|Non-Confidential - Beta||PDF version||101375_0000_00_en|
|Home > Functional Description, AXI5 to AHB5 bridge > Burst conversions|
The AXI5 to AHB5 bridge converts the AXI transactions into AHB transfers. To indicate the type of AHB transfer, the bridge sets the value of the hburst, hsize, and htrans signals.
For AXI transfers with a length that matches natural AHB bursts such as 4, 8, or 16, the AXI5 to AHB5 uses fixed-length bursts.
For AXI transfers with a length of 1 and for fixed address transfers, the bridge uses AHB bursts of type SINGLE.
For other transfer lengths, the bridge uses AHB INCR bursts.
The burst boundary for AXI is 4KB, but in AHB it is 1KB. Therefore, if an AXI burst crosses the 1KB boundary, then the bridge sets the AHB burst type to INCR and at the crossing point it changes the transfer type from SEQ to NONSEQ.
The following table shows the mapping of AXI5 burst types to AHB5 burst types.
Table 2-3 AXI5 burst type to AHB5 burst type mapping
|AXI burst type & length||hburst[2:0]||Conditions|
|FIXED, any length||SINGLE||Never crosses the 1KB border, but can be sparse.|
|INCR, length 1|
|INCR, length 4||INCR4||
The mapping is possible only when all the following conditions apply:
|INCR, length 8||INCR8|
|INCR, length 16||INCR16|
|WRAP, length 4||WRAP4|
|WRAP, length 8||WRAP8|
|WRAP, length 16||WRAP16|
This mapping occurs when any of the following conditions occur:
If the bridge splits a burst, then it issues multiple INCR bursts.
The bridge sets hsize[2:0] to the value of axsize[2:0] unless it has to split an AXI transaction to multiple AHB transfers when handling sparse writes or Non-modifiable reads. In these situations, the bridge sets hsize[2:0] to the largest possible aligned size for the next AHB transfer.
The htrans[1:0] signal indicates the transfer type and whether a beat continues or splits a burst. The following table lists htrans[1:0] settings for various conditions.
Table 2-4 htrans[1:0] mapping
|Conditions for htrans[1:0] mapping||htrans[1:0]||Comment|
|An AXI burst is in progress but the relevant AXI channel (W, R, or B) is not ready. No AHB burst is in progress.|
|All AXI beat strobes are LOW.|
|First beat of an AXI transaction.||NONSEQ||-|
|hburst == SINGLE||-|
|1KB border crossing beat.||-|
|Sparse AXI beat.||NONSEQ for all AHB transfers belonging to the AXI beats.|
|Previous AXI beat was sparse.|
|Wrap-back beat.||Wrap-back beat of an AHB-INCR-converted AXI WRAP transaction. See the Any row in the hburst[2:0] mapping table.|
|AHB burst is in progress, but the relevant AXI channel (W, R, or B) is not ready.||BUSY||hburst != SINGLE|
|Any other cases.||SEQ||-|