2013. július 26., péntek

Double multiplex

My head is full with planed new electronic projects. Most of these contain some - especially LED based - display. Even with my Frequency Counter project caused some headache driving the display. If I want to use the minimum amount of external components than an 8 digit 7-Segment multiplexed display requires 16 I/O pins, what is problematic with a small pin count MCU. In addition, it requires external components because an MCU is not able to supply the current required by a whole digit. The lack of enough I/O pins driven me to use an additional IC (74HC238), 8 transistor and 16 resistance  for the display (the transistors and half of the resistors could be replaced by a darlington array, while we using common cathode displays. For common anode this is not an option because PNP array or common collector NPN does not exists). As I was searching new ideas for one of my new project on the net, accidentally came across an EDN article. This article is about the "double multiplexing". The point is that at any given moment we drive only single LED from the entire LED matrix. This reduces current consumption of the circuit, the number of required external components, and the amount of required I/O pins. The display mentioned above can be driven by nine pieces of resistance and 9 I/O pins. The question we should answer, if we can generate enough current for brightness and fast enough code to avoid flickering.
We'll see.I definitely want to try the stuff, so I designed a test PCB (I hate to put this amount of jumper wire into the breadboard).


I'll try to put it together during the weekend and writing the test code is the next.

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