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AR# 6459

Block Ram simulation show outputs on a read port as 'X', even though doing read only (BRAM)


Keywords: BRAM, block ram, simulation, unknown, X, output,

Urgency: Standard

General Description:

I am doing a VHDL simulation using the file created from ngd2vhdl (A1.5),
and I have Virtex Block Ram in my design. When I try and simulate the
BRAM I am seeing 'X' on the output of the read port when I write to the
other port, starting at the same address for both ports, but then
increment the write port address. Only when the write enable on the
write port goes low does the data on the read port go valid.

I am doing the following:

- Same clock for Port A and Port B
- Writing to one port (Port A), with WEA high
- Reading on the other port (Port B) WEB low
- Adress for Port A starts at 0, and increments, loading the memory.
- Port B address held static at 0 (same starting address)


The problem you are experiencing is an error in the VHDL simulation model
for the RAMB4. The following is the change that will fix the problem. This
fix is implemented in the Xilinx 2.1i release. After making the following
changes the library will have to be re-compiled.

In every model for [X_]RAMB4_S*_S* in simprim and unisim libraries in
Port B code, the following lines should change:

line 11832(In my copy) in 1.5 was

if (VALID_ADDRB AND Tviol_CLKB_CLKA_posedge /= 'X') then
downto (ADDRESS_B*DIBW));
DOB_zd := (others => 'X');
end if;

should change to:

if (VALID_ADDRB ) then
if ( WEA_ipd = '0' OR Tviol_CLKB_CLKA_posedge
/= 'X' OR (HAS_OVERLAP = FALSE)) then
downto (ADDRESS_B*DIBW));
end if;
DOB_zd := (others => 'X');
end if;

The vhd file will need to be re-compiled for the changes to take

This is fixed in 2.1i
AR# 6459
Date Created 08/31/2007
Last Updated 09/30/2008
Status Archive
Type General Article