Hi,

I'm new to PIC (and electronics really) and I've been reading
through John Morton's book 'PIC Your Personal Introductory Course'.
It's really helping me to learn PIC programming, but I do have a
question from one of the example circuits in the book.

The circuit in question (fig. 2.22 page 62 for anyone with the book
following along) has four 7-segment LED displays. The seven led
anode pins of each display are connected to RB1-RB7 via a resistor,
and then the common cathode pin of each display is connected via a
transistor to RA0-RA3, again by a resistor. All fine so far. So I
set a cathode pin high to select which display I want to light up,
and then set the RB pins for that display. The book then suggests
that I cycle round all the displays very quickly in order to keep
them lit up, again fine.

OK, so the book suggests that I need to give each of the LEDs 10mA
to light it up properly and given that I am cycling through each
display in turn, each one is only going to get a quarter of the
time and therefore I should quadruple that amount to 40mA for the
short burst. Therefore the book calculates an 82R resistor to be
used on each of the LEDs. Now, my question is that whilst none of
the LEDs will get a sustained 40mA current, surely if I'm cycling
through all the displays and they all have the same number on them,
then some of the pins on the PIC will be having 40mA drawn through
them? (cycling through the four displays, lighting up the same
segment, drawing 40mA each time and thus sustained pretty much) And
isn't that too much for the PIC?

Thanks in advance for your help,
Matt

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Matt N. Marsh
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