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Chapter 03

Circuitry for the Propeller PCB

Page 1

Some concepts about switching



"Only those who have the patience to do simple things perfectly
 will acquire the skill to do difficult things easily."
                                           -Johann Von Schiller




Notations used:
Resistance and LED

In order to understand the POVD circuit, let us first understand the basics of switching theory. The example shown below, consists of 4 Red coloured LEDs. The cathodes of all the LEDs are connected together. Hence it is called Common Cathode (CC) configuration. Ground voltage (0V) is applied to the common cathode. A Logical 1 (equivalent to +5v) is applied to the anodes. All LEDs glow, as shown below. A nibble is a group of 4-bits. In this case the value of the Nibble is binary 1111 or hexadecimal F. Hence providing +5V to all anodes.
LED Sequence

If only +5V is supplied to anode and nothing is connected (N/C) to the cathode the LED will not glow, as shown below:
LED Sequence

In this example, GND is applied to CC of all LEDs. Still, only those LEDs will glow which are receiving +5V at anode. The LEDs receiving a logical 0 (i.e., 0 Volt) on anode will not glow, as shown below:
LED Sequence

From now onwards the binary pattern (Nibble) applied to anodes will be termed as onSequence (ON Sequence). The onSequence defines which LEDs should glow.

The onSequence defines which LEDs should glow, BUT CC serves as a "master-control" for all LEDs. Irrespective of the onSequence, if CC is N/C then LEDs will not glow, as hown below:
LED Sequence

Question: If we need to switch-ON the Rightmost LED, what will be the value of onSequence (Nibble) and CC ?
LED Sequence




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