Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Highlighted
Valued Contributor III
813 Views

SoPC - VHDL questions

Hi, 

 

I ran into a couple of questions: 

I'm building a new component for the SOPC. 

It generated a template that seems to me not 'generic' VHDL 

 

entity pio_TERAchip_data is  

port ( 

-- inputs: 

signal address : IN STD_LOGIC_VECTOR (1 DOWNTO 0); 

signal clk : IN STD_LOGIC; 

signal in_port : IN STD_LOGIC_VECTOR (31 DOWNTO 0); 

signal reset_n : IN STD_LOGIC; 

 

-- outputs: 

signal readdata : OUT STD_LOGIC_VECTOR (31 DOWNTO 0) 

); 

end entity pio_TERAchip_data; 

 

-> Here I haven't seen declaring a signal in a port (like in signal clk : in std_logic;) 

Is this the same as: 

"clk : in std_logic;"? 

 

2nd question concerns and odd construction (I guess it's a casting, but I'm not sure) 

 

readdata <= std_logic_vector'("00000000000000000000000000000000"); 

 

Why this " ' " behind the std_logic_vector? And why this rather diffucult construction? 

 

wasn't it easier to write: 

readdata <= (others => '0'); ? 

 

Thanks in advance, 

Luc
0 Kudos
7 Replies
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

Good questions! 

 

The word "signal" in the entity is used to distinguish the port as a signal rather than say a file. 

It is assumed to be a signal if omitted, which in 99% of cases that I have seen it usually is! 

 

http://tams-www.informatik.uni-hamburg.de/vhdl/tools/grammar/vhdl93-bnf.html#concurrent_assertion_st... 

 

Has a great VHDL syntax browser. 

 

 

As to the type cast attribute question, I have no idea why the template is coded as shown and I agree that your suggested way is much neater. :confused:  

 

Maybe this is something to do with the rules used by the tool-set to automate code generation. 

 

I would be tempted to modify the template to suit your coding style.
0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

Do you mean I could 'probably' alter the code to the simpler version or should I leave as is. 

Unfortunately it's not easy to read .... 

 

Luc
0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

I have seen an explanation of the std_logic_vector' thing before as sometimes you see string'("tally ho"). I can't find the explanation I came across but basically it is to explicitly specify the type of your literal e.g. "0000000" could be std_logic_vector or a string. Here obviously there is no doubt as to what type you want but if for example you had some function which was overloaded for string and std_logic_vector then you may need to ensure one or the other was called when you passed in your literal. 

 

Bit of a vague explanation and sorry I can't find the proper reference for you. Basically I wouldn't worry about it and follow vernmid's advice instead. 

 

Is this template something that will get frequently updated? If you have automatically generated code from say a state machine editor that gets updated every time you make a change to the state diagram then I'd leave it as it is. If it's just a starting point and you are going to manually maintain the file as you would any other VHDL file then change it so it makes sense to you. (others => '0') is like you suggest a much more generic and maintainable way of writing it.
0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

I have a related problem. 

A line from a test bench generated by the SOPC builder: 

 

active_and_waiting_last_time <= std_logic'('0'); 

 

Ncsim doesn't like this. Here is the warning message: 

 

"white space assumed after apostrophe [13.5]" 

 

The same construct for std_logic_vector is accepted: 

No warning or error for this line: 

 

address_from_the_master_0_last_time <= std_logic_vector'("00000000000000000000000000000000"); 

 

Can anyone explain this? I don't like warnings that I don't understand.
0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

What's your intention to use a qualified expression in this place? It seems meaningless here.

0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

 

--- Quote Start ---  

What's your intention to use a qualified expression in this place? It seems meaningless here. 

--- Quote End ---  

As I mentioned, the code is generated by Altera tools. There are many meaningless qualified expressions in the code, but ncsim only complains for "std_logic'('0')" and "std_logic'('1')". 

 

It seems to be legal VHDL code, so I don't understand the warning from Ncsim.
0 Kudos
Highlighted
Valued Contributor III
12 Views

Re: SoPC - VHDL questions

No warning from modelsim 

Raise an issue with incisive?
0 Kudos