2.3.4. Downsizing data width function

The downsizer 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 CoreLink NIC-400 Network Interconnect converts INCR bursts that fall within the maximum payload size of the output data bus to a single INCR burst. It converts INCR bursts that are greater than the maximum payload size of the output data bus to multiple INCR bursts. Table 2.7 shows how the network converts INCR bursts when it downsizes them.

Table 2.7. Conversion of INCR bursts by the downsizer function

INCR burst typeConverted to
Aligned INCR4INCR8
Unaligned INCR4INCR7[a]
Aligned INCR9INCR16 + INCR2

[a] This is only valid if the address is aligned to the destination width, and is not aligned to the source width. For example, if 0x4 is placed on a 64-32 bit downsizer, then 0x1 still requires an INCR8.


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

The CoreLink NIC-400 Network Interconnect packs INCR bursts with a SIZE smaller than the output data width to match the output width whenever possible, using the upsizer transfer function. See Upsizing data width function.

Downsize

When downsizing and protocol converting to or from AXI3 and AXI4, then the following maximum INCR burst lengths are shown in Table 2.8.

Table 2.8. Maximum INCR burst lengths

RatioAXI3 to AXI3AXI3 to AXI4AXI4 to AXI4AXI4 to AXI3
1:2Maximum Len16 outputMaximum Len16 outputMaximum Len256 output[a]Maximum Len16 output
1:4Maximum Len16 outputMaximum Len16 outputMaximum Len256 outputMaximum Len16 output
1:8Maximum Len16 outputMaximum Len16 outputMaximum Len256 outputMaximum Len16 output

[a] In AXI3 to AXI4 downsizing, you can create long bursts from an AXI3 burst input. If a restriction to short bursts is required, you can control this by a programmable register. For more information, see the registers for the ASIB, IB, and AMIB in Chapter 3 Programmers Model.


WRAP bursts

The CoreLink NIC-400 Network Interconnect always converts WRAP bursts to WRAP bursts of twice the length, up to the output data width maximum size of WRAP16, in that case, it treats the WRAP burst as two INCR bursts that can each map onto one or more INCR bursts.

Note

If a wrap transaction is aligned to the wrap boundary, it is converted into an INCR transaction.

FIXED bursts

The CoreLink NIC-400 Network Interconnect converts FIXED bursts to one or more INCR1 or INCRn bursts depending on the downsize ratio. Table 2.9 shows how the network converts FIXED bursts when it downsizes them.

Table 2.9. Conversion of FIXED bursts by the downsizer function

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

The CoreLink NIC-400 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 downsizer 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 upsizer function. See Upsizing data width function.

Acceptance capability

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

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