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Arbitration
Flex ROM III provides hardware based arbitration support. This hardware works with your target’s
hardware to affect arbitration, allowing you to read and write the emulator’s memory while the target is
accessing that same memory. This may be useful for updating data tables, strings or constants on-the-fly.
The full-screen editor will even allow you to look into memory to see if the target left any messages there
for you.
Regardless of arbitration settings, FlexROM III will ALWAYS grant access to the HOST whenever the
target is powered down or being held in reset.
Flex ROM III supports several different arbitration methods.
Request/Grant
In Request/Grant arbitration, Flex ROM II will assert a /REQUEST before each access to the memory space.
The Flex ROM II hardware arbiter will then wait until the target asserts a /GRANT signal before generating
the memory cycle. At the completion of the access, Flex ROM II will release the /REQUEST signal and
return control of the memory space back to the target.
Flex ROM III does not actually access the target’s bus, but requesting access (and waiting for a grant)
insures that the target will not attempt to interrupt our access to our memory.
These arbitration signals (/REQUEST and /GRANT) could be connected to your target’s BUS
request/grant, DMA request/grant or similar control lines. Of course many micro-controllers do not support
any type of bus mastering or DMA operations.
Note that this is hardware based arbitration and very efficient. Flex ROM III will release /REQUEST within
300ns after /GRANT is asserted.
Ready
In Ready Arbitration, Flex ROM III inserts wait states into target cycles to affect arbitration. NOTE that this
is radically different than simply slowing down your target with wait-states. Flex ROM III will insert wait-
states ONLY DURING COLLISIONS. Your target runs at full speed except for during the brief moment in
which the target attempts to access Flex ROM III’s memory while it is being accessed by the Flex ROM II
itself.
This is a hardware based arbitration method. The HOST posts a request to read or write to the emulator’s
memory space. Flex ROM III hardware watches the target’s accesses and waits for the end of a target access
before starting its own access. This minimizes the chances of a collision (minimizing the impact of
arbitration). Once Flex ROM III sees a target access complete, it isolates its SRAM from the target and
starts its own access. If the target starts an access before the Flex ROM III has completed its access,
Flex ROM III asserts the /READY signal. This signal is held active until Flex ROM III has completed its
access, re-enabled the target side buffers and the memory has had time to complete the current target access.
At this point, /READY is released and the HOST is notified that the access completed.
The /READY line should be connected to your target’s wait-state circuitry.
FlexROM III User’s Manual
18
Copyright ? 2001, TechTools
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