SRAM bidirectional bus

Feb 24, 2004 3 Replies
A

Hi,



I have a question concerning the VHDL description of a bidirectional bus.



This bus comes from (goes to) an SRAM which I try to simulate with a corresponding VHDL model.



Now I have an INOUT pin at my SRAM-Controller : Sram_data : inout(7 downto 0);



Within my SRAM-Controller I have the local signals l_sram_data_out : std_logic_vector(7 downto 0); l_sram_data_in : std_logic_vector(7 downto 0);


l_sram_data_out describes the data which I want to write into the SRAM. l_sram_data_out is a registered signal.



l_sram_data_in describes the data I want to read from the SRAM.



The signal which is responsible for writing to the SRAM or reading out of the SRAM is WE_bar. WE_bar='0' & CS_bar='0' & OE_bar='1' ---> Write to the SRAM WE_bar='1' & CS_bar='0' & OE_bar='0' ---> Read from the SRAM



How can I connect l_sram_data_out and l_sram_data_in in a appropriate way to the bidirectional bus?



Does l_sram_data_in has to be synchronized? (It is used within the controller in a synchronous environment)



I would appreciate any helpful hint.



Kind regards André V.


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M

My goal was to make Xilinx ISE use all the FF in the io-blocks ... one ff for data_in, for the tristate-buffer and for data_out

the following has to be placed in the toplevel and was the only way to make ISE do what I wanted ...

(Options: keep_hierarchy=true, use io-buffers=all/auto... can't remember)

if clk'event and clk = 1 then l_sram_data_in

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A

That works? The synthesis tool doesn't choke on the (others => 'Z')?

The usual way to do it is (sorry for Verilog here, also assume active HIGH output enable):

assign Sram_data = OE ? l_sram_data_out : 'hz;

-a

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M

well - yes ... there are a few things I should have mentioned:

- Im using Xilinx fpgas with ISE and XST.

- The signals are all std_logic_vector and Sram_data should be std_logic_vector with INOUT

- (others => 'Z') is only a vhdl shortcut for "ZZZZZZZZZZZZ"

this looks similar to Sram_data 'Z');

if you allow FFs to be placed in io-blocks then XST puts the data-FFs in the iob ... but if you take a close look at xilinx fpgas then you can see another FF for controlling the Tristate-Buffer ... the only way (I know) to make XST use this FF was the above description ...

bye, Michael

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