I'm using a Dallas DS89C450 microcontroller to control a TFT monitor. I have a large lookup table with the appropriate scaling requirements for the different sources the monitor accepts (NTSC/PAL/VGA/etc).

This is currently stored as a constant in code memory and the monitor works fine. I'd like to add an option to change this on the fly, however, so this will need to be stored as a variable. The table is too large to be stored into data memory so I wish to store it in xram.

When I do this, however, the monitor doesn't work properly and displays random noise on the screen (sometimes looks like memory contents, sometimes it's a constant colour) instead of the input source or a blank screen in the case of no signal.

Can you write a simple routine to dump the contents of the table, just to see if it's getting set up in RAM properly? Or maybe do the same thing with an ICE or a debugger or whatever?

If the table appears intact (and remains so as you run your program), then the problem may be that you are not accessing it correctly. On the other hand, if the table looks wrong, then maybe your startup code is incorrect, or your linker settings are incorrect, or the table is getting clobbered before you try to use it, or ...

-- Russ

The ultra-high-speed microcontroller incorporates an internal 1kB SRAM that is usable as data, program, or merged program/data memory. Upon a power-on reset, the internal 1kB memory is disabled and transparent to both program and data memory maps.