MIPS R4000 Microprocessor User's Manual 331
System Interface
NOTE: The external agent must not assert the signal ExtRqst* for the
purposes of returning a read response, but rather must wait for the un-
compelled change to slave state. The signal ExtRqst* can be asserted
before or during a read response to perform an external request other
than a read response.
5. The processor releases the SysCmd and the SysAD buses one SCycle
after the assertion of Release*.
6. The external agent drives the SysCmd and the SysAD buses within
two cycles after the assertion of Release*.
Once in slave state (starting at cycle 5 in Figure 12-17), the external agent
can return the requested data through a read response. The read response
can return the requested data or, if the requested data could not be
successfully retrieved, an indication that the returned data is erroneous. If
the returned data is erroneous, the processor takes a bus error exception.
Figure 12-17 illustrates a processor read request, coupled with an
uncompelled change to slave state, that occurs as the read request is
issued. Figure 12-18 illustrates a processor read request, and the
subsequent uncompelled change to slave state, that occurs sometime after
the read request is issued.
NOTE: Timings for the SysADC and SysCmdP buses are the same as
those of the SysAD and SysCmd buses, respectively.
Figure 12-17 Processor Read Request Protocol
SCycle
1 2 3 4 5 6 7 8 9 10 11 12
SClock
SysAD Bus
Addr
SysCmd Bus
Read
ValidOut*
ValidIn*
RdRdy*
WrRdy*
Release*
2
3
4
Master
Slave
5
1
6
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