The schematic for this has no current limiting for the LED segments meaning that if a segment was to draw more than 25mA the PIC would most likely burn out as if memory serves, that is the maximum source/sink current for the uC's pins. Also, since there is no transistor switching or anything of the like on the digit select pins (be they common anode or cathode), ALL of the current for each digit is either sourced or sunk into the select pins, the implications of this are two fold, if the displays are common anode the displays will dim as each additional segment is turned on, also increasing the current draw and likelyhood of destroying the PIC. If the display is common cathode, each additional segment forces the PIC to SINK more current and increases the likelyhood of destroying it. For the sake of 11 resistors and 4 transistors (7 resistors for the segments, 4 for base current limiting) all of this could be avoided...
The maximum allowed continuous output current from a PIC pin is only 25mA. The current into seven segments without current limiting resistors is about 60mA. POOF! It might be destroyed immediately or after a few minutes.
PIC16F84 9999 counter
Posted - Jul 14 2006 : 5:04:17 PM
