.. _HDLs:Verilog:

Verilog on Fomu
---------------

“Hello world!” - Blink a LED
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The canonical “Hello, world!” of hardware is to blink a LED.
The directory :repo:`hdl/verilog/blink <hdl/verilog/blink>` contains a Verilog example of a blink project.
This takes the 48 MHz clock and divides it down by a large number so you get an on/off pattern.

Enter the :repo:`hdl/verilog/blink <hdl/verilog/blink>` directory and build the demo by using ``make``:

**Make sure you set the** ``FOMU_REV`` **value to match your hardware!** See :ref:`required-hardware`.

.. session:: shell-session

   $ make FOMU_REV=$FOMU_REV
   ...
   Info: Max frequency for clock 'clk': 73.26 MHz (PASS at 12.00 MHz)

   Info: Max delay posedge clk -> <async>: 3.15 ns

   Info: Slack histogram:
   Info:  legend: * represents 1 endpoint(s)
   Info:          + represents [1,1) endpoint(s)
   Info: [ 69683,  70208) |**
   Info: [ 70208,  70733) |
   Info: [ 70733,  71258) |**
   Info: [ 71258,  71783) |**
   Info: [ 71783,  72308) |**
   Info: [ 72308,  72833) |**
   Info: [ 72833,  73358) |
   Info: [ 73358,  73883) |**
   Info: [ 73883,  74408) |*
   Info: [ 74408,  74933) |**
   Info: [ 74933,  75458) |**
   Info: [ 75458,  75983) |*
   Info: [ 75983,  76508) |*
   Info: [ 76508,  77033) |**
   Info: [ 77033,  77558) |**
   Info: [ 77558,  78083) |*
   Info: [ 78083,  78608) |
   Info: [ 78608,  79133) |*************************
   Info: [ 79133,  79658) |**
   Info: [ 79658,  80183) |***
   22 warnings, 0 errors
   icepack blink.asc blink.bit
   cp blink.bit blink.dfu
   dfu-suffix -v 1209 -p 70b1 -a blink.dfu
   dfu-suffix (dfu-util) 0.9

   Copyright 2011-2012 Stefan Schmidt, 2013-2014 Tormod Volden
   This program is Free Software and has ABSOLUTELY NO WARRANTY
   Please report bugs to http://sourceforge.net/p/dfu-util/tickets/

   Suffix successfully added to file
   $

You can then load ``blink.dfu`` onto Fomu by using the same ``dfu-util -D`` command we’ve been using so far.
You should see a blinking pattern of varying color on your Fomu, indicating your bitstream was successfully loaded.


Reading Input
^^^^^^^^^^^^^

There is another small example in :repo:`hdl/verilog/blink-expanded <hdl/verilog/blink-expanded>` which shows how to read
out some given pins.
Build and flash it like described above and see if you can enable the blue and red LED by shorting pins 1+2 or 3+4 on
your Fomu (the pins are the exposed contacts sticking out of the USB port).