VHDL array question

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Hi there, I'm having a bit of confusion using arrays in VHDL.

I know I can:

type heap is array (0 to 800) of std_logic;
signal que: heap;

to set 8 bits of an array, but is there a way of setting all 8 bits at
once? Something like que(0 to 7)='11111111' which I thought I could do,
but can't.
And how does the string type work? I was first trying to do this using
it ( que(0)='a' ) , but I don't understand how to address single bits
(I'm making an RS232 port so I need to.).

Also.. if anyone can recommend some good (hobbyist/student priced)
books, that would be appreciated.

Another question I've been meaning to ask for a while - on
optimization. Is it the general rule in VHDL that making the entire
project in a schematic would be more efficient, but hellishly complex?
Or is it (similar to C, arguably) the consensus that the compiler (is
that the right word?) can optimize more effectivly at that level, and
the coder should concentrate on algorithms? Or is it a religeous
argument I shouldn't get in to? ;)

Ta muchly.


Re: VHDL array question
what about



Re: VHDL array question
Thanks, I'll have  a read through it. I did search for the answer to my
problems via google - my apologies if i've asked a really simple

Ta anyway :)

Re: VHDL array question

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Howdy Alan,

Don't forget to search in groups.google.com... I found several
discussions on strings (which probably aren't what you think they are),
good VHDL books, and arrays - mostly in comp.lang.vhdl.

To answer your "compiler" question, if the schematic tool generates
moderately decent VHDL, then most synthesis tools (which is our term
for compiler) will generate decent stuff.  The last schematic tool I
used was one made by Mentor (before it changed its name several times
[the tool, not Mentor]).  I was pretty happy with its output, but I'm
sure it is way out of your price range.  You can search in
google.groups.com for more dicussion on this as well (in

BTW, from the wording of your original question (especially the
que(0)='a' part), it seems like you are approaching VHDL as a typical
programming language.  You know that it is not, right?  When coding in
an HDL (either VHDL or Verilog), you have to think in terms of the
physical hardware you want (FF's, memories, counters, shift registers,
and individual bits being OR'ed or AND'ed together).

Have fun,


Re: VHDL array question
Yeah, I'm trying very hard to visualise how the statements I make will
map to hardware. Finding it kind of hard though, so I'm hoping some
nice books will help me out.
Ta for the searching-google-groups tip, btw!

So are schematics compiled to VHDL which is then synthesised? I always
assumed the HDL was 'compiled' into a schematic which was then mapped
into the chip.

I'm determined to master this fpga marlarkey.. even if it takes me a
log time.. :) Thanks.


Re: VHDL array question

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Howdy Alan,

Until you have the hardware drawn (in your head, on paper, or in a
schematic tool), I dare say there is little chance of getting the
desired result(s) out of the tools.  I'm not saying this to make it
sound difficult or to keep you from trying - just that even people who
know what hardware they want find it easy to screw up HDL's.

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The Mentor schematic tool created VHDL.  I don't know if Xilinx's
schematic tool does or not - but it wouldn't have to.  It could
directly synthesize the schematic into a netlist, if that's how they
designed it.

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If you have some digital design experience, it shouldn't take too long
to get a fair understanding of it.

Good luck,


Re: VHDL array question

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que<= (others=>'1');
que(0 to 7)<= "11111111";

std_logic_vector is an array (<> natural range) of std_logic.  You can
use that instead of explicitly defining a type!

--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
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