Strange FPGA problem

Hello Guys,

I am working on some verfication of the SoC IP. To verify these IP we
are implementing it on FPGA.
I am facing an strange problem.

One of the IP which i have implemented as Master and slave AHB
interface. Since the AMBA interface
i am having is a multiplexed bus i need to multiplex the master and
slave interface signal. So to do that
i am using grant signal of AHB to multiplex common signal (of master
and slave). But the problem is in
slave mode (grant signal is zero), external HBURST[2:0]=000 (seen
through logic analyzer), i have tapped
the internal HBURST to test point, i observed that the tapped internal
signal value is different from the external HBURST.

Internally i am connecting the HBURST to slave HBURST port and to
multiplex HBURST i am using the below assign for master mode

assign SGNTi = HGRANT[2];

always @(posedge iHCLKBUF or negedge nSYSRST)
begin : p_MasterEnable
  if (nSYSRST == 1'b0)
      AHBMasterEnable <= 1'b0;
  else
    if (HREADY == 1'b1)
      AHBMasterEnable <= SGNTi;
end
assign HBURST = (AHBMasterEnable == 1'b1) ? iHBURSTOut : 3'bzzz;

iHburstOut is signal which is driven by the Master interface of the IP.

I am not able to understand how does the HBURST signal tapped (test
point) changed to something other then the external HBURST (HGRANT=0)?

One more doubt:
Is there any problem in assign the incoming HBURST directly to test
point? 

waiting for ur reply,
Thanks and regards
williams

Hi, first I would like to suggest to get rid of this construction
"always @(posedge iHCLKBUF or negedge nSYSRST)"
as it is not really a synthesised construction, so your multiplexer will  
never operate properly,
you can see it in PP&R simulations.

Second, your code is not really a multiplexer, but a FF.
So I would recommend to think more on architecture.
Hope it will help.
Regards,
Alex

> Hello Guys,
>
> I am working on some verfication of the SoC IP. To verify these IP we
> are implementing it on FPGA.
> I am facing an strange problem.
>
> One of the IP which i have implemented as Master and slave AHB
> interface. Since the AMBA interface
> i am having is a multiplexed bus i need to multiplex the master and
> slave interface signal. So to do that
> i am using grant signal of AHB to multiplex common signal (of master
> and slave). But the problem is in
> slave mode (grant signal is zero), external HBURST[2:0]=000 (seen
> through logic analyzer), i have tapped
> the internal HBURST to test point, i observed that the tapped internal
> signal value is different from the external HBURST.
>
> Internally i am connecting the HBURST to slave HBURST port and to
> multiplex HBURST i am using the below assign for master mode
>
> assign SGNTi = HGRANT[2];
>
> always @(posedge iHCLKBUF or negedge nSYSRST)
> begin : p_MasterEnable
>   if (nSYSRST == 1'b0)
>       AHBMasterEnable <= 1'b0;
>   else
>     if (HREADY == 1'b1)
>       AHBMasterEnable <= SGNTi;
> end
> assign HBURST = (AHBMasterEnable == 1'b1) ? iHBURSTOut : 3'bzzz;
>
> iHburstOut is signal which is driven by the Master interface of the IP.
>
> I am not able to understand how does the HBURST signal tapped (test
> point) changed to something other then the external HBURST (HGRANT=0)?
>
> One more doubt:
> Is there any problem in assign the incoming HBURST directly to test
> point?
>
> waiting for ur reply,
> Thanks and regards
> williams
>



-- 
Alex