Display multiplexing

[davt]

Hi all

I have a Pic base project with 7 segment led displays - 0.56" size.
At present I use transistors to switch the common cathodes to ground to turn the relevant digit on. Because the display is not on continuously ie. it is scanned can I dispense with the transistors and use the port pins to ground the cathodes without damaging the pic or reducing its lifespan?
Thanks.

[Humberto]

First answer is no.
Not sure what do you try to do, considering how cheap a transistor is.
To light-on each segment of the display you need at least 7 - 10mA,
taking the worst case, wich is to display the number '8', you need
7mA x 7seg = 49mA/digit.

Microchip datasheet spec for almost all the line stated a maximum
allowable current of 25mA for driving/sinking.

Hope this help.

Humberto

[davt]

Thanks Humberto for your reply.

I do realize that the max current is 25ma but if I am scanning through the 7 seg displays they will only be on for about 2ms.

I thought that the port pins might withstand a 2ms pulse rather than be continuously on.

Dave

[Humberto]

Hi davt,

I´m sure that if you use a PIC output to sink such pulsed current would
work, but it´s a matter of taste + experience, I´m very conservative and
never will take the risk, just to save $0.02 in a transistor.

Best wishes,

Humberto

[drh]

If you don't use transistors AND you are debugging with an ICE or ICD, you could accidentally stop the code with the cathodes enabled and blow the PIC output. I would use transistors.
_________________
David

[davt]

Point taken!
Thanks for all input.

Dave