Testing
Several Verilog test benches are included to verify the
operation of the video and SRAM test modules. The NTSC
test benches generate a BMP file to simulate viewing the
output on a television screen, as seen in Figure 7. The
code converts the eight-bit 3-3-2 RGB color to a 24-bit
RGB color to prevent having to deal with indexed palettes
in the BMP file.
Conclusion
There are some issues with the
NTSC shield design. At this point,
there is no way to turn on and off
the video signal via the FPGA since
the AD723 enable is hard-wired.
The same is true for the SRAM, as
we discussed earlier. Additionally,
the NTSC sync/pixel generator
depends on a lot of combinational
logic. This can be tightened up with
more registers and it’s something I’m
planning on fixing in the future.
There are also some issues with
the video output. You will notice
dot crawl on the composite output
when looking at the color bars. This
is normal at the boundary between
two different colors, but it can be
distracting.
Another issue is pixel jitter.
This happens because the AD723
is operating asynchronously from
the Mojo V3. To fix this, the NTSC
shield would have to be redesigned
to send the 14.31818 MHz color
burst oscillator to the Mojo V3.
Clock domain crossing logic would
be needed to synchronize the two
systems. You know what? I’ll leave
that as a fun project for you!
So, what can we do with this
thing, anyway? Well, you already have
a 256 font ROM at your disposal.
It wouldn’t be too tough to write
a video interface for the Mojo V3
serial port. With the 1 MiB SRAM,
you could certainly make a video
game system on the Mojo V3. Oh, I
forgot to mention! The NTSC shield is
compatible with the Embedded Micro
SDRAM shield. Put a soft processor
core on the FPGA and design a
miniature computer using the NTSC shield as the video
sub-system!
I’d like to acknowledge Dr. Pong P. Chu’s book,
FPGA Prototyping by Verilog Examples. The book was
instrumental to me understanding how to implement a
SRAM controller. The book also provided insight on how to
implement a video controller. Although Dr. Chu references
the VGA standard, it was helpful for me to translate it to
the NTSC standard. Until next time, have fun! NV
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