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MPC106 PCIB/MC User's Manual
MOTOROLA
As a master, if the MPC106 does not detect the assertion of DEVSEL within four clocks
following the address phase (five clocks after asserting FRAME), it terminates the
transaction with a master-abort. On reads that are aborted, the MPC106 returns all 1s
(0xFFFF_FFFF). On writes that are aborted, the data is lost.
7.4.3.2 Target-Initiated Termination
By asserting the STOP signal, a target may request that the master terminate the current
transaction. Once asserted, the target holds STOP asserted until the master negates
FRAME. Data may or may not be transferred during the request for termination. If TRDY
and IRDY are asserted during the assertion of STOP, data is transferred. However, if TRDY
is negated when STOP is asserted, it indicates that the target will not transfer any more data,
and the master therefore does not wait for a final data transfer as it would in a completion
termination.
When a transaction is terminated by STOP, the master must negate its REQ signal for a
minimum of two PCI clocks, one being when the bus goes to the idle state (FRAME and
IRDY negated). If the master intends to complete the transaction, it must reassert its REQ
immediately following the two clocks or potential starvation may occur. If the master does
not intend to complete the transaction, it can assert REQ whenever it needs to use the PCI
bus again.
There are three types of target-initiated termination:
Disconnect
Disconnect refers to termination requested because the target is
temporarily unable to continue bursting. Disconnect implies that some
data has been transferred. The master may restart the transaction at a
later time starting with the address of the next untransferred data.
(That is, data transfer may resume where it left off.)
Retry
Retry refers to termination requested because the target is currently in
a state where it is unable to process the transaction. Retry implies that
no data was transferred. The master may start the entire transaction
over again at a later time. Note that the
PCI Local Bus Specification,
Revision 2.1 requires that all retried transactions must be completed.
Target-abort Target-abort is an abnormal case of target-initiated termination.
Target-abort is used when a fatal error has occurred, or when a target
will never be able to respond. Target-abort is indicated by asserting
STOP and negating DEVSEL. This indicates that the target requires
termination of the transaction and does not want the transaction
retried. If a transaction is terminated by target-abort, the received
target-abort bit (bit 12) of the master’s status register and the signaled
target-abort bit (bit 11) of the target’s status register is set. Note that
any data transferred in a target-aborted transaction may be corrupt.