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Multi-Master AHB

 
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Neel



Joined: 13 Jun 2009
Posts: 5

PostPosted: Wed Mar 16, 2016 2:00 am    Post subject: Multi-Master AHB Reply with quote

I am trying to use the AHB bus provided in the BSV_Source/Contrib folder.I am facing certain issues while implementing a 2-master single slave instance of the bus. I have explained the problem in detail below. Wanted to know if there is an updated AHB library source available anywhere?

Problem - 2 masters (Instruction-fetch and data-fetch) - 1 slave (memory)
the instruction-fetch continuously (on every cycle) generates a request on the bus for fetching instructions. This generates a "SINGLE" transaction on the bus with burst_length 1 and burst_size of 4-bytes.
Since the entire the path (TLM2+AHB) as of now is pipelined - we get an instruction every cycle back to the processor (assuming a dummy memory at the slave giving highest throughput).

My second master which is the data-fetch generates requests only when a Load/store is generated.
The problem is, that the arbitration logic is not able to grant the bus to the data-fetch master since the bus is always granted to the instruction-fetch master. In the entirety of the simulation the bus is in the "NONSEQ" transfer state.

Fix - having spent the last 2 days on the code I have realized that the bus can change arbitration output only under two conditions -
1) when no master generates a request - in this situation a new request generated by any master is taken up.
2) Only at the end of the current on-going transfer will there be an arbitration change. This is where I believe is the bug. the condition for end of transfer is when there are no remaining transfers i.e. remaining_reg (AHBBus.bsv) ==0. But in the code in the library this condition is placed as "remaining_reg == 1". the condition for update_value is never true and thus the instruction-fetch is always the master and the bus is always in "NONSEQ" state.

Changing the condition for remaining_reg to 0 does make my functionality to work but I wanted to know if this indeed is the right fix or am I missing something very basic about AHB?
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