2.2 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.

hburst[2:0] mapping

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:

  • The burst does not cross 1KB border.
  • For writes, awsparse is LOW or hwstrb is in use.
  • A read is not unaligned Non-modifiable.
  • AXI transaction size is not 1024.
  • AXI burst type is not WRAP length 16.
INCR, length 8 INCR8
INCR, length 16 INCR16
WRAP, length 4 WRAP4
WRAP, length 8 WRAP8
WRAP, length 16 WRAP16
Any INCR

This mapping occurs when any of the following conditions occur:

  • The incoming AXI burst is of a type and length combination that is not previously listed.
  • The burst crosses a 1KB border.
  • For writes, awsparse is HIGH.
  • An AXI WRAP burst that has a length of 2.
  • An AXI WRAP burst has a length of 16.
  • An AXI data width of 1024 bits.
  • The read is unaligned Non-modifiable.

If the bridge splits a burst, then it issues multiple INCR bursts.

hsize[2:0] mapping

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.

htrans[1:0] mapping

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
Default IDLE -
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 -
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