UBB-VGA
This is an effort led by Werner Almesberger to use the
UBB
(Universal Breakout Board), a few resistors, and a VGA cable to create a
workable VGA signal on the
Ben Nanonote.
What it can do
The video signal timing is controlled by software. Available resolutions
include 640x480 and 800x600.
At present, the maximum resolution is 1024x768 with a refresh rate of 50 Hz:
The 16 colors are generated from a binary channel for red, green, and blue
each, plus a binary luminance channel that affects all colors.
The hardware
The current design is based on the
Universal Breakout Board (UBB)
that plugs into the Ben's
8:10 card
slot. The MMC controller in the Ben's Jz4720 CPU is tricked into
considering the UBB-VGA board as a MMC/SD device and streaming the pixel
data at a rate of up to 56 MHz.
The pictures below show a prototype that connects directly to a VGA cable:
The circuit is very simple. In its simplest form, it consists of UBB,
ten resistors, and the VGA cable:
Each of the three analog color channels uses three resistors for lowering
the 3.3 V logic voltage to the 0.7 V analog VGA level and for mixing in the
luminance (Y) signal. The tenth resistor pulls down the HSYNC/CMD
line, to fake the "not busy" signal the MMC controller expects to receive
from a MMC device when starting a block write.
The software
The driver currently runs in user space (with a lot of dirty tricks) and
blocks all other system activity while running.
It can display the test image shown above, the content of the LCD display,
or a PNM image.
The source code can be found
here.
