Archive for May 24th, 2009
I recently attached an ancient Optrex DM16117 LCD to an Arduino and discovered that the standard LiquidCrystal library routine wouldn’t initialize it properly. After turning on the power, the display would be blank. Hitting the Reset button did the trick, but that’s obviously not the right outcome.
It turns out that initializing one of these widgets is trivially easy after you realize that the data sheet is required reading. If you do everything exactly right, then it works; get one step wrong, then the display might work most of the time, sorta-kinda, but most likely it won’t work, period.
The catch is that there’s no such thing as a generic datasheet: what you must do depends on which version of the HD44780 controller lives on the specific LCD board in your hands and what oscillator frequency it’s using. The LiquidCrystal library seems to be written for a much newer and much faster version of the HD44780 than the one on my board, but, even so, the code may not be following all the rules.
Fetch the Optrex DMC16117 datasheet, which includes the HD44780 timings for that family of LCD modules. There’s also a datasheet for just the Optrex LCD module itself, which isn’t quite what you want. You could get a bare Hitachi HD44780 datasheet, too, but it won’t have the timings you need.
Pages 32 and 33 of the DMC16117 datasheet present the 8-bit and 4-bit initialization sequences. Given that no sane engineer uses the 8-bit interface, here’s the details of the 4-bit lashup.
Two key points:
- The first four transfers are not standard command sequences
- The delays between transfers are not negotiable
The starting assumption is that the LCD has not gone through the usual power-up initialization, perhaps because the supply voltage hasn’t risen at the proper rate. You could drive the LCD power directly from a microcontroller pin for a nice clean edge, but most designs really don’t have any pins to spare for that sort of nonsense: code is always cheaper than hardware (if you ignore non-recurring costs, that is, as many beancounters do).
The Arduino LiquidCrystal library routine initialization sequence (in /opt/arduino/hardware/libraries/LiquidCrystal/LiquidCrystal.cpp) looks like this:
command(0x28); // function set: 4 bits, 1 line, 5x8 dots command(0x0C); // display control: turn display on, cursor off, no blinking command(0x06); // entry mode set: increment automatically, display shift, right shift clear();
The four-bit version of the command() function sends both nibbles of its parameter, high followed by low, which simply isn’t correct for the first few values the DMC16117 expects. Worse, the timing doesn’t follow the guidelines; there’s no delay at all between any of the outgoing values. Again, this is most likely due to the fact that LiquidCrystal was written for a newer version of the HD44780 chip.
After a bit of fiddling around, I decided that the only solution was to create a new library routine based on LiquidCrystal with the proper delays and commands: LCD_Optrex. It might not work for newer LCDs, but at least it’ll play with what I have in my parts heap.
Memo to Self: The protracted delay after the first Clear is absolutely vital!