At 10:03 AM 3/17/2004 +0200, you wrote:
>
>Hi,
>
>For the display of my project I'm using four seven segment displays (common
>anode), using 8 IO pins for the segments, and another 4 to drive the
>individual displays. The four displays are driven through a BC557 transistor
>(with a 1K resistor on the gate), and each segment is connected to an IO pin
>through a 330 Ohm resistor (all segments are common to all displays, going
>through the same resistor. I hope this is all clear :-)
>
>To drive the displays I repeat the following via a timer interrupt
>(Prescaler set to 1:1, 8 bit timer, 4 Mhz clock):

Better if you turn off all displays, set correct segments, then
turn on display X. Prevents "ghosting".

>1) Turn on display X, turn off all other displays
>2) Set the correct segments for display X
>3) Add one to X
>4) If (X > 4), set X = 1
>
>So each time the ISR executes it turns on the next display, etc.
>
>Problem is, the segments on the displays are very dim if I do this. If I
>only have one display on, then it is the correct brightness. This is
>understandable since each display is only on for 1/4 of the time now. I was
>thinking maybe I could just use a quarter of the value resistor instead
>(replace 330 Ohm with 100 Ohm, for example). But then, won't it draw too
>much current even if its only for a fraction of second, or will it all
>average out? Also, if something happens which cause the pic to stop
>multiplexing the displays, there is the possibility of only one display
>being on for a long period of time (clocking stops, transistor failure,
>etc).
>
>Any suggestions on how to overcome this?

Use a better filter in front of the display, use more average current,
or use a better display. Don't exceed the PIC specifications. Usually if
you run a good display at a fairly conservative current you'll get good
brightness and no worries if the multiplexing stops (say someone touches
the crystal leads), at least for a short period of time at room
temperature.

Here's the ones we have:
http://www.trexon.com/leds/trexon_led_display.pdf

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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