DP8344 datasheet(88/184 Pages) NSC

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40 Remote Interface and Arbitration System (RIAS) (Continued)

The BCP is now shown executing a local memory write with

remote data still pending in the latch At the end of this

instruction the BCP begins executing a series of internal

operations which do not require the bus RASM therefore

takes over and without waiting the Timing Control Unit exe-

cutes the Remote Write

426 Remote Rest Time

For the BCP to operate properly remote accesses to the

BCP must be separated by a minimal amount of time This

minimal amount of time has been termed ‘‘rest time’’

There are two causes for remote rest time The first cause is

implied in the functional state machine forms for remote ac-

cesses and can be explained as follows At the beginning of

every T-state the validity of a remote access is sampled for

that T-state To guarantee that the BCP recognizes the end

of a remote cycle the time between remote accesses must

be a minimum of one T-state plus set up and hold times

In the case of Latched Read and Fast Buffered Write the

validity of a remote access is not sampled on the first rising

edge of the CPU-CLK following XACK rising However on

all subsequent rising edges of the CPU-CLK the validity of

the remote access is sampled As a result if the remote

processor can terminate its remote access quickly after

XACK rises (within a T-state) up to a T-state may be added

to the above equation for Latched Read and Fast Buffered

Write modes (ie a second remote access should not begin

for two T-states plus set up and hold times after XACK rises

in Latched Read and Fast Buffered Write modes) On the

other hand if the remote processor does not terminate its

remote access within a T-state of XACK rising the above

equation (one T-state plus set up and hold times between

remote accesses) remains valid for Latched Read and Fast

Buffered Write modes

If these specifications are not adhered to the BCP may

sample the very end of one valid remote access and one

T-state later sample the very beginning of a second remote

access Thus the BCP will treat the second access as a

continuation of the first remote access and will not perform

the second readwrite The second access will be ignored

(Reference

Figure 4-24 for the timing diagrams which dem-

onstrate how two remote accesses can be mistaken as

one)

The second source of remote rest time is due to the manner

in which the BCP samples the CMD signal CMD is sampled

once at the beginning of each remote access Due to the

manner in which CMD is sampled CMD will not be sampled

again if a second remote access begins within 15 T-states

plus a hold time after the BCP recognizes the end of the

first remote access If this happens the BCP will use the

value of CMD from the previous remote access during the

second remote access If the value of CMD is the same for

both accesses the second access will proceed as intended

However if the value of CMD is different for the two remote

accesses the second remote access will readwrite the

wrong location

The reader should note that the timing of the second source

of rest time begins at the same time that the BCP first sam-

ples the end of the previous remote access Thus when the

first source of rest time ends the second source of rest time

begins (Reference

Figure 4-25 for timing diagrams for rest

time in all modes except Latched Write mode)

Latched Write Mode

Latched Write mode is a special case of rest time and

needs to be discussed separately from the other modes

The first cause of rest time affects every mode including

Latched Write In regards to the second source of rest time

Latched Write mode was designed to allow a second re-

mote access to start while a write is still pending (ie

WR-PEND e 0) Thus when WR-PEND rises (signaling the

end of the previous write) the value of CMD is sampled for

the second remote access This allows Latched Write to

avoid the second cause of rest time discussed above

However if a remote access begins within one half a

T-state after WR-PEND rises CMD will not be sampled

again For this case if the value of CMD changes just after

WR-PEND rose and at the same time the remote access

begins the BCP will readwrite the wrong location (Refer-

ence

Figure 4-26 for timing diagrams of rest time for latched

write mode)

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