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The SPLIT and RETRY responses provide a mechanism for slaves to release the bus when they are unable to supply data for a transfer immediately. Both mechanisms allow the transfer to finish on the bus and therefore allow a higher-priority master to get access to the bus.
When a master initiates a transaction on the AMBA bus, if the target detects that the transfer will take a large number of cycles to perform, it can issue a SPLIT signal. What happens now is that the arbiter can grant the bus to other masters even before the SPLIT transaction is complete. The master to which the SPLIT has been issued has to then wait and complete the entire transaction.
During the address phase of a transfer the arbiter generates a tag, or bus master number, on HMASTER[3:0] which identifies the master that is performing the transfer. Any slave issuing a SPLIT response must be capable of indicating that it can complete the transfer, and it does this by making a note of the master number on the HMASTER[3:0] signals.
Later, when the slave can complete the transfer, it asserts the appropriate bit, according to the master number, on the HSPLITx[15:0] signals from the slave to the arbiter. The arbiter then uses this information to unmask the request signal from the master and in due course the master will be granted access to the bus to retry the transfer. The arbiter samples the HSPLITx bus every cycle and therefore the slave only needs to assert the appropriate bit for a single cycle in order for the arbiter to recognize it.
The basic stages of a SPLIT transaction are:
1. The master starts the transfer in an identical way to any other transfer and
issues address and control information
2. If the slave is able to provide data immediately it may do so. If the slave
decides that it may take a number of cycles to obtain the data it gives a SPLIT
transfer response. During every transfer the arbiter broadcasts a number, or
tag, showing which master is using the bus. The slave must record this number,
to use it to restart the transfer at a later time.
3. The arbiter grants other masters use of the bus and the action of the SPLIT
response allows bus master handover to occur. If all other masters have also
received a SPLIT response then the default master is granted.
4. When the slave is ready to complete the transfer it asserts the appropriate bit of
the HSPLITx bus to the arbiter to indicate which master should be regranted
access to the bus.
5. The arbiter observes the HSPLITx signals on every cycle, and when any bit of
HSPLITx is asserted the arbiter restores the priority of the appropriate master.
6. Eventually the arbiter will grant the master so it can re-attempt the transfer. This
may not occur immediately if a higher priority master is using the bus.
7. When the transfer eventually takes place the slave finishes with an OKAY
transfer response.
For a SPLIT transfer the arbiter will adjust the priority scheme so that any other
master requesting the bus will get access, even if it is a lower priority. In order
for a SPLIT transfer to complete the arbiter must be informed when the slave has
the data available.
For RETRY the arbiter will continue to use the normal priority scheme and
therefore only masters having a higher priority will gain access to the bus.
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