how to display zero on LTP-305?

Discussion in 'Embedded Systems and Microcontrollers' started by viva_unix, May 11, 2010.

  1. viva_unix

    Thread Starter New Member

    Apr 24, 2010
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    Hello!

    I am trying to get my microcontroller to display zero on matrix display and seems cannot understand how the display works. This is what I have:

    http://www.datasheetarchive.com/pdf/Datasheet-013/DSA00229414.pdf
    LTP-305G (G- Green, R - Red, Y - Yellow)

    Can someone tell me how to display zero? Looks like I am able to power the whole column/row only. In case of zero all leds are one and it hardly look like a zero... :(

    Thank you!
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    You need to pulse the x/y coordinates that match the led you want to light.

    Using PWM, you can light each led, one at a time, so fast, it looks like a zero.

    If you use constant current, you will light up every led in the row from the top most column to the bottom most.

    google 'persistence of vision with matrices'.

    here is the first result:
    http://www.best-microcontroller-projects.com/led-dot-matrix-display.html

    It will explain what you need to do with PWM (Pulse Wave Modulation)
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    PWM is a bit misleading, though technically accurate.

    You need to study Multiplexing of displays, it's the same principal as with a multi-digit seven segment LED or whatever, but for each common line you are outputting a column or row of data rather than a 7 segment pattern.

    As you have full access to both the row and column pins, you can decide which to use as common lines and which to use as the pattern data; it depends how you want to organise the character generator (patterns) in your program.
     
  4. viva_unix

    Thread Starter New Member

    Apr 24, 2010
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    Is 74HC595B1 will help? Am I looking into right direction?
    Thank you!
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    Yes, the 74595 is a shift register. It will help you multiplex the display so you can choose what dots to light.

    There are hundreds of examples listed on google. I would search for 'multiplex led matrix circuit'
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    That looks familiar. Didn't I link that in post #2? Great minds, i suppose ;)

    viva_unix, have you read the linked site, and if so, is it starting to make sense?
     
  8. viva_unix

    Thread Starter New Member

    Apr 24, 2010
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    Yes, I belive I am 50% to understand the concept...
    I understand how to drive the positive charge (high voltage from PIC to shift register and to the led matrix). But what about ground? They will be always connected.

    I understand the concept, the CRT mmonitors work this way, I just do not get the idea of Anodes and Cathodes...
     
  9. retched

    AAC Fanatic!

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    Cathodes are connected to ground and anodes are connected to positive supply.
     
  10. viva_unix

    Thread Starter New Member

    Apr 24, 2010
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    yes, so, I am connecting all the cathodes to the ground, In this case when I connect at least one Anode the whole row will light up... At least it is on my bread board now. So, when I get a high voltage on my PIC the whole row will be lightning not one led. Am I missing something?

    It works if I connect one cathode and one anode.
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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    yes you are missing an important part. You must run all the cathodes and all anodes through the multiplexer. You have to be able to switch each anode and each cathode to choose which led to light. If you leave all cathodes tied to ground, the whole line will light.
     
  12. viva_unix

    Thread Starter New Member

    Apr 24, 2010
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    OK, I think I got it now. I will try and report later if I still have the problem.
    Thank you!
     
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