Parallel interface Character LCD gotchas!

Peter Jakacki

Recently I designed a prop board to interface to a standard parallel interface character lcd module. This is so ho-hum, another lcd module, woo-hoo. But it wasn't so. No matter what I tried the lcd module would not respond . Tried another from the same batch, same thing. Checked my timing, scoped it out etc and still couldn't get it going. Then I tried a different brand module. Ah ha! now it works! (so it should).

What was the problem? Well that troublesome lcd module had been used in production for years with no problems. Only difference was that it was run from 5V logic, maybe that was the problem. The easiest way I could test it was to up the supply voltage to the propeller so I connected an infrared led with a voltage drop of around 1.2V (under load) from +5V to VDD bypassing the 3.3V regulator (the prop's absolute max is +4V!!! ). Lo and behold, the beast behaves.

Anyway I've emailed the lcd manufacturer to find out more about their controller as it's a chip-on-board blob but behaves similar to the standard 44780.

BTW, I use my lcd's in write-only mode by tying the r/w low so that I don't have any voltage interfacing issues plus the operation is deterministic anyway.

*Peter*

Bergamot

Just don't draw much more than that LED's max current, or bad things will happen.

Peter Jakacki

IR leds are normally rated for higher currents (actually they dissipate less heat due to lower Vf) but this was only done as a quick way of getting a higher VDD for the prop. The 3.3V regulator is still in-circuit as well so if the current was too much the forward voltage drop would increase to the point where the regulator would take the current. I don't really like the idea of running the prop at this higher voltage though so I wouldn't recommend it to anyone, this is more of an exercise in troubleshooting and to alert others to these incompatible displays (YMS202).

*Peter*

Fred Hawkins

Peter,
Can you illustrate this hack with a quick schematic?
Thanks, Fred