With respect to the resistors on the backlight. It depends on the module.
Some do need them and some don't. Some will actually burn out instantly with
no resistor. So I use a resistor all the time just to be safe.
Measure the current and see if falls within the spec
(if you can really believe the spec).
For several of mine I inserted a larger than necessary resistor because I power my circuits from USB and to keep the power under the USB spec
the display needs to be limited more than usual.
(most USB ports can supply way more than the spec - which is how
all these small external USB hard drives work - they use more than
double the spec when powering up)
In my experiments for using limiting resistors, I learned that the human
does not see brightness linearly. What that means is that it sees a change
in brightness (light energy level) easier when the level is low rather the bright
and that once you achieve a certain level of brightness, the eye really
can't detect it any more. So at least for my eyes, I can limit the current
a bit and not tell any noticeable difference.
Also some of my circuits are battery powered which benefit from the reduced
current. I noticed that often you can power it with half or less than the spec
and still see things quite well when indoors. - Outdoors, well that is
another matter.
And I'll agree with you that the only real way to learn about this stuff is to really
dig down into it by writing a real project.
I'm impressed with your i/o expander. Neat project.
On the graphics and speed optimization yet keeping things simple,
one thing that I've also thought about doing is to make a very slimmed
down "lean and mean" library that uses the CP437 font.
(if you haven't seen this google it).
With CP437, you could do many graphics like capabilities
(horizontal and vertical lines) without actually doing graphics.
There are even characters/glyphs that allow doing horizontal and vertical bars
for bar graphs on pixel boundaries.
And if you force the font to be 8x8 and always
land on row boundaries, you can slam the characters out and remove
the reads, even when doing graphic like line operations.
With CP437 and a 8x8 font you will not insert any pixel padding
between characters (it isn't needed for this type of font).
Sure the graphics are limited to some straight line functions, but
you can do quite a bit using only the CP437 font.
And it is much faster than plotting lines.
Also, it is easy to support a "wide/bold" mode and a "tall" mode.
Wide and tall modes simple double each pixel of the font.
Wide is really easy as you simply double each page/byte as
you go.
Tall is a little bit more complicated as you have to stretch the byte
into two bytes and interleave the bits.
Its kind of like was don't back in the late 70's and early 80's before
we really had the ability to set individual pixels.
What would be great would be a library that sat on top of the
cp437 font to provide the "graphic" functions.
Things to draw rectangles/boarders, horizontal and vertical lines,
and bar graphs etc...
For many applications cp437 would work fine.
I'm surprised that it isn't more widely
available on the character only lcds.
--- bill