2.3.2. Downsizing data width function

The downsize function reduces the data width by the following ratios:

The downsizer does not merge data narrower than the destination bus if the transaction is marked as non-cacheable.

This section describes the following:

INCR bursts

The AMBA Network Interconnect converts INCR bursts that fall within the maximum payload size of the output data bus to a single INCR. It converts INCR bursts that are greater than the maximum payload size of the output data bus to multiple INCR bursts. Table 2.5 shows how the network converts INCR bursts when it downsizes them.

Table 2.5. Conversion of INCR bursts by the downsize function

INCR burst typeConverted to
Aligned INCR4INCR8
Unaligned INCR4INCR7
Aligned INCR9INCR15 + INCR2

INCR bursts with a size that matches the output data width pass through unconverted.

The AMBA Network Interconnect packs INCR bursts with a SIZE smaller than the output data width to match the output width whenever possible, using the upsize transfer function. See Upsizing data width function.

WRAP bursts

The AMBA Network Interconnect always converts WRAP bursts to WRAP bursts of twice the length, up to the output data width maximum size of WRAP16, in which case, it treats the WRAP burst as two INCR bursts that can each map onto one or more INCR bursts.

FIXED bursts

The AMBA Network Interconnect converts FIXED bursts to one or more INCR1 or INCRn bursts depending on the downsize ratio. Table 2.6 shows how the network converts FIXED bursts when it downsizes them.

Table 2.6. Conversion of FIXED bursts by the downsize function

FIXED burst typeConverted to
FIXED1INCR2
FIXED2INCR2 + INCR2 + ...

The AMBA Network Interconnect optimizes unaligned fixed bursts. If an unaligned input fixed burst maps onto a single output beat, then the output is a fixed burst of the optimal size.

Bypass merge

You can configure the downsizer function to have a programmable bit named bypass_merge. If bypass_merge is asserted, the network does not perform any packing of beats to match the optimum SIZE, up to the output data width SIZE.

An aligned input burst means that the address is aligned to the input data width word boundary after the network aligns it to the transfer size. An unaligned input burst means that the address is not aligned to the input data width word boundary, even after the network aligns it to the transfer size.

If a transaction passes through, this means that the downsize function does not change the input transaction size and type.

Note

  • If an exclusive transaction is split into multiple transactions at the output of the downsizer, the exclusive flag is removed and the master never receives an EXOKAY response. Response priority is the same as for the upsize function. See Upsizing data width function.

  • In the following example, the input data width is 128-bit and the output data width is 64-bit unless otherwise stated.

Acceptance capability

You can configure the acceptance capability to 1-32 read transactions and 1-32 write transactions. The maximum issuing capability is (ratio x acceptance capability +1).

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