How to Control more than 80 7seg LED Displays through a 8051

Discussion in 'Embedded Systems and Microcontrollers' started by blue_flare, Feb 6, 2008.

  1. blue_flare

    Thread Starter New Member

    Feb 6, 2008
    8
    0
    Hello All,
    I am a beginner in microcontrollers. I am building a display panel, which has almost 80 or more 7seg display and an IR input circuitry. My choice of microcontroller is 8051 as I am not familiar with any other. I have decided to multiplex the display using two microcontrollers and a pair of 4x16 decoders.
    I need your invaluable inputs on the following:
    1. If I activate each digit for 1ms every 50ms, will I notice any flicker?
    2. Is there a better option to use a single 8051 and be able to control this diplay, yet keeping the cost low.

    The block diagram of the circuit which i have in mind is something like this:

    IR ---------- IR Remote
    |
    8 | 8
    |----/---------------------8051--SPI--8051-------------------/---|
    | | | |
    | | | |
    2x(4x16 (decoder)) 2x(4x16 (decoder))
    | | | |
    /(16+16) /(16+16)
    | | | |
    Driver (4 BCD+4 BCD)/ /(4 BCD+4 BCD) Driver
    | | | |
    /(Control) / (Data) / (Control)/
    | | | |
    (Upto 32 pair of 7seg digits) (Upto 32 pair of 7seg digits)

    Thanks,
    Ather
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    There would be flicker with that 1:50 duty cycle. I would suggest using serial or parallel latches to buffer the data for the 7-segments. Load the data (could be multiplexed) and then latch them to the output to update the display.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Actually, with the displays only getting current for so little time, they would probably be too dim to see any flicker. The idea of using external latches to store the display data is a good way to go. With so many individual displays, a standard display latch/driver board would be economical.
     
  4. mik3ca

    Active Member

    Feb 11, 2007
    189
    0
    But before we go on, why would you need 80 digits?
    and do you NEED 7-segment displays?

    Definitely use only one 8051.

    You should probably use an LCD instead. I can get a text-based LCD that can display 40 alpha-numeric characters for $10, or I could get 80 7 segment displays for $120 provided each one is $1.50. Since you are asking for 80 characters, let's add another LCD. after all that, you save at least $100.

    Also, interfacing is much easier, and you will save yourself alot of time and wires. Go to 8052.com and click on the SBC link (in a green rectangle) down the left to see how they make their system with an 8051 chip.

    an 8052 is the same as an 8051, except that timer 2 and interrupt 2 don't exist in 8051.

    The best way to answer this is to make yourself an oscillator circuit with the LED for output. An astable multivibrator will work the best. Configure the capacitors and resistors so that the time is 1ms and 50ms respectively.

    then watch the LED to see if it flickers. there are many astable multivibrator circuits on the web.
     
  5. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    One reason would be visibility. There are environments where only LED-based brightness of 7-segment would do. There are also big-sized 7-segments available for extra distance visibility.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,145
    1,791
    Maxim has some interesting 7-segment display chips. They handle the multiplexing, duty cycle, and provide a constant current source. I've used multiple MAX6954 chips on an 8051 with excellent success. They also implement the Charliplexing Algorithm cuts doen on the number of lines you need for a given number of digits. I also recommend trying to run the controller and the displays at less than +5VDC to lower the power consumption.

    http://datasheets.maxim-ic.com/en/ds/MAX6954.pdf

    You might inhale at the cost of the chip, but trust me you will make it back in system performance and time to market because it just solves so many problems. As a bonus I can tell you that the datasheet is excellent.
     
  7. blue_flare

    Thread Starter New Member

    Feb 6, 2008
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    0
    The idea of latching the data so as to increase the duty cycle sounds logic. As [[papabravo]] has suggested, MAX6954 does ease off a lot of complexity in terms of hardware as well as software, tihs way I dont need more than a single 8051. I am going to enquire the cost of the component in the local market. I will keep you guys updated with the schematics that i might come up with.

    Thanks for all your help.
    Regards,
    Ather
     
  8. blue_flare

    Thread Starter New Member

    Feb 6, 2008
    8
    0
    the fact that i cannot go back to LCD is that, this is going to be a large outdoor kinda display panel
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If size or brightness becomes a factor, you can always use individual LED's to make the displays. It will keep you busy drilling holes, though.
     
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