Problem with DUT DDR input -> SDR output

OVM
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1

Hi,
I am doing verification of VHDL component using OVM and ran into serious problems. I have found that problem is in one specific component and created environment specifically for it. It’s a RGMII to internal interface converter (from DDR to SDR) using Xilinx UNISIM to simulation.


BUFG_inst : BUFG
	port map (
		O => INTERNAL_CLK,
		I => RGMII_CLK
	);

	IDDRDATA0_inst: IDDR
	generic map (
		DDR_CLK_EDGE => "SAME_EDGE_PIPELINED",
		INIT_Q1 => '0',
		INIT_Q2 => '0',
		SRTYPE  => "SYNC"
	)
	port map (
		Q1 => data(0),
		Q2 => data(4),
		C  => INTERNAL_CLK,
		CE => CE,
		D  => RGMII_DATA(0),
		R  => RST,
		S  => '0'
	);
-- several more instantiations

I generate transactions and send them to DUT using driver:


forever
begin
	my_transaction tx;
			
	@(posedge dut_vi.clock); 
	seq_item_port.get(tx);

	dut_vi.ctl   = tx.ctl;
	dut_vi.reset = tx.reset;
	dut_vi.data  = tx.data;
			
	@(negedge dut_vi.clock); 
	seq_item_port.get(tx);
			
	dut_vi.ctl   = tx.ctl;
	dut_vi.reset = tx.reset;
	dut_vi.data  = tx.data;
end

In Modelsim wave, this input (watching DUT input ports) is just fine… But internally, it takes previous value of data - for example previous value of CTL was 0, on rising edge it’s 1, but DUT samples with rising edge that previous 0 as it didn’t actualize inside modelsim.

I tried using “little hack”:


TMP_CLK <= RGMII_CLK after 1ps;

BUFG_inst : BUFG
	port map (
		O => INTERNAL_CLK,
		I => TMP_CLK
	);

It solved the problem (partially). DUT was now sampling correct values… But it created problem with output and monitor. Now everything is correct in wave, but monitor sees wrong values (but only sometimes - usually with beginning of packet - valid signal changed from 0 to 1). So it’s similar problem as with input. My monitor looks like this:


forever
begin
	dut_out_transaction tx;
	
	@(posedge dut_out_vi.clock);          
	tx = dut_out_transaction::type_id::create("tx");
	
	tx.dv    = dut_out_vi.dv;
	tx.err   = dut_out_vi.err;
	tx.sof   = dut_out_vi.sof;
	tx.eof   = dut_out_vi.eof;
	tx.data  = dut_out_vi.data;
	
	$display("DUT_OUT EOF: %b", dut_out_vi.eof);
	
	if (dut_out_vi.eof == 1)  // this never happens
		ovm_report_info("out_monitor", "EOF");

	aport.write(tx);
end

Since it’s similar bug, I tried similar hack - I added #1ps right after @(posedge dut_out_vi.clock); and it truly helped… But new problem arisen - now monitor never sees EOF signal in 1 even though it is in 1 in wave several times.

I use Modelsim 10.5 with resolution of 1ps (as requested by unisims). Any ideas what could be causing it or how to fix it?

2

In reply to arakan94:

I finally solved it - problem was in Xilinx unisim library. For some reason, there is 100ps delay (which shouldn’t be in behavioral simulation) in IDDR… So solution is to wait in monitor (I have actually two - for input and output of DUT):


forever
begin
	my_transaction tx;
	
	@(posedge dut_vi.clock);
	#1ps
	tx = my_transaction::type_id::create("tx");
	
	tx.ctl   = dut_vi.ctl;
	tx.reset = dut_vi.reset;
	tx.data  = dut_vi.data;

	aport.write(tx);
	
	@(negedge dut_vi.clock);
	#1ps
	tx = my_transaction::type_id::create("tx");
	
	tx.ctl   = dut_vi.ctl;
	tx.reset = dut_vi.reset;
	tx.data  = dut_vi.data;

	aport.write(tx);
end

and second one:


forever
begin
	dut_out_transaction tx;
	
	@(posedge dut_out_vi.clock);
	#101ps   // simulation hack
	tx = dut_out_transaction::type_id::create("tx");
	
	tx.dv    = dut_out_vi.dv;
	tx.err   = dut_out_vi.err;
	tx.sof   = dut_out_vi.sof;
	tx.eof   = dut_out_vi.eof;
	tx.data  = dut_out_vi.data;
end

In the input monitor, I have to wait 1ps (didn’t figure out why) and in output, I have to wait 101ps due to delay inside IDDR (I checked the code). This way, scoreboard gets correct data.