how to correctly assign to these variables

Hi,

These are very basic things. I recommend you find a good VHDL book and learn the syntax.

constant ff_tx_mod : IN STD_LOGIC_VECTOR (1 DOWNTO 0) := '00';

should be:

constant ff_tx_mod : STD_LOGIC_VECTOR (1 DOWNTO 0) := "00";

You use IN to define an input to an entity.

variable gm_tx_d2 :STD_LOGIC_VECTOR (7 DOWNTO 0) := '00';

should be:

variable gm_tx_d2 :STD_LOGIC_VECTOR (7 DOWNTO 0) := "00000000";

because your vector has eight elements.

constant ff_tx_mod : IN STD_LOGIC_VECTOR (1 DOWNTO 0) := '00';

get rid of the IN since this is not an input to the entity.

constant preamble : STD_LOGIC_VECTOR (7 DOWNTO 0) := '5555555555555555';

You define an eight bit vector and then try to stuff a huge number into it. The biggest unsigned number that can be represented by eight bits is 255.

Mark.

really thank you very much, your reply very helpful

preamble in mac address using 55555555 , is it a hex number or integer in double colon?

Hi,

The Ethernet preamble is 14 instances of 0x5 (binary 0101), for which you'll need 7 bytes. This can be written many different ways in VHDL, but to write it as hex use:

x"55555555555555"

which is interpreted as an unsigned vector. You could also write it as:

16#55555555555555#

which is interpreted as an integer literal.

Mark.

is STD_LOGIC_VECTOR (7 DOWNTO 0) assumed assign is integer ?

if in hex number, can it be written in

constant preamble : STD_LOGIC_VECTOR (7 DOWNTO 0) := x'55555555555555';

Hi,

> constant preamble : STD_LOGIC_VECTOR (7 DOWNTO 0) := x'55555555555555';

That would still be wrong as you're trying to assign a 56 bit number to an eight bit vector. Do this:

constant preamble : STD_LOGIC_VECTOR (55 DOWNTO 0) := x'55555555555555';

Good luck.

Mark.

You need to use double quotes, " for a string.

constant preamble : STD_LOGIC_VECTOR (55 DOWNTO 0) := x"55555555555555";

Also, this is just a hex string, not a decimal integer. a std_logic_vector is not an integer, it is a collection of bits. To assign it from an integer, you need to convert it via the numeric_std package

constant preamble : STD_LOGIC_VECTOR (55 DOWNTO 0) := std_logic_vector( to_unsigned( 555555, 56 ) );

--- Quote Start ---

You need to use double quotes, " for a string.

--- Quote End ---

Ah, my bad. I just copied his line and didn't notice he'd used single quotes.

--- Quote Start ---

Also, this is just a hex string, not a decimal integer. a std_logic_vector is not an integer, it is a collection of bits. To assign it from an integer, you need to convert it via the numeric_std package

constant preamble : STD_LOGIC_VECTOR (55 DOWNTO 0) := std_logic_vector( to_unsigned( 555555, 56 ) );

--- Quote End ---

True for older VHDL, but VHDL-1993 onwards will correctly convert the hex string to a std_logic_vector without needing to convert it explicitly. Not that yours is wrong, just two ways to skin this cat :). Personally I prefer the cleaner look of the direct assignment without lots of explicit type conversions.

Mark.

--- Quote Start ---

True for older VHDL, but VHDL-1993 onwards will correctly convert the hex string to a std_logic_vector without needing to convert it explicitly. Not that yours is wrong, just two ways to skin this cat :). Personally I prefer the cleaner look of the direct assignment without lots of explicit type conversions.

--- Quote End ---

the x"12345" is just that, a hex string, its not an integer or anything, its just a string of bits.

The OP started talking about integers, which is not a hex string, and thus requires the type conversion.

Aren't integers limited to 32-bit values in VHDL? I think the standard just says "at least" 32 bits but I don't know what Quartus actually uses.

Anyway I'm not sure it is safe to use something like 16#55555555555555#. I think it's better to stick to vectors (including the signed and unsigned types) when dealing with values that need more than 32 bits.