RE: AW: [avr-chat] LCD duty cycles, help please

[Ben Mann]

Hi James,

Wow, looks like a fun project.

Initially, you can generate the "negative" voltage for VLCD by simply tying
it to VSS. This voltage is in the range VDD to VDD-28, so you are just
supplying it -5V which should be enough to display 'something'; you can
supply it (indirectly) via a PWM from the AVR to vary between VDD and VSS.
Eventually you may want to step-up the voltage so that VSS for your
microcontroller and the display is +23V, VDD is +28V and the PWM is driven
between 0V and 28V. However I can't imagine it's a priority (and I imagine
the LCD will work fine without worrying about it).

You talk about using external RAM on your AVR - have you a plan as to how to
do this? The ATmega8 doesn't have a parallel memory interface. You might buy
SPI RAM, which you can access via the SPI peripheral on the mega8. I would
imagine that your computer<->RAM interface would be via a serial link such
as RS232? So maybe another mega8 and 
arbitration of the SPI bus between them.

Unless I'm mistaken the 372ns (1/70/242/160) refresh of the display data in
the timing diagram assumes a 70fps refresh - completely optional and too
fast (6ck at 16MHz) for the avr to bit-bash.  Instead I imagine all you need
to do is set the outputs at regular interval, and the 1/242 business is
simply a product of the number of lines that you transmit. Using a timer
interrupt, and taking care to only manage the SPI in the dead time between
pixels should work fine.

There are a grillion ways of interfacing the chip - if I was bent on using
the atmega8, I would start by forgetting about external RAM, and just
programming a beginner's driver that displays a pattern or 'hello world' at
the top of the lcd, since the bit pattern is fixed, a small amount of code &
data can remove the need for the 38k ram which is otherwise needed.
(Thinking about it, making a text based driver might require no external RAM
at all if you can fit the text and state variables in the mega8's RAM).

While I agree that it's not a project to be completed overnight, getting
started is probably not as painful as you might expect. Of course, I don't
have one of these modules so I'm just guessing ;)

Sorry that I go on and on but it sounds like a truly interesting project.
Good luck!

Ben Mann


-----Original Message-----
--snip!--
I'm aware it's not trivial, but I'd rather use the ATmega8s I have now than
buy anything else for the moment.

Thanks,
James