Thursday, June 11, 2015

Prototype 14-segment-display shield

There are many ways (*) to drive the 6-digit 14-segment common cathode display from Seeed Studio.
This time I chose to multiplex two MAX7221, a method described here (but used for driving a bi-color 8x8 LED matrix).


The code is based on LedControl library, which I extended to cover the definition and display of 14-segment characters (digits, upper case letters, and a few specials). Below is a relevant fragment of the code I added:

/*
* Segment names in the 14-segment (plus DP) display:
*
*     -     A
*   |\|/|   F,I,J,K,B
*    - -    G,H
*   |/|\|   E,N,M,L,C
*     -  .  D,P
*/
// my wiring:
//            GFEDCBAx
// 1st byte: B11111111
//
//            NHJIKMLP
// 2nd byte: B11111111

const static byte charTable14Seg[43][2] = {
    {B01111110,B10001000},  // 0
    {B00001100,B00001000},  // 1
    {B10110110,B01000000},  // 2
    {B00011110,B01000000},  // 3
    {B11001100,B01000000},  // 4
    {B11010010,B00000010},  // 5
    {B11111010,B01000000},  // 6
    {B00000010,B00001100},  // 7
    {B11111110,B01000000},  // 8
    {B11011110,B01000000},  // 9
    {B00000000,B01000000},  // :
    {B00000000,B01000000},  // ;
    {B00000000,B01000000},  // <
    {B00000000,B01000000},  // =
    {B00000000,B01000000},  // >
    {B00000000,B01000000},  // ?
    {B00000000,B01000000},  // @
    {B11101110,B01000000},  // A
    {B00011110,B01100100},  // B
    {B01110010,B00000000},  // C
    {B00011110,B00100100},  // D
    {B11110010,B01000000},  // E
    {B11100010,B01000000},  // F
    {B01111010,B01000000},  // G
    {B11101100,B01000000},  // H
    {B00000000,B00100100},  // I
    {B00111100,B00000000},  // J
    {B11100000,B00001010},  // K
    {B01110000,B00000000},  // L
    {B01101100,B00011000},  // M
    {B01101100,B000100L0},  // N
    {B01111110,B00000000},  // 0
    {B11100110,B01000000},  // P
    {B01111110,B00000010},  // Q
    {B11100110,B01000010},  // R
    {B11011010,B01000000},  // S
    {B00000010,B00100100},  // T
    {B01111100,B00000000},  // U
    {B01100000,B10001000},  // V
    {B01101100,B10000010},  // W
    {B00000000,B10011010},  // X
    {B00000000,B00011100},  // Y
    {B00010010,B10001000},  // Z
};
...
void setChar14Seg(byte pos, byte ascii)
{
  if (pos>7)
    return;

  if (ascii>90 || ascii<48)
    return;

  byte index = ascii - 48;
  for(byte seg=0; seg < 8; seg++)
  {
    SetLed(SEG_AG, pos, seg, charTable14Seg[index][0] & 1 << seg);
    SetLed(SEG_GN, pos, seg, charTable14Seg[index][1] & 1 << seg);
  }
}

This method (hardware and software) can be used for up to 8 14/16-segment displays.


(*) Should be the topic of a future post.


4 comments:

  1. Can you post the code for arduino to use this library to write characters on that display?

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    Replies
    1. You can find the sketch here:
      https://drive.google.com/file/d/0B01cjIbSk11NaENFU01taHNrZ0U/view?usp=sharing

      The wiring I used to connect the display is documented in the sketch.

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  2. I am trying to use 2 max 7219s to control 4 double 14-segment displays. I am using digit 0 of 1st max to control outer ring of digit 0, and digit 0 of 2nd max to control inner segments, digit 1 to control outer digit 1, etc etc. The problem is, I have ghosting on every digit. If segment "A" of digit 0 is lit, the segment "A" of every digit is lit. Please help me!!!

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