Tristate Bridge

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What is Tristate Bridge?. Does it convert the Avalon MM slave interface to standard Local bus which can be interfaced to SRAM?.

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Yes.

It bridges Avalon MM slave to a standard asynchronous local bus, like the ones used for sram, parallel flash and so on.

It is Tristate because you can connect to the same bus data lines several devices and use appropriate chip select signals to access them singularly.