Recently impulsive, I bought a very low cost electronic tweezers kit. The assembly process is still quite classical. The design style of this circuit is still in the 70s: a CD4017, a NE555, a pair of transistors. Of course, I started thinking about how to achieve it in the most efficient way. Of course, this means that a microcontroller is needed; and we have to achieve the goal of making our resources as low as possible. So can you implement ATTIny10 with 3 I/O ports?

The pattern of the dice contains 7 LEDs, but you may soon find that six of the LEDs are in pairs, so we have 3 pairs of LEDs plus an additional LED to control. This requires four I/O ports - but it's still too much!

To reduce the number of I/O ports needed, one obvious use is Charliplexing. You can find a lot of circuit diagrams on the Internet for Charlie multiplexing technology. In simple terms, Charlie multiplexing technology allows n2-n LEDs to be controlled using n I/O ports. So when we use Charlie multiplexing, we need three I/O ports. But for the ATTIny10 we use it is still too much, because we also need an extra I / O port to "shake the dice".

Charlie multiplexing uses the tristate nature of the microcontroller I/O pins. There are only two I/O ports activated at the same time - one high and one low - while the other pins are high impedance. Only LEDs that are directly connected to the correct polarity and activated pins can illuminate. LEDs on paths that are not directly connected to the pins—such as two series-connected LEDs connected to the active pins—these LEDs do not illuminate because of the nonlinear current-voltage characteristics of the diode. of.

Now, you may be wondering what happens when only one I/O port (not two) is activated. Nothing happens in the Charlie reuse technology solution, but we can take advantage of this.

Charlie reuse

The above circuit shows how to connect LEDs in different ways. In addition to the anti-parallel pairs between the two I/O pins, as a usage habit of Charging Multiplexing, the LEDs need to be connected to VCC (5V) and GND. The sum of the forward voltages of the four LEDs in series (LED1-4 and LED5-8) exceeds 5V, so when PB0 and PB2 are in the high impedance (Z) state, these lamps will not emit light.

When one of PB0 and PB2 is in a high or low state and the other pin is in a high impedance state, then a pair of LEDs will illuminate. When PB0 is high and PB2 is low or vice versa, LED9 or LED10 will be in parallel with a pair of LEDs. At this time, most of the current will flow through a single LED, so only LED9 or LED10 will emit light.

The above table is a possible encoding. see it? Two pins can control six parts of the LED! This is exactly what we need and the task is done.

But one thing to note: the brightness of a single LED is different from the two LEDs in series. This may require corrections in the Charging Multiplexing technique. But in fact, because the human eye is not particularly sensitive to the difference in brightness, the difference between the two can only be said to be barely visible.

Simply test it (LEDs are not arranged like dice). It seems that our new multi-multiplex technology works well. Note that this circuit requires the internal impedance of the AVR I/O interface. This works, but it should be avoided in a "real" design.

Electronic tweezers

We can talk about this new solution as "Charlieplex Plus". Analysis shows that relying on such a technology, an I/O port can control two LEDs more than ordinary Charlie multiplexing technology. That is to say, the number of LEDs that can be controlled by n I/O ports is 2n+n2-n = n2+n. The details are as shown in the table above.

Should you adopt this approach in your design? I am afraid it should not, because the flaws and limitations of this approach are even more than the existing Charlie reuse. But this is very interesting!

So back to the beginning, how many I/O ports do you need to control the electronic dice?

The answer is 2.

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