Implementing multiple registers with one single input output bus and address select in VHDL.

Sep 30, 2003 1 Replies

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James Williams 22 years ago

Hello,

I am just now learning VHDL and am wondering how I work write code to use one single 8bit in/out pins, which is mapped to 4 internal 8bit registers, which can either be read or written too. It of course would has a 2 bit address select. I know how to do the entity declaration, I'm not quite sure how to do the arch process. Here is what I am thinking, please tell me if I am doing this wrong.

Note also that this is just an example on not my actual project.

entity myMemory is port (Clk : in std_logic; DataIo :inout std_logic_vector (7 downto 0) Bus :=(others=>'Z'); OutEn : in std_logic_vector; --Enables the output buffers. Write : in std_logic; --Write to memory " Active low" Read : in std_logic; --Read from memory "Active low" Addr : in std_logic_vector (1 downto 0)); end myMemory; architecture Behavioral of myMemory is signal Reg0 : std_logic_vector (7 downto 0); signal Reg1 : std_logic_vector (7 downto 0); signal Reg2 : std_logic_vector (7 downto 0); signal Reg3 : std_logic_vector (7 downto 0); begin process(Clk) begin if(Clk'event and Clk='1') then case Addr is when "00"=>

if(Read='0' and Write='1' and 'OutEn='1') then DataIo

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Jim Lewis 22 years ago

James, Before coding, you want to start with a good picture of the hardware in your mind (or even better on paper).

Your code probably has a few surprises in it. There is a register on DataIO and on OutEn. Is this what you wanted?

First a couple of HDL principles

1) Any signal assignment in a clocked process (one that has "wait until Clk='1'", or an if expression with 'event or rising_edge). 2) Always code tristates separate from all other logic (avoids a number of issues). 3) Don't code things that your hardware implementation already guarantee's (such as read and write being mutually exclusive - if you need to check them, then use an assert statement).

What I would suggest is to write it as three separate pieces of code.

1) A register bank that accepts writes to the correct address 2) A Multiplexer for read back 3) A Tristate

Recoding your design a little:

-- Writing to register bank RegBankProc : process(Clk) begin if rising_edge(Clk) then if Write = '1' and ChipEnable = '1' then case Addr is when "00"=> Reg0 Reg1 Reg2 Reg3 -- report not required, but good for validation report "address invalid during valid write" severity error ; end case ; end if ; end if ; end process ;

-- Readback Multiplexer ReadBackMuxProc : process(Addr, Read) begin DataIo_int 'X') ; -- Default value if not read if Read = '1' and ChipEnable = '1' then case Addr is when "00"=> DataIo_int DataIo_int DataIo_int DataIo_int -- report not required, but good for validation report "address invalid during valid read" severity error ; end case ; end if ; end process ;

-- Tristate (process also ok) with OutEn select DataIo 'Z') when '0', (others => 'X') when others ;

-- Check for bad things, -- not required, but good for validation -- addresses issues in your code without creating hardware assert not (ChipEnable='1' and Read='1' and Write='1') report "Read and Write asserted simultaneously" severity ERROR ;

Cheers, Jim

-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Jim Lewis Director of Training mailto: snipped-for-privacy@SynthWorks.com SynthWorks Design Inc.

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1-503-590-4787

Expert VHDL Training for Hardware Design and Verification ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

James Williams wrote:

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