I'm using an Okaya RC2004ARS-DSC-A (RC2004D, the baby 20x4) display in a 3.3V system. I originally planned on using a negative voltage charge pump driven by a PWM pin on a PIC (a circuit I've used before successfully.) When I couldn't get that to work I tried a ICL7660 neg chg pump. No luck on anything. Everything logic-wise appears to be operating normally, it init's fine, etc. When I put 5V on the system I can actually get the contrast up enough to start to see the characters.
There is an Okaya appnote that speaks of internal and external contrast adjustment on their modules. However, none of the jumpers they describe appear on the RC2004. So I'm assuming it doesn't have that capability (which could be causing the problem if somehow the internal contrast selection was jumpered on.)
When I probe the Vo pin, I get an impedance of about 3K ohms. When I test the ICL7660 without it connected to the V0 pin I get a good V of about -3.2V from a 3.3V supply. However, when I connect the ICL7660 output to the Vo pin all I get is -0.7V when scoped. The 4V bias obviously isn't enough when I need at least 5+ volts to start to see the characters when tested with a 5V Vdd. BTW, one would expect that when using the ICL7660 that would produce a bias of about 6.5V and the screen would be over contrasted and be dark. No such luck, it's as if there's no bias at all.
Any ideas or suggestions would be hugely appreciated,