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Dafasheet can now always stay on. So the enable signal only controls the input of that gate. But with the hardware debouncing applied here we get a clean, digital output singal nevertheless.

While the display got its own little board, the encoder connected directly with the solar charger where its signals aredebounced in hardware and then routed to the PIC. Testing First of all nothing worked at all. Again, the reason was simple.

74HC Datasheet PDF –

While it can supply up to 35mA according to the data sheet, its output voltage is nowhere near its positive supply rail when it supplies that much current. This was a bit harder to fix than the previous error. 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.

Even less pretty but problem solved anyway. The digipot should always be either at its minimum or its maximum. datasheett

But getting them initialized with the right settings is always a lengthy trial-and-error process. And the commands for this black-on-white display are different to the otherwise identical white-on-blue display. Like in the solar charger designa 74HC quad tri-state buffer is used together with some resistors and capacitors. I asserted the enable signal and nothing happened.

Its datasheet specifies a maximum static current consumption dayasheet 8 microamps worst at room temperature.

Next steps After the two hardware fixes this user interface works quite as intended. Then ok, power was there but when I tried to turn off the reset singal i. Toghether with the positive feedback via the k resistors that works very well. But when the user interface is not in use, we need a way to put it in a very low power state where the display as well as the rotary encoder is off but the push button still works in order to wake up the microcontroller.


First of all nothing worked at all. I made a design error with the result that the respective gate was permanently in a high-impedance state. While that was really hard to notice on the schematic I should have noticed the problem when I laid out the board.

They make nice, pretty and affordable displays. As a result, the display as well as all the other components only get to see about 2.

SGS Thomson Microelectronics – datasheet pdf

Here you find the eagle files as well as PDFs of the schematic and board. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay. That eliminates the need to design the same functionality think debouncing again and again. Datasheef there will be a Rev B of this board with those issues solved. One then needs to wait for 5ms before the initialization sequence is sent.

The LM is inexpensive and I had quite a few left from another project so I used it without giving it much thought. 74hc216 the two hardware fixes this user interface works quite as intended. When I measured it, this is what I got.

(PDF) 74HC126 Datasheet download

That way I get a quite universal, easy to use user interface that I can use for other projects as well. I expected to tweak those values a bit but found that they work so well that I just leave them as they are. When in low-power mode only the push button pull-up and the 74HC is supplied with power. When the enable signal is high, the display is on, when enable is low, the user interface goes into its low-power state. Another issue I noticed was that the LM op amp I had chosen is not a rail-to-rail op amp.


So you face the challenge of debouncing the signal while still letting relatively fast transitions pass. Since there are 4 gates on the 74HC and I only need to debounce 3 signals, there is still one gate left.

Debouncing something like a pushbutton is quite easy. And as mentioned, the op-amp was not a great choice for the job datashfet hand. On the positive side the debouncing works just perfect with the chosen component values. Another design requirement was to keep this a very 47hc126 design.

And it was not very elegant that the display was powered off by just cutting the entire power supply. That will introduce some delay in the output signal but as datasbeet rule of thumb anything below 50ms is not noticable. 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. There was obviously no connection to that pin.

The basic circuit is exactly the same but choosing the right component values is more difficult. Nor the timing of the reset singnal. More on that later.

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.