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Large BRAM on Zynq 7000 : Read Failure

Visitor
Posts: 5
Registered: ‎04-16-2017

Large BRAM on Zynq 7000 : Read Failure

Hello,

 

I faced a problem while designing a system with large bram size (32 x 256K - WIDTH x DEPTH) on Zynq 7000 series (ZC706 board).

 

I tried to generate a single bram instance of simple dual port bram using block memory generator with width of 32 bits and depth around 256K which utilizes 232 block ram slices (total available 545). I uploaded data from DRAM to BRAM using bursts of 256 length (write operation working fine). While reading back the data from bram I got the data in anonymous order and also data bus flipped when address bus is carrying the same address. 

 

So, I thought of having some sort of timing issue with the memory design. I tried to lower the operating frequency and tried upto 500 kHz but still the results are same i.e. data is anomalous and and bits getting flipped in non-uniform fashion.

 

Experiments performed to debug :

1. Lower frequency - 500KHz - 32 x 256K  - Result - Failing to read 

2. Nominal Frequency - 500KHz - 32 x 256K  - Result - Failing to read 

 

 

Banking Structure implementation:

I also tried to lower the bram size and analyze the behaviour and I found that we can have reliable read operations with bram size (32 x 64K) and data is in orderand works well even at 50MHz .

 

So I tried to bank the overall required memory i.e. (32 x 256K) into four banks of (32 x 64K) as I was getting correct data for lower bram size. But still after instantiating four small bram modules, It followed similar anomalous operation and unable to provide the correct data.

 

I think the problem is because of overall bram utilization by the system and it corrupts the data once a limit is reached but system uses less than 50% of total bram available and I couldn't figure out the root cause of the problem.

 

If anyone has encountered similar problem or can help me with this problem. Any help would be appreciated.

 

- Ayush 

Teacher
Posts: 5,143
Registered: ‎03-31-2012

Re: Large BRAM on Zynq 7000 : Read Failure

@parth.7012121 did you simulate your design? how reliable is your timing constraints and does your design pass timing? If you have hold violations, lowering the clock speed doesn't help. Simulate and time your design properly.

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Visitor
Posts: 5
Registered: ‎04-16-2017

Re: Large BRAM on Zynq 7000 : Read Failure

@muzaffer While going through post-synthesis and post-implementation timing reports, I figured out that post-synthesis timing report shows hold violations of '0.091 ns' in write data path. Whereas post-implementation timing report shows all timing constraints met without any violation.

 

I referred to one of your post where you have mentioned that small hold violations are taken cared by Vivado and after implementation there will be no violations (similar happened in my case). But It is expected that after implementation it should work properly if there are no timing violations left. In my case, although there are no timing violations after implementation but on reading data from BRAM, it is providing anomalous data (because of mis-matched timing).

 

Can you provide any help with this.

Thanks 

Teacher
Posts: 5,143
Registered: ‎03-31-2012

Re: Large BRAM on Zynq 7000 : Read Failure

@parth.7012121 the questions are: do you have multiple clock domains in your design, or is it single clock synchronous design? Do you simulate the RTL ? If the answers are single clock domain with RTL simulations passing, then your only remaining choice is to simulate the post-implementation design with timing and see where the issue is coming from.

 

By the way, what do you mean by: "on reading data from BRAM, it is providing anomalous data (because of mis-matched timing)."

 

how do you know the result is because of "mis-matched timing"? And what does that mean exactly?

 

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Visitor
Posts: 5
Registered: ‎04-16-2017

Re: Large BRAM on Zynq 7000 : Read Failure

@muzaffer Designed system is a single clock synchronous design. I have performed logic simulation on the RTL and getting correct output there. I have also checked post-implementation timing report and checked at the exact nets but according to report and timing at that particular net, there is no timing violation in the design.

 

Earlier I mentioned that "on reading data from BRAM, it is providing anomalous data (because of mis-matched timing)." because when I use debugger to check functionality on board the data I get from BRAM is flipped at different places. At times, data bus gets flipped even if the address bus carrying the same address. Also at few positions, I can observe that data bits getting shifted as well as flipped. So I thought if having an timing issue while interacting with BRAM. Also, in post synthesis report, we can observe a hold violation in write datapath.

 

Please let me know if there is any solution

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Teacher
Posts: 5,143
Registered: ‎03-31-2012

Re: Large BRAM on Zynq 7000 : Read Failure

@parth.7012121 as you mention, what matters is the post-implementation timing, not post-synthesis. At this point the only suggestion I can make is to run post-implementation timing simulations to see if you can duplicate the issue and find the root cause if yes. If post-implementation simulation doesn't agree (with respect to timing) you should open a case with Xilinx.

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Visitor
Posts: 5
Registered: ‎04-16-2017

Re: Large BRAM on Zynq 7000 : Read Failure

@muzaffer Earlier I have performed behavioral simulation of the design only. 

 

Can you please pass on some material to perform post-implementation timing simulation for zynq (ZC706) board if possible. I tried to perform post-implementation timing simulation but as I'll be using DRAM and software core for transferring the data. Can you please let me know about the simulation possibilities.

 

Thank you

Teacher
Posts: 5,143
Registered: ‎03-31-2012

Re: Large BRAM on Zynq 7000 : Read Failure

@parth.7012121 post-implementation on zynq needs to be done in coordination with a BFM to access the various axi busses into and out of the PL which should simulate the behavior of PS & the DDR ports (through HPx I assume). Alas it's not a very easy thing to setup.

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