appropriate analog NTSC signal as both composite and
As a tradeoff between color depth and ease of
construction, I decided on an eight-bit 3-3-2 RGB color
palette. What does that mean? Well, it means there are
three bits for red, three bits for green, and two bits for
blue, totaling 256 colors. (As a side note, the reason
that blue only gets two bits is because the human eye is
less sensitive to the color blue.) The RGB input must be
between 0-714 mV, which we can easily achieve with a
binary-weighted resistor DAC (digital-to-analog converter).
Okay, so we’ll generate an NTSC signal, but I think
we need more. Wouldn’t it be nice to have some video
memory to play with? Although the Xilinx Spartan- 6 on the
Mojo V3 has a bunch of onboard Block RAM ( 64 KiB), it’s
not enough for a reasonable frame buffer. No problem!
We’ll just add external RAM to the shield! We need that
video RAM to be fast, but it also needs to have enough
Let’s target a resolution of 640x480, which is
equivalent to VGA. At one eight-bit byte per pixel, we
require 300 KiB of storage. I personally think it would be
nice to have two frame buffers so that we can page flip.
That requires 600 KiB of storage, which leads us to a 1 MiB
RAM device. Long story short, I settled on the Alliance
AS7C38096A 1M x 8-bit 10 ns SRAM. See Figure 1 for the
Fair warning here! If you don’t like soldering SMT
(surface-mount technology) devices, then I have bad news:
The entire shield — except for the expansion headers and
output jacks — is SMT. It’s really not that bad, honest! The
hardest part to solder is the 14.31818 MHz color burst
oscillator (U2). It’s lead-less and the footprint isn’t exactly
huge, but with a little bit of patience it doesn’t take much
PRO-TIP: Be careful when soldering the SRAM. It has
many fine-pitch leads and it’s very easy to bridge them
while drag soldering (this may or may not have happened
to me). If you have the ability to reflow solder, then this is
all mostly trivial.
There’s no preferred sequence to solder the shield.
Figure 1. NTSC Shield schematic.
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70 September/October 2018