In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

To explain this, simply think of an SX I/O pin configured as an input with a pull-up resistor connected to it and the other end connected to positive supply (Vdd), and a normally open pushbutton connected to that pin, with the other end connected to the negative supply (Vss).

When an I/O pin is configured, it has a high impedance, or high input resistance. Nevertheless, the input also acts as a small capacitor. When you connect the input pin to Vdd, this capacitor is charged. When you then connect the input to Vss, the capacitor is discharged.

When you apply power to the SX, the input capacitor is charged through the pull-up resistor. When you then press the pushbutton, the capacitor is discharged through the closed pushbutton. When you release the pushbutton, the capacitor is charged again through the pull-up resistor. The time-constant for charging is experessed by R * C, where R is the value of the pull-up resistor, and C the input capacity. As you can see, when you increase R, the time-constant also increases, i.e. it take more time to charge the capacitor when R is increased (the same is true for C, but you can't change its value, unless you would attach an additional parallel capacitor).

The faster the signal speeds are that control the input, the more is it important that the input capacitor is charged and discharged fast enough in order to maintain steep signal edges. This is why you should select lower values for the pull-up resistor in such cases 10 kOhm, down to 1 kOhm are values that are fine in most cases.

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