Charlieplexing - Problems With Charlieplexing - Refresh Rate

Refresh Rate

Because only a single set of LEDs, all having a common anode or cathode, can be lit simultaneously without turning on unintended LEDs, charlieplexing requires frequent output changes, through a method known as flickering. When flickering is done, not all LEDs are lit quite simultaneously, but rather one set of LEDs is lit briefly, then another set, then another, and eventually the cycle repeats. If it is done fast enough, they will appear to all be on, all the time, to the human eye. In order for a display to not have any noticeable flicker, the refresh rate for each LED must be greater than 50 Hz. Suppose 8 tri-state pins are used to control 56 LEDs via charlieplexing, which is enough for 8 7-segment displays (without decimal points). Typically 7-segment displays are made to have a common cathode, sometimes a common anode, but without loss of generality suppose it is a common cathode. All LEDs in all 8 7-segment displays cannot be turned on simultaneously in any desired combination via charlieplexing. It is impossible to get 56 bits of information directly from 8 trits (the term for a base-3 character, as the pins are 3-state) of information, as 8 trits fundamentally comprises 8*log(3)/log(2) or about 12.7 bits of information, which falls far short of the 56 bits required to turn all 56 LEDs on or off in any arbitrary combination. Instead, the human eye must be fooled by use of a flicker. Only one 7-segment display, one set of 7 LEDs can be active at any time. The way this would be done is for the 8 common cathodes of the 8 displays to each get assigned to its own unique pin among the 8 I/O ports. At any time, one and only one of the 8 controlling I/O pins will be actively low, and thus only the 7-segment display with its common cathode connected to that actively low pin can have any of its LEDs on. That is the active 7-segment display. The anodes of the 7 LED segments within the active 7-segment display can then be turned on in any combination by having the other 7 I/O ports either high or in high-impedance mode, in any combination. They are connected to the remaining 7 pins, but through resistors (the common cathode connection is connected to the pin itself, not through a resistor, because otherwise the current through each individual segment would depend on the number of total segments turned on, as they'd all have to share a single resistor). But to show a desired number using all 8 digits, only one 7-segment display can be shown at a time, so all 8 must be cycled through separately, and in a 50th of a second for the entire period of 8. Thus the display must be refreshed at 400 Hz for the period-8 cycle through all 8 segments to make the LEDs flash no slower than 50 times per second. This requires constant interruption of whatever additional processing the controller performs, 400 times per second.

Read more about this topic:  Charlieplexing, Problems With Charlieplexing

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