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So the measurement works and it seems that the 8 microamps given in the data sheet are just a very conservative worst case. I mean CMOS is close to 74hv126 when static and only a single tri-state gate is active so essentially no current flows. The latest version uses a 4 lines x 20 characters LCD that connects via I2C as well as a rotary encoder with a push button.
Tag Archives: 74HC126
Even less pretty but problem solved anyway. As long as the push button is not pressed, no power flows there so the only power consuming device is the 74HC On the schematic the wire touched the respective pin but there was no connection.
As a result, the display as well as all the other components only get to see about 2. Its datasheet specifies a maximum static current consumption of 8 microamps worst at room temperature. Debouncing something like a pushbutton is quite easy. Furthermore the encoder signals must assume a high impedance state when the user interface is disabled. Using the spare gate to supply the rest of the circuit with power was not as good an idea than I thought.
Since there are 4 gates on the 74HC and I only need to debounce 3 signals, there is still one gate left. Add a dual I2C digipot to control both the reset signal as well as the backlight. So the enable signal only controls the input of that gate. The logic is pretty much the same here except that I use different non-inverting gates here.
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. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay.
Then ok, power was there but when I tried to turn off the reset singal i. Ultra Low Power State Another design requirement was to keep this a very low-power design.
Like in the solar charger designa 74HC quad tri-state buffer is used together with some resistors and capacitors.
74HC Datasheet(PDF) – NXP Semiconductors
The digipot should always be either at its minimum or its maximum. I expected to tweak those values a bit but found that they work so well that I just leave them as they are. The debouncing is basically unchanged. But I like the look and feel of this universal user interface and think the overall concept is good.
First of all nothing worked at all. On the positive side the 74uc126 works just perfect with the chosen component values. But getting them initialized with the right settings is always a lengthy trial-and-error process.
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.
Nothing was powered on. Just use a relatively long time constant i.
The error was soon found. I still need a way to control both the backight as well as the reset signal without needing any more singnals. The LM is inexpensive and I had quite a few left from another project so I used it without giving 74hcc126 much thought. 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.
More on that later. While that was really hard to notice on the schematic Daatsheet should have noticed the problem when I laid out the board.
The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet. This was a bit harder to fix than the previous error. Testing First of all nothing worked at all. I then decided to design 74hx126 new board that also includes the rotary encoder with all the necessary debouncing. Power consumption Very low standby power dtasheet was one of the key design goals. The reason for that is the need for 3-state outputs as mentioned before.
74hc datasheet pdf
Next steps After the two hardware fixes this user interface works quite as intended. Another design requirement was to keep this a very low-power design. One then needs to wait for 5ms before the initialization sequence 74hv126 sent. Well, if you think so too, think again. Turning on the backlight basically worked as intended but the load was a bit much for the dataasheet They make nice, pretty and affordable displays.
The reset signal needs to be pulled low for 10ms. That will introduce some delay in the output dataaheet but as a rule of thumb anything below 50ms is not noticable. When in low-power mode only the push button pull-up and the 74HC is supplied with power. My final solution was as follows: When I measured it, this is what I got.