Category: Electronics Workbench

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.

To begin…

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.

Next, the gory details…

Memo to Self: The protracted delay after the first Clear is absolutely vital!

Most of the time you need just a single PWM output, but when you’re driving a transformer (or some such) and need twice the primary voltage, you can use a pair of PWM outputs in push-pull mode to get twice the output voltage.

A single PWM output varies between 0 and +5 V (well, Vcc: adjust as needed), so the peak-to-peak value is 5 V. Drive a transformer from two out-of-phase PWM outputs, such that one is high while the other is low, and the transformer sees a voltage of 5 V one way and 5 V the other, for a net 10 V peak-to-peak excursion.

A 50% duty cycle will keep DC out of the primary winding, but a blocking capacitor is always a good idea with software-controlled hardware. A primary winding with one PWM output stuck high and the other stuck low is a short circuit that won’t do your output drivers or power supply any good at all.

An Arduino (ATMega168 and relatives) can do this without any additional circuitry if you meddle with the default PWM firmware setup. You must use a related pair of PWM outputs that share an internal timer; I used PWM 9 and 10.

#define PIN_PRI_A   9    // OCR1A - high-active primary drive
#define PIN_PRI_B   10   // OCR1B - low-active primary drive

I set push-pull mode as a compile-time option, but you can change it on the fly.

#define PUSH_PULL   true // false = OCR1A only, true = OCR1A + OCR1B

The timer tick rate depends on what you’re trying accomplish. I needed something between 10 & 50 kHz, so I set the prescaler to 1 to get decent resolution: 62.5 ns.

// Times in microseconds, converted to timer ticks
//  ticks depend on 16 MHz clock, so ticks = 62.5 ns
//  prescaler can divide that, but we're running fast enough to not need it

#define TIMER1_PRESCALE   1     // clock prescaler value
#define TCCR1B_CS20       0x01  // CS2:0 bits = prescaler selection

Phase-Frequency Correct mode (WGM1 = 8) runs at half-speed (it counts both up and down in each cycle), so the number of ticks is half what you’d expect. You could use Fast PWM mode or anything else, as long as you get the counts right; see page 133 of the Fine Manual.

// Phase-freq Correct timer mode -> runs at half the usual rate

#define PERIOD_US   60
#define PERIOD_TICK (microsecondsToClockCycles(PERIOD_US / 2) / TIMER1_PRESCALE)

The phase of the PWM outputs comes from the Compare Output Mode register settings. Normally the output pin goes high when the PWM count resets to zero and goes low when it passes the duty cycle setting, but you can flip that around. The key bits are COM1A and COM1B in TCCR1A, as documented on page 131.

As always, it’s easiest to let the Arduino firmware do its usual setup, then mercilessly bash the timer configuration registers…

// Configure Timer 1 for Freq-Phase Correct PWM
//   Timer 1 + output on OC1A, chip pin 15, Arduino PWM9
//   Timer 1 - output on OC1B, chip pin 16, Arduino PWM10

 analogWrite(PIN_PRI_A,128);    // let Arduino setup do its thing
 analogWrite(PIN_PRI_B,128);

 TCCR1B = 0x00;                 // stop Timer1 clock for register updates

// Clear OC1A on match, P-F Corr PWM Mode: lower WGM1x = 00
 TCCR1A = 0x80 | 0x00;

// If push-pull drive, set OC1B on match
#if PUSH_PULL
 TCCR1A |= 0x30;
#endif

 ICR1 = PERIOD_TICKS;           // PWM period
 OCR1A = PERIOD_TICKS / 2;      // ON duration = drive pulse width = 50% duty cycle
 OCR1B = PERIOD_TICKS / 2;      //  ditto - use separate load due to temp buffer reg
 TCNT1 = OCR1A - 1;             // force immediate OCR1x compare on next tick

// upper WGM1x = 10, Clock Sel = prescaler, start Timer 1 running
 TCCR1B = 0x10 | TCCR1B_CS20;

And now you’ll see PWM9 and PWM10 running in opposition!

Memo to Self: remember that “true” does not equal “TRUE” in the Arduino realm.