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noise from mux-ing 7-seg displays

Discussion in 'Electronic Basics' started by johnT, Feb 22, 2007.

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  1. johnT

    johnT Guest

    I have a 4 digit 7-segment display that I mux through a MPU (illuminate one
    digit, wait a couple ms, go to the next digit... like this: This switching is
    causing terrible noise on the power bus that is really messing with my
    analog circuit (which is running on a separate circuit board and MPU). I've
    loaded the display 'leg' of the power bus with gobs of capacitors (1000uf
    elect, 33uf tantalum, .1uf ceramic) and it helped alot but not enough. I
    use a 9V battery and a LT1111-5 as a power supply.
  2. Eeyore

    Eeyore Guest

    You most likely have 'ground noise' from a common connection with the display
    current hrowing through it.

    It might also be RFI. Is the display driver right next to the display itself ?

  3. johnT

    johnT Guest

    The display driver (MPU) pcb is mounted directly to the back of the 4x7
    display module. There is 4" of wire (+5, G, SDA, SCL) going to the display
    pcb, however, when assembled the 2 pcbs are 1/2" apart.

  4. Eeyore

    Eeyore Guest

    How does that assembly get its power ?
    How does the analogue section get its power ?

  5. johnT

    johnT Guest

    A 9V battery goes to a LT1111 regulator on the analog board. The analog
    board has a header where I run power and coms (TWI/I2C) to the display board
    (through 4" of 22AWG wire).
  6. Ben Jackson

    Ben Jackson Guest

    If you're sure it's not ground bounce, try making your LED VCC separately.
    If they're common anode displays, use 9V directly.
  7. Eeyore

    Eeyore Guest

    There's your problem. What does I * R equal ?

    What's the R of a pcb trace ?

  8. Eeyore

    Eeyore Guest

    Make the LED *RETURN* seperately more like ! Noise on the plus volts in comparison
    is easily filtered.

  9. johnT

    johnT Guest

    Thanks, Ben and Eeyore. I got it working.

    For some reason (noise, ground bounce, V sag, ...) when I was feeding 5V to
    the LED module the display MPU was going haywire and drawing way too much
    current (290mA!). Once I ran 9V to the display presto, 40mA (@9V), and
    clean (enough) analog signals.
  10. Lionel

    Lionel Guest

    Not that I suppose it matters any more, but you can reduce noise & RFI
    by slowing down the multiplexing. The amount of noise is directly
    related to the how often the drive signals change state, so the slower
    you switch them, the fewer the switching transients. For 7-segment
    displays, 50 refreshes/second (20mS each, divided by the number of
    digits) is plenty. In your circuit, that works out to 5ms per digit,
    or a mux frequency of 200Hz. I haven't looked very closely at your
    code, but it seems to be refreshing way more than neccessary, which
    will increase your power requirements, generate lots of noise, & waste
    lots of CPU cycles you could be using for something else.

    Pseudo-code: (All straight out of my head, & completely untested.)

    ; **************************************************
    ; **** 7 segment LED display multiplexing stuff. ***
    ; **************************************************
    DISP_MUX = 0
    ; DISP_0-3 are the segments for each
    ; 7 segment display, Bits 0-6 = Seg's
    ; A-F, bit 7 = DP (if used)
    ; Main prog can update patterns any time.

    DISP_0 = 0xFF ; All segments lit for debugging.
    DISP_1 = 0xFF : **Assuming that '1' = on, '0' = off*
    DISP_2 = 0xFF ; **Invert if neccessary! **
    DISP_3 = 0xFF
    ; Output Ports:
    DISP_SEGS = PORTA ; Or whatever.
    DISP_DIG = PORTB ; Or whatever.
    DISP_BLANK = 0x00 ; All digits disabled.

    ; Setup:

    conf TIMER for 5ms (5ms/digit * 4 digits = 20ms = 50Hz)
    set TIMER interrupt to DISPLAY subroutine
    enable TIMER interrupt

    ; Main:
    ; Your main program goes here.

    ; DISPLAY: ; Multiplexing IRQ routine.
    OUT DISP_DIG, DISP_BLANK ;Blank display until
    ;segment data valid.
    IF DISP_MUX = 0 ; DIGIT 0 (Left)
    OUT DISP_DIG b'00000001' ; Enable Digit 0

    IF DISP_MUX = 1 ; DIGIT 1
    OUT DISP_DIG b'00000010' ; Enable Digit 0

    IF DISP_MUX = 2 ; DIGIT 2
    OUT DISP_DIG b'00000100' ; Enable Digit 0

    IF DISP_MUX = 3 ; DIGIT 3 (Right)
    OUT DISP_DIG b'00001000' ; Enable Digit 0
    AND DISP_MUX, 3 ; Roll counter over if needed
    RETI ; Return from interrrupt.
    ; *************************************************************

    Oh well, even if you don't need this, hopefully someone else will find
    it useful. ;)
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