Using LPM Megafunctions in
your own VHDL
Ian McCrum
• The Megafunctions wizard in Altera gives you a powerful
and easy way to build custom parts
• It creates a .vhdl file but also a .cmp file and a _inst.vhdl file
– The .cmp file is the VHDL you insert to declare a component
– The optional _inst.vhdl file is a template to instantiate the
component – it lists the port names in a convenient way
– The VHDL file is a simple wrapper that feed the appropriate
values from the wizard to the altera libsray part
• There are several ways to use these.
• If you are a beginning VHDL modeller (programmer) then
the first is the easiest
• If you are an experienced VHDL programmer – familiar with
packages and all the VHDL constructs then method 2 is
handy.
Example Design
If a ROM contains contents and these are
latched through a register a counter can be
designed. Assume the ROM contents are such
that
address 0 contains 1,
address 1 contains 2
address 2 contains 3,
….
address 31 contains 1,
Thus on power up the Q output
is 0000 and the ROM has
address inputs sitting at 0000.
The ROM contents of 00001 are
sitting at the D-type inputs but
are not latched into the D-types
until the clock.
After the clock the output is
now 00001 and the next ROM
location is addressed.
To use the megafunction wizard
Start the Megafunction option under the tools menu or the BDF designer (below)
Start a project, e.g called v_lpm. Use a foldercalled v_lpm. All relevant files end up
here.
Create a bdf file and select the insert component menu item
Select the Megawizard Plug-in Manager at the bottom
Make sure you give a sensible filename – in the correct folder
pick VHDL as the output file
Pick the correct part
Fill in the next few screens
Sensibly (read them and think!)
Memory contents
A .mif file is a simple text file, you can create this
within Quartus by using the “NEW” menu item
under the File menu. Or use a text editor and
place the file in the project folder and specify it in
the window on the left
A mif file can have 2 types of comments. Single line comments begin with a ; or multiline comments by % delimiting symbols. The radix can be unsigned decimal, binary or
hex. You must specify the DEPTH (number of addresses) and WIDTH (in bits.)
Don’t
use …
At the end of the wizard…
You can tick the _inst.vhd if you like, but make sure you have the .cmp file.
You can actually delete, or not save, the bdf file, we used it to give us access
to the wizard, it has created the .vhd and .cmp files for us
Now: create a top level VHDL file and set as the top level entity
Type in the following : (and think about what it means)
The contents
Of the .cmp
file
This is an
instance
of our LPM
part
You could
insert the
_inst.vhd
here
Here is the actual commented file spread over a couple of pages
The megawizard actually created the file below
Gives access to a
package of altera
parameterizable
parts
This is the text the wizard
produces – it sets various generic
data that is passed to the library
part.
The body of the .vhd file –
This instantiates a altsyncram
part and gets it to behave like
a ROM.
The code just passes data to
the component.
If you have sufficient data
about the LPM parts you can
directly instantiate them into
your code.
Provided you include the
library and uses clauses of
altera_mf you can avoid the
two step way of doing things
Second example of a MEGAfunction
- created with the wizard – direct from the tools menu
This is a suitable replacement for part of the datamemory module.
The CYCLONE III ep3C16488 allows use of 56 M9K memory blocks. Each block
can be 9kbits and allows widths of 32.
We create a datamemory of 4096 locations each of 32 bits width. This uses
16 M9K blocks – less than a quarter of the available resource.
We can create a register file and a instruction memory as well.
The datamemory is a a one port RAM cell and does not need output
clocking. The input clocking may be a nuisance and we may need to generate
and gate the clock in out logic.
Look and think about the circled items on the next few slides.
Some screenshots of the MEGAWizard