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On the positive side the debouncing works just perfect with the chosen component values. Even less pretty but problem solved anyway. The entire initialization sequence needs to be written in one single I2C transmission. On the schematic the wire touched the respective pin but there was no connection. Power can now always stay on.
Its datasheet specifies a maximum static current consumption of 8 microamps worst at room temperature. Furthermore the encoder signals must assume a high impedance state when the user interface is disabled.
When I measured it, this is what I got. I made a design error with the result that the respective gate was permanently in a high-impedance state. Since the display connects via Datxsheet it was quite straight forward to use I2C communication for those purposes as well. 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.
So you face the challenge of debouncing the signal while still letting relatively fast transitions pass. I ended up soldering a thin wire accross the digipot to a resistor that is connected to VCC on the upper end. I expected to tweak those values a bit but found that they work so well that I just leave them as they are. More on that later. As a result, the display as well as all the other components only get to see about 2.
The latest version uses a 4 lines 74c126 20 characters Dataseet that connects dxtasheet I2C as well as a rotary encoder with a push button. Another issue I noticed was that the LM op amp I had chosen is not a rail-to-rail op amp.
SGS Thomson Microelectronics
They make nice, pretty and affordable displays. So there will be a Rev Datssheet of this board with those issues solved.
And it was not very elegant that the display was powered off by just cutting the entire power 74hc12. I wrote an email to their customer support asking if there is a list of commands the display supports. Like in the solar charger designa 74HC quad tri-state buffer is used together with some resistors and capacitors.
74HC | soldernerd
The debouncing is basically unchanged. And the commands for this black-on-white display are different to the otherwise identical white-on-blue display. The error was soon found.
Turning on the backlight basically worked as intended but the load was a bit much for the 74HC That eliminates the need to design the same functionality think debouncing again and again. 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.
(Datasheet) 74HC pdf – Quad buffer/line driver 3-state (1-page)
But getting them initialized with the right settings is always a lengthy trial-and-error process. Here you find the eagle files as well as PDFs of the schematic and board.
The reason for that is the need for 3-state outputs as mentioned before. Toghether with the positive feedback via the k resistors that works very well. Actually the measurement fluctuated between about 0 and 15 nanoamps. Then ok, power was there but when I tried to turn off the reset singal i. Add a dual I2C digipot to control both the reset signal as well as the backlight.
The logic is pretty much the same here except that I use different non-inverting gates here. I then decided to design an new board that also includes the rotary encoder with all the necessary debouncing. While that was really hard to notice on the schematic I should have noticed the problem when I laid out the board. This was a bit harder to fix than the previous error. Power consumption Very low standby power consumption was one of the key design goals.
When in low-power mode only the push button pull-up and the 74HC is supplied with power.