microcontrolleroutput
I often see those phrases in tutorials with microcontroller I/O. An example use is "set pin 3 high", or "set pin 5 low for 20 ms".
I also recall reading several different definitions on what that terminology means. Does the low in "set low" mean low impedance? Does it mean low logic level (0 volts)? Or does it mean low as in negative voltage? Or is there even a universal meaning for those phrases?
First, let's say we work with these two functions:
\$L(n)\$ is the maximum amount of LEDs that can be driven from \$\mathsf{n}\$ pins.
\$p(n)\$ is the minimum amount of pins needed to drive \$\mathsf{n}\$ LEDs.
This one is easy:
$$L(n)=n$$
$$p(n)=n$$
At first, we need to determine the most efficient diode matrix. For example, you could divide 4 pins into two sets of 2, or one set of 1 and one of 3. Obviously, the amount of LEDs is given by \$\mathsf{length\cdot{}width}\$. We can say \$\mathsf{width=n-length}\$, so the amount of LEDs is: \$\mathsf{length\cdot{}(n-length)=-length^2+n\cdot{}length}\$. Given an \$\mathsf{n}\$, this is a parabola, which has a maximum when \$\mathsf{length=\frac{n}{2}}\$. You can also do this on gut feeling. So, the maximum amount of LEDs is reached when the two sets have an equal amount of pins, or differ only 1, in case of an odd number of pins. We can now say:
$$L(n)=\lfloor{}\frac{n}{2}\rfloor{}\cdot\lceil{}\frac{n}{2}\rceil{}$$
Also, we can now easily understand the function \$\mathsf{p(n)}\$:
$$p(n)= \begin{cases} 1&\text{ for }n=1\\ \lceil{}\sqrt{n}\rceil&\text{ for }n\gt1 \end{cases}$$
I just included the cases for 1, as this is a special case. Normally, you can just use the second function.
In this method, we have two LEDs between every set of two pins. We can calculate the amount of sets of two pins with:
$$(n-1)+(n-2)+\dots+1 = \frac{n\cdot(n-1)}{2}=\frac{n^2-n}{2}$$
Now we can say that:
$$L(n)=2\cdot\frac{n^2-n}{2}=n^2-n$$
We saw that the amount of pairs of pins equals \$\mathsf{n\cdot(n-1)}\$. With some reverse thinking, this leads to:
$$p(n)= \begin{cases} 1 &\text{ for } n=1\\ 2\cdot\lfloor\sqrt{n}\rfloor-1 &\text{ for } n\gt1 \end{cases}$$
I just included the cases for 1, as this is a special case. Normally, you can just use the second function.
I'm not aware of any other methods, as of Tuesday march 12, 2013.
It's right there, in section 21. See it?
\$\frac{0.25\text{V}}{10\text{mA}}= 25\Omega\$
\$\frac{5\text V - 4.75\text{V}}{10\text{mA}}= 25\Omega\$
Therefore yes, the output has a resistance of 25\$\Omega\$ at 25\$^{\circ}\$C.
Best Answer
In the root of the expression, it is "set the bit high/low (=1/0)"
It usually means set the corresponding pin "high logic level" and "low logic level", accordingly.
You would have to consult the documentation of your microcontroller, but it almost always is "set the output voltage to Vss/Vdd" (VssIO/VddIO if your micro has separate IO and core power supplies).