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.
Other articles related to "refresh rate, refresh, rate":
... Also the screen has low resolution and refresh rate compared to the very high resolution and instantaneous refresh provided by an optical path in the OVF of DSLRs ... A slow refresh rate means that the image seen on the screen will have a fraction of a second lag or delay from the real scene being photographed ... size, resolution, visibility, magnification and refresh rate ...
... The refresh rate (most commonly the "vertical refresh rate", "vertical scan rate" for CRTs) is the number of times in a second that a display hardware draws the data ... This is distinct from the measure of frame rate in that the refresh rate includes the repeated drawing of identical frames, while frame rate measures how often a video source can feed an entire ... runs at 24 frames per second, but has a 48 or 72 Hz refresh rate ...
... market today those which "flash" a picture for a short part of the refresh period (CRT, cinema projector), and those which display an essentially static image between the ... "flashing" displays must be driven at least 48 Hz, although today, a rate significantly below 85 Hz is not considered ergonomic ... is usually displayed at 2x, 3x, or 4x the capture rate ...
Famous quotes containing the words rate and/or refresh:
“At the rate science proceeds, rockets and missiles will one day seem like buffaloslow, endangered grazers in the black pasture of outer space.”
—Bernard Cooper (b. 1936)
“To look backward for a while is to refresh the eye, to restore it, and to render it the more fit for its prime function of looking forward.”
—Margaret Fairless Barber (18691901)