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As long as the push button is not pressed, no power flows there so the only power consuming device is the 74HC As so often, google was my friend and others have faced similar problems before so Datashert managed to find some commands that make the display work properly.
The reset signal needs to be pulled low for 10ms. In that state the user interface must not use more than a few microamps. Use the 74h1c26 backlight PWM signal as 74hc1126 enable signal. Debouncing something like a pushbutton is quite easy. And PWM control of the backlight would require a great deal of I2C communication which puts a burdon on the microcontroller. So you face the challenge of debouncing the signal while still letting relatively fast transitions pass.
Next steps After the two hardware fixes this user interface works quite as intended.
(Datasheet) 74HC pdf – Quad buffer/line driver 3-state (1-page)
Just use a relatively long time constant i. Very low standby power consumption was one of the key design goals. They make nice, pretty and affordable displays. I still need a way to control both the backight as well as the reset signal without needing any more singnals.
I ended up soldering a thin wire accross the digipot to a resistor that is connected to VCC on the upper end. Its datasheet specifies a maximum static current consumption of 8 microamps worst at room temperature. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay.
Here you find the eagle files as well as PDFs of the schematic and board. But getting them initialized with the right settings is always a lengthy trial-and-error process. When I measured it, this is what I got. That way I get a quite universal, easy to use user interface that I can use for other projects as well.
M54/74HC datasheet & applicatoin notes – Datasheet Archive
The logic is pretty much the same here except that I use different non-inverting gates here. Like in the solar charger designa 74HC quad tri-state buffer is used together with some resistors and capacitors.
The LM is inexpensive and I had quite a few left from another project so I used it without giving it much thought. 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. Again, the reason was simple. 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 ddatasheet as the rotary encoder is off but the push button still works in order to wake up the microcontroller.
Another issue I noticed was that the LM op amp I had chosen is not a rail-to-rail op amp. But I like the look and feel of this universal user interface and think the overall concept is good. Testing First of all nothing worked at all. 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.
I asserted the enable signal and nothing happened. Since the display connects via I2C it was quite straight forward to use I2C communication for those purposes as well. Then ok, 74c126 was there but 74hv126 I tried to turn off the reset singal i.
(PDF) 74HC126 Datasheet download
That will introduce some delay in the output signal but as a rule of thumb anything below 50ms is not noticable. While that was really hard to notice 74h126 the schematic I should have noticed the problem when I laid out the board. Furthermore the encoder signals must assume a high impedance state when the user interface is disabled.
Datassheet mean CMOS is close to zero-power when static and only a single tri-state gate is active so essentially no current flows. The error was soon found. The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet.
The reason for that is the need for 3-state outputs as mentioned before. So the enable signal only controls the input of that gate. Actually the measurement fluctuated between about 0 and 15 nanoamps. So the measurement works and it seems that the 8 microamps given in the datzsheet sheet are just a very conservative worst case. This was a bit harder to fix than the previous error. 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.
Power consumption Very low standby power consumption was one of the key design goals.
There was obviously no connection to that pin. The size of the board is mainly given by the size of the display so there is plenty of board space. So there will be a Rev B of this board with those issues solved. On the positive side the debouncing works just perfect with the chosen component values. Ultra Low Power State Another design requirement was to keep this a very low-power design.
Since there are 4 gates on the 74HC and I only need to debounce 3 signals, there is still one gate left. One then needs to wait for 5ms before the initialization sequence is 74hc26. Another design requirement was to keep this a very low-power design.