Cheap & good don't go together on this afaik. One of the less advertised
ways to do it is to use a byte wide bus inexpensive mcu with a piece of
RAM. By using clever address decoding the actual display is done by
mapping bytes off of the data bus into a latch that drives one side of a
matrix, the other side being driven by a shift register + buffers (or not)
that shifts a '1' or a '0' driven by the same chip select logic. The shift
register uses very few interface pins (two or one). The CPU is made to
read a large table or something like that. It also communicates by serial
bus with other CPUs.

The latch can be 8 bits or interleaved for 16, 24 etc. By using some
intelligent double buffering schemes the curtain effect with running
displays can be avoided entirely. Not that anybody seems to bother about
that.

This may use a larger PIC too but the 8051 derivatives are very popular
for this.

With a matrix display it pays to put as much of a matrix as you can per
module even if the module gets more complex because the number of display
points increases with the square (the product) of the line and coulmn
drivers.

There are also power issues. Driving a matrix can get complex very fast.
F.ex. a 8 x 8 matrix requires 8 x nominal LED current per pulsed line (200
mA or so). Switching these currents is not a problem but if it stops the
display will burn. Also the electrical noise and the power supply become
very significant. The same 8 x 8 display consumes a whopping 1.6 A with
all lights on. Not enough, if you do not use special LED drivers or a
special power supply for the lights, the controller will dissipate the
remainder of the power after lighting the LEDs (about 2V per red LED at
that current afair). Both the display and the driver will run hot and that
needs to be taken care of eventually.

Note that many manufacturers skimp on the psu and on heat considerations
because they use 'average' display data which assumes that up to 50% of
the lights are on on the average (and much less if displaying only text).

Peter

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