Length = AxLEN[7:0] + 1, to accommodate the extended burst length of the INCR burst type in AXI4.AXI has the following rules governing the use of bursts:• for wrapping bursts, the burst length must be 2, 4, 8, or 16• a burst must not cross a 4KB address boundary• early termination of bursts is not supported.No component can terminate a burst early. However, to reduce the number of data transfers in a write burst, themaster can disable further writing by deasserting all the write strobes. In this case, the master must complete theremaining transfers in the burst. In a read burst, the master can discard read data, but it must complete all transfersin the burst.
Best regards,Yasuhiko Koumoto.
Hi Utkarsh,
In both AXI3 and AXI4, WRAPx bursts can only have lengths 2, 4, 8 or 16 transfers, so exactly as the later comment describing wrapping bursts states. The concept of a WRAP burst allows you to read in a cache line with the critical word first, and then fetching the remainder of the cache line after the first access. Cache lines are (usually) powers of 2 in length, hence these specific lengths.
The earlier comment about burst lengths of 1 to 16 transfers for all burst types was trying to be generic about the burst length limitations of AXI3 before then stating what AXI4 has added.
So FIXED bursts can have any length from 1-16 transfers in both AXI3 and AXI4.
And WRAP bursts can have lengths 2, 4, 8 or 16 in both AXI3 and AXI4.
But INCR bursts can have lengths 1-16 in AXI3 and 1-256 in AXI4.
JD
Thanks JD and yasuhikokoumoto
I found it a bit ambiguous, so wanted to clarify.