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StackBook Part 4.1 - More More screen.

25 08 09 - 22:55 Rule #2 of electronics according to gm: Just because it works (or appears to) doesn't mean it's right.

I was experiencing problems with the third segment of the display - namely anything drawn on it going onto the other two and general pixel corruption caused by the third panel not being activated in the correct sequence. Remember last time, when CS1 and CS2 had to be swapped to make the graphics appear the right way round? It wasn't because they were wired up wrong. This is the chip select sequence:

__inline__ void ks0108::SelectChip(uint8_t chip) {
//static uint8_t prevchip;
if(chipSelect[chip] & 1)
fastWriteHigh(CSEL1);
else
fastWriteLow(CSEL1);

if(chipSelect[chip] & 2)
fastWriteHigh(CSEL2);
else
fastWriteLow(CSEL2);
}

Can you see what this assumes about the LCD, and why it would work with CS1 and CS2 swapped if this assumption was wrong?

This display has active-low CS lines - in other words the chip is selected when the enable line is grounded. In the previous case where we were only driving the left 128 pixels, when chip 1 was supposed to have been selected, chip 2 was, and vice versa. However, attempting to use 3 chips with this results in something different - chip 1 is off when selected, chips 2 and 3 are on - very very bad. Hence why the stuff for chip 3 was being drawn over the other two. So after fixing the fastWriteHigh/Low problem and adding an if statement to turn chip 3 on and off, it worked.

I'll release the modified ks0108 code when I've got it cleaned up a bit - it's a little hackish at the moment - and I've probably broken stuff in the code for other setups. But anyway, I can now draw to the whole 192x64 LCD.