ben-blinkenlights/ubb-vga

UBB-VGA - VGA-like output via UBB
=================================

Sources
-------

Timing and the idea for the voltage divider is from:
http://faculty.lasierra.edu/~ehwang/public/mypublications/VGA Monitor Controller.pdf

More timing parameters:
http://tinyvga.com/vga-timing/640x480@60Hz


Signal	8:10	VGA
-------	-------	---
R	DAT2	1
VSYNC	DAT3	14
HSYNC	CMD	13
G	DAT0	2
B	DAT1	3
GND	GND	5

http://en.wikipedia.org/wiki/VGA_connector


Timing
------

Since the Ingenic CPUs take about 8.5 PCLK cycles for a GPIO set or clear,
and we can only set or clear a set of signals in GPIO operation, but not
set some and clear others, we cannot have a real 320 horizontal pixels.

Instead, set and clear operations alternate. This means that the best-case
resolution is equivalent to 320 pixels (if the original pixel boundaries
coincide with the set/clear phases), but it can be as low as 160 pixels if
the boundaries don't match.

Furthermore, timing is still a bit too tight. We therefore use a pixel
clock that's about 10% slower than the original. Luckily, most monitors
don't mind.


Single/double mode
------------------

In single mode, only one set/clear can be performed per pixel. They are
arranges such that the first pixel can only turn on channels, the
following pixel can only turn off channels, and so on.

In double mode, a set and a clear is performed per pixel. This changes
the timing such that it differs substantially from VGA, but some
monitors can still synchronize with this.

The resulting signals for various bit patterns are shown in mapping.fig
Each group shows the input pixels, and signal for single mode, and the
signal for double mode. "S" indicates a point where the channel can be
turned on, "C" indicates a point where it can be turned off.

Note that there are probably other mapping algorithms that would yield
better results. The mapping is not a very performance-critical part of
ubb-vga. Improvements are welcome.


Compatibility
-------------

ubb-vga in single mode has been tested with the following monitors:

Display			Monitor size	Quality
-----------------------	---------------	----------------------------------
Xenon XEN-1510T		15", 1024x768	good
Samsung 206NW		20", 1680x1050	poor (horizontal instability)
LG W2243C		22", 1920x1080	acceptable (slight instability)
LG W2243L		22", 1920x1080	acceptable (slight instability)

In double mode:

Display			Pixels missing	Quality
			left/right
-----------------------	---------------	------------------
Xenon XEN-1510T		5 / 25		good


To do
-----

- use timer half interrupt and WAIT to synchronize (less bus traffic and
  hopefully better granularity)
- recover from DMA lockup
- use color cube map (5x5x5 bits)
- read/write raw frame buffer
- consider prefetching first word of each line, to make DRAM controller
  open the row and reduce DMA startup latency
- add option to not disable the LCD (works with some modes if reducing
  the LCD controller's burst size)
- find out why 640x480 has so much FIFO noise
- fix modes 800x600/54 and 800x600/72, broken by the timer change