Motorola MVME172 - Manual (Page 76)

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Board Description and Memory Maps
1
and when VMEbus mastership has been granted. Because we have found
in the past that some VME systems can become very busy, we recommend
this time-out be set at a large value, such as 32 msec.
Once the VMEbus has been granted, a third timer takes over. This is the
global VMEbus timer. This timer starts when a transfer actually begins
(DS0 or DS1 goes active) and ends when that transfer completes (DS0 or
DS1 goes inactive). This time should be longer than any expected
legitimate transfer time on the bus. We normally set it to 256
µ
sec. This
timer can also be disabled for debug purposes. Before an MVME172
access to another MVME172 can complete, however, the VMEchip2 on
the accessed MVME172 must decode a slave access and request the local
bus of the second MVME172. When the local bus is granted (any
in-process onboard transfers have completed) then the local bus timer of
the accessed MVME172 starts. Normally, this is also set to 256
µ
sec.
When the memory has the data available, a transfer acknowledge signal
(TA) is given. This translates into a DTACK signal on the VMEbus which
is then translated into a TA signal to the first requesting processor, and the
transfer is complete. If the VMEbus global timer expires on a legitimate
transfer, the VMEbus to local bus controller in the VMEchip2 may become
confused and the VMEchip2 may misbehave; therefore, the bus timers’
values must be set correctly. The correct settings depend on the system
configuration.
MVME172 MC68060 Indivisible Cycles
The MC68060 performs operations that require indivisible
read-modify-write (RMW) memory accesses. These RMW sequences
occur when the MMU modifies table entries or when the MPU executes a
TAS, CAS, or CAS2 instruction. TAS cycles are always single-address
RMW operations, while the CAS, CAS2, and MMU operations can be
multiple-address RMW cycles. The VMEbus does not support
multiple-address RMW cycles and there is no defined protocol for
supporting multiple-address RMW cycles which start onboard and then
access off-board resources. The MVME172 does not fully support all
RMW operations in all possible cases.