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Memory Controller
MOTOROLA
MPC801 USER’S MANUAL
15-31
15
15.2.3.5 LOOP CONTROL SIGNAL.
allows you to run repetitive subpatterns included in a memory cycle pattern a specific
number of times. The memory controller marks the first word that the LOOP bit is found
asserted in a pattern as the loop start word. At this time, the memory LOOP counter is
loaded with the corresponding contents of the LOOP field. The next word that the LOOP bit
is found asserted in the same pattern is marked as the loop end word in the memory
controller. At this time, the memory LOOP counter is decremented by one.
The LOOP bit in the user-programmable machine
Whether or not the word following the loop end word is run depends on the value of the
memory LOOP counter. If it is not zero, the next word is the loop start word. If it is zero, the
user-programmable machine continues with the word after loop end word. After exiting a
loop, the next word read in the user-programmable machine with a LOOP bit set, is marked
as the new loop start word of the new loop. Every time the LAST bit is found in a pattern, the
loop condition is reset. The LOOP field is loaded into the loop counter when the
user-programmable machine services a request. The decoding of the loop bits is shown in
Table 15-4.
15.2.3.6 EXCEPTION HANDLING.
MPC801 under UPM control on the memory controller, the external device may assert the
TEA or RESET signal. The MPC801 tries to close the bus transfer immediately. The
user-programmable machine in the memory controller provides a mechanism by which you
can handle the memory control signals to meet the timing requirements of the device and
assume no data is lost. When one of the exceptions mentioned above is recognized and the
EXEN bit in the user-programmable machine is set to 1, the next word read and run by the
user-programmable machine is found at the fixed address “exception start address”—EXSA
(RAM ADDRESS = 0x’3C). Normally, there is a pattern here that allows immediate negation
of the control signals. If the EXEN bit is 0, the user-programmable machine continues with
the remaining words until the EXEN bit is 1 and a branch to the exception start address is
performed or until the LAST bit is read by the user-programmable machine. When the
“branch” to the EXSA is performed, the user-programmable machine continues reading from
successive locations until the LAST bit is equal to 1 in a UPM word.
When an access to a memory device is initiated by the
Table 15-4. Loop Field For UPM Service Requests
REQUEST SERVICED
UPM LOOP FIELD LOADED
Read Single Beat Cycle
RLFx
Read Burst Cycle
RLFx
Write Single Beat Cycle
WLFx
Write Burst Cycle
WLFx
Periodic Timer Expired
TLFx