74HC126 DATASHEET PDF
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There are various contrast settings that must be set properly via I2C commands but there is very little information on those commands in the data sheet.
First of all nothing worked at all. The reason for that is the need for 3-state outputs as mentioned before. So the measurement works and it seems that the 8 microamps given in the data sheet are just a very conservative worst case. As so often, google was my friend and others have faced similar problems before so I managed to find some commands that make the display work properly.
When I measured it, this is what I got. Power consumption Very low standby power consumption was one of the key design goals. With rotary encoders things get much trickier. And PWM control of the backlight would require a great deal of I2C communication which puts a burdon on the microcontroller.
Another issue I noticed was that the LM op dataasheet I had chosen is not a rail-to-rail op amp. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay. The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet. So the enable signal only controls the input of that gate.
74HC126 Datasheet PDF
Next steps After the two hardware fixes this user interface works quite as intended. But when the user interface is not in use, we need a way to put it in a very low power dtasheet where the display as well as the rotary encoder is off but the push button still works in order to wake up the microcontroller. The mistake was corrected by cutting the ground connection and conecting the respective pin to the enable singal itself which was available from the pin right next to it.
Add a dual I2C digipot to control both the reset dayasheet as well as the backlight. More on that later. The size of the board is mainly given by the size of the display so there is plenty of board space.
Again, the reason was simple. If you use a time constant of a few milliseconds like with the push button you will not be able to detect when the encoder is turned quickly.
Very low standby power consumption was one of the key design goals. I wrote an email to their customer support asking if there is a list of dtaasheet the display supports. Its datasheet specifies a maximum static current consumption of 8 microamps worst at room temperature.
Not pretty see below but problem solved. I mean CMOS is close to zero-power when static and only a single tri-state gate is active so essentially no current flows. I also considered adding a sub-dollar PIC16 which could be programmed as an I2C slave and control both the backlight and the reset signal as well as giving the option of getting the encoder input via I2C.
Using the spare gate to supply the rest of the circuit with power was not as good an idea than I thought. Well, if you think so too, think again. The debouncing is basically unchanged.
74HC Datasheet(PDF) – NXP Semiconductors
I ended up soldering a thin wire accross the digipot to a resistor that is connected to VCC on the upper end. I made a design error with the result that the respective gate was permanently in a high-impedance state. And the commands for this black-on-white display are different to the otherwise identical white-on-blue display. Toghether with the positive feedback via the k resistors that works very well.
They make nice, pretty and affordable displays. The basic circuit is exactly the same but choosing the right component values is more difficult. Turning on the backlight basically worked as intended but the load was a bit much for the 74HC I expected to tweak those values a bit but found that they work so well that I just leave them as they are.
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As a result, the display as well as all the other components only get to see about 2. The entire initialization sequence needs to be written in one single I2C transmission. But I like the look and feel of this universal user interface and think the overall concept is good. That will introduce some delay in the output signal but as a rule of thumb anything below 50ms is not noticable.
After the two hardware fixes this user interface works quite as intended. I then decided to design an new board that also includes the rotary encoder with all the necessary debouncing. Debouncing something like a pushbutton is quite easy.