purpuse of waitrequest

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I've designed a custom component which acts as an Avalon Slave. This component has a 32-bit result-register. If data are valid or not is indicated by Bit 31 (true (1) = data valid). Now I want to upgrade the component in such a way that read access to the result register clears the valid bit (write 0 to bit 31).

How can i avoid a write acces at the same time to the valid bit? I mean, the avalon master and my IP-Core could try to write at the same time to the register.

Can I use "waitrequest"? The IP-Core updates the register is in a specific state of the fsm. Can I set the waitrequest signal high in this state and everything would be fine? Or has waitrequest an other purpose?

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The read or write command to the register is accepted by the component deasserting waitrequest. If you assert waitrequest when your other logic is modifying the register, then you will stall any concurrent access by an Avalon-MM master.

So yes, you can use waitrequest to implement the functionality you want.

Cheers,

Dave

Thank you for your help.

After studing the avalon specification again I have a few question more.

1) Is it correct that a "normal" read or write-access needs two cycles?

2) Can I simulate the read and write behaviour with ModelSim? Is there a master aviable?

Thank you

brunjak

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After studing the avalon specification again I have a few question more.

1) Is it correct that a "normal" read or write-access needs two cycles?

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No. If a device has waitrequest low, then a write will be accepted in one cycle. The read command will also be accepted in one cycle. The Avalon specification indicates that a slave cannot deassert wait and assert readdatavalid in the same clock cycle, so the soonest a slave can return read data is one clock after the waitrequest is deasserted, i.e., two clock cycles for a single-read. However, a burst read can overlap the acceptance of the next read command while delivering the read data from a previously accepted command, if your read burst is long enough, then reads are essentially one clock too.

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2) Can I simulate the read and write behaviour with ModelSim? Is there a master aviable?

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Yes, there is a SystemVerilog JTAG-to-Avalon-MM BFM. If you are using the Altera Starter Edition, then your testbench will also need to be in SystemVerilog (or Verilog).

Cheers,

Dave

Thank you for your explanation.

Ragards

brunjak