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VFD display interface question — Parallax Forums

VFD display interface question

Don MDon M Posts: 1,653
edited 2011-06-22 08:07 in Propeller 1
The VFD display operates @ 5 VDC. I know all the connections to the display will be configured as outputs from the prop. Should resistors still be used in between? If so what value?

Thanks.
Don

Comments

  • kwinnkwinn Posts: 8,697
    edited 2011-06-21 20:36
    Depends on the display specifics. Post the data sheet or part number.
  • AleAle Posts: 2,363
    edited 2011-06-22 04:48
    Depending on which pins you use (say if they are on the same side of the package, for instance) the use of a HCT245 as level shifter poses minimal overhead :). Some displays (with CMOS inputs) require more than 3.3V to be recognized as high... post the datasheet or the minimum VIH :)
  • Don MDon M Posts: 1,653
    edited 2011-06-22 06:16
    Right now I am using it with 2.2K resistors and it has been working ok for a week or more but just wanted to be safe than sorry. I have attached the data sheets for both the display assembly and the IC on the display itself.

    Thanks.
    pdf
    pdf
    PT6314 VFD controller IC.pdf
    1M
    pdf
    pdf
    VFM202SDA1 VFD Display.pdf
    300K
  • AleAle Posts: 2,363
    edited 2011-06-22 06:47
    The VIH is 0.7*5.0 V (assuming 5.0 VCC) = 3.5 V, but it has been working... (I'd use a HCT245 or similar as level shifter anyways just to be safe as you are working out of spec already)
  • Mike GreenMike Green Posts: 23,101
    edited 2011-06-22 07:04
    The display datasheet says that 10K pullup resistors (to +5V) are provided on all logic inputs. When the Prop I/O pin is set to input mode or output high mode, the 2.2K resistor helps limit the current through the protection diode on the I/O pin, but the 10K resistor in the display should be adequate for this.

    When the Prop I/O pin is set to output low mode, you have a circuit like this:

    Prop (0.3V) ---- 2.2K ---- 10K ---- +5V

    The display input is sensing the voltage between the 2.2K and 10K resistors which, by Ohms Law, would be roughly 1.2V. The datasheet says that the maximum logic low input voltage is 1.0V with a +5V supply. The current setup works, but is slightly out-of-spec. You could leave it as it is or change the 2.2K resistor to something like 1K or use a buffer like the HCT245 or even just use a couple of NPN or MOSFET switching transistors as level shifters.
  • Don MDon M Posts: 1,653
    edited 2011-06-22 07:41
    Mike- Using Ohms Law I come up with a different number-

    +5v supply - .3v prop low = 4.7v
    4.7v / 12200 ohms (total both resistors) = .00039 Amps (.39 mA)
    .00039 x 2200 ohm = .85v

    Even if you don't figure the .3 voltage drop on prop pin the low would still be less than 1 volt. Am I doing something wrong here? If not then it would be right in spec.

    I'll probably use the resistors since that appears to be ok according to the math here.

    Thanks for your help guys.

    Don
  • Mike GreenMike Green Posts: 23,101
    edited 2011-06-22 07:50
    The drop across the 2.2K resistor is indeed 0.85V, but you have to add that to the 0.3V Prop output voltage (= 1.15V).
  • Don MDon M Posts: 1,653
    edited 2011-06-22 08:07
    Oops.. my bad. Thanks Mike. :) Looks like 1K is the winner here!
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