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LED multiplexing tutorial.

Discussion in 'Microcontrollers' started by Nigel Goodwin, Sep 15, 2006.

  1. williB

    williB New Member

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  2. bbarney

    bbarney New Member

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    Mike
    Unfortunatley I'am stuck with the cc matrix's.so my stp16c596 is no good what would you suggest use two 74hc595 in place of it and still use the 2803a?
     
  3. philba

    philba New Member

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    Amen!
    that does seem low. Are you certain the duty cycle for each row is 1/8 (minus a little for blanking during row change)? based on your numbers, it should be about double that. with some voltage drop from the pic, I'd expect more like 100-110 mA.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Just measured it again, at home this time, still 62mA?.

    Certainly the multiplexing was 1/8th the other day when I scoped it, I'll try it again and see what's going on!. Might not be till Monday now though.
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    OK, I've added two more sections to the tutorial, the first adds full text capability, and the second uses that to scroll a text message across the display.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    OK, I've got the scope round it - with all 64 LED's lit, the PIC pins are only pulling the bottom of the LED's down to 1.65V above 0V, there's 1.75V dropped across the LED's, and 1.6V across the 150 ohm resistors. So this is about 10mA peak through the LED's.

    As you say, adding sink drivers would easily increase that, but it's bright enough as it is.
     
  8. tomekkicaj

    tomekkicaj New Member

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    LED capacitor

    I was just wondering how the multiplexing of the LED matrix would affect the system power supply? Wouldn't such frequent high current draw(sink) feed noise to the Vcc? Have anyone experimented with that? I'm using an ADC in my system with multiplexed leds and wondering if I shouldn't add a parallel capacitor with each LED?
     
  9. Mr RB

    Mr RB Well-Known Member

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    62mA for all LEDs lit up is about what I would expect. At any point there are only 8 leds going, which is about <8mA per LED.

    The PIC output pin sink driver is a NFET with similar properties to any NFET that you would use in a PSU design etc, Rds-on and they are really rated for average power disipation so you can run quite a high current IF it is a low duty cycle.

    Just because Microchip are conservative (and have to guarantee their products) they like to rate them with the convenient "absolute max current" figures. But in reality they are a power FET like any other type.

    Juts watch out for the new "shrunken die" PICs have much smaller and weaker output FETs than the old large die chips. If you look in the electrical specs in the datasheet your PIC will be one of these 2 types;

    Max current pin=25mA, max for ALL ports 200mA (old PIC with large FETs)
    Max current pin=25mA, max for all ports 90mA (new shrunken PIC with weak FETs)

    The old large die has bigger FETs on bigger silicon with more heat dissipation, larger tracks, lower Rds-on of the FETs themselves etc etc. But the new smaller die PICs are cheaper and use less power.

    The PIC power supply should be fine to source 62mA (max) pulsed, i've got some 22uF tantalums for the PIC power pins to use for PIC direct multiplexing. Don't expect a 0.1uF cap to do the job. ;)
     

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