"Your question is so vague as to be useless.

What LCD are you using?

And what LCD is bidirectional -- in other words, what makes you think you can read from LCD the value that it displays in a particular position?"

I haven't heard back from the dude, but maybe his English parser exploded on my third sentence.

Anyways, the Hitachi 44780 LCD controller does have a "read the character at the current cursor position" command, which I'd forgotten about (hence the rather nasty tone, which I should probably apologize for but won't) and which our drive-by e-mailer Zahid would know had he bothered to RTFDS. It presumes that the hardware doesn't tie the R/W signal to Write. As to WHY you'd want to read it back, dunno.

I'm futzing with this 480-by-128 pixel GLCD I got from Marlin P Jones for $7. It requires a controller, so I built one using a Xilinx FPGA (gotta LOVE dual-port block RAMs with one side being 8 bits wide and the other being 1 bit wide and both sides run off of different, asynchronous clocks!) and an SiLabs '340 (USB, don'tcha know). The current implementation is a dumb VT-100-type display. The dual-ports are the display buffer; the '340 simply copies a character map from a table to the correct locations in the display buffer. Readback is useful, say if you wanted to have a cursor (all black) and when the cursor is over a character you're already displaying, that character is shown inverse. So you need to read back what's at the current cursor location and invert it to show the cursor.

Anyways, it's all fun and at some point I'm going to put it up on the web somewhere.

-a