Parallax Propeller Book - Seven Segment Display

In this Chapter we will discuss how to use Seven Segment Display to show digits:

7 Segment Digits — Fig 36-1 Seven Segment Digits

as well as Letters:

7 Segment Letters — Fig 36-2 Seven Segment Letters

Seven Segment Display

All the various versions of Seven Segment Display are usually wired as shown in the Figure 36-3 below.

Both common Anode (+) or common Cathode (-) versions exist for LED displays as well as LCD displays.

7 Segment Designations — Fig 36-3 Seven Segment Designations

Segment Activation Table

Displaying	Segments ON	Segments OFF	Binary Code	Case	Note
0	a,b,c,d,e,f	g	1111110	0
1	b,c	a,d,e,f,g	0110000	1
2	a,b,d,e,g	c,f	1101101	2
3	a,b,c,d,g	e,f	1111001	3
4	b,c,f,g	a,d,e	0110011	4
5	a,c,d,f,g	b,e	1011011	5
6	a,c,d,e,f,g	b	1011111	6
7	a,b,c	d,e,f,g	1110000	7
8	a,b,c,d,e,f,g		1111111	8
9	a,b,c,d,f,g	e	1111011	9

C	a,d,e,f	b,c,g	1001110	10	Cold
E	a,d,e,f,g	b,c	1001111	11	Empty
F	a,e,f,g	b,c,d	1000111	12	Full
H	b,c,e,f,g	a,d	0110111	13	Hot/High
L	d,e,f	a,b,c,g	0001110	14	Low
	a,d,g	b,c,e,f	1001001	15	3 bars
		a,b,c,d,e,f,g	0000000	16	OFF

Port Assignment / Connection Table

Pin	Segment	Port
7	a	P19
6	b	P20
4	c	P21
2	d	P22
1	e	P23
9	f	P24
10	g	P25
5	Decimal Point	P26
3,8	Common

7 Segment Wiring Diagram — Fig 36-4 Seven Segment Wiring Diagram (Common Cathode)

7 Segment Pinout — Fig 36-5 Seven Segment Pinout

The following Methods are for Seven Segment Display with common Cathode, wired as shown in Figure 36-4 above. All resistors (R1 to R8) are same and of any value from 150 Ohm to 470 Ohm, depending on current draw of the Segment LEDs and the desired brightness of the displayed character.

Any individual I/O pin can source or sink about 40 mA however, care should be taken to ensure that the entire chip does not exceed 1 W of power dissipation.
See the Propeller P8X32A Datasheet for more information.

See the specifications in Datasheet for the Seven Segment Display that you plan to use, most are limited to 20 mA per segment, however we recommend that you select the resistors so initially the current per Port / Segment does not exceed 10 mA.

It is also important to note that some versions of Seven Segment Displays may have more than one LED connected in series per each individual Segment, thus the minimum forward Voltage may be significantly higher than what is normal for a single LED. Therefore, if the minimum forward Voltage is higher than 3.3 V the Segment many not light up.

That is why it is important to either get the specifications from Datasheet or test the Seven Segment Display and determine the minimum required Voltage for its operation.

The Common Cathode of the Seven Segment Display is permanently grounded, i.e. attached to (-). Most Displays have two Common Cathode Pins, and either or both can be used as they are internally connected together. (usually Pin 3 and 8)

Each segment lights up when the appropriate Port / Segment combination is provided with +3.3V DC.

The Method below lights up Segment “a” when Port P19 is HI.

'' 36-1_Segment-a-ON-P19.spin       Common Cathode (-)

PUB SegmentOn                     ' Public Method declaration

    dira[19] := outa[19] := 1     ' Set P19 to output; Set P19 high (HI=ON)

    repeat                        ' Endless loop prevents program from ending

Above Spin lights up segment “a” on the Seven Segment Display.

To test other segments change the Port number as per Port Assignment Table above.

The Spin below lights up each segment in sequence for one second and repeats forever.

'' 36-2_Segment-x-ON-Test.spin    Common Cathode (-)

VAR

    byte Segment                 'Set up a number to identify a segment Port

PUB SegmentTest                    'Name this object: SegmentTest

    Segment := 19                   'begin with 1st segment on port P19

    dira[19..25] := %11111111       'all Segment Ports set as outputs

    repeat                       'create an endless loop

       outa[Segment] := 1           'turn on Segment x at Segment

       waitcnt(clkfreq + cnt)       'wait one seconds

       outa[Segment] := 0           'turn same Segment off

          Segment++              'increase Segment number by one--next Segment

           if Segment > 25           'ensures count doesn't go past last Segment Port

              Segment := 19        'if count > 25 (last Segment g position)
                                     'reset count to P19 (Segment a)

The Spin below lights up each digit or letter character in sequence for one second and repeats forever.

'' 36-3A_Segment-Display-Test.spin      Common Cathode (-)

    CON

        SegmentA = 19           'Set up a constant number to identify A segment Port

    VAR

    byte SegmentG 'Set up a number to identify G segment Port

    byte A 'define CASE counter A as Byte (0-255) sized global variable

    PUB Start

        Ini

        XStep

    PUB Ini

        SegmentG := SegmentA + 6                    'Define Segment G Port

        dira[SegmentA..SegmentG] := %11111111        'all Segment Ports set as outputs

        outa[SegmentA..SegmentG] := %00000000        'display is Blank

        waitcnt(clkfreq + cnt)                       ' wait - one second delay

        A := 0                                       ' Set A to zero 0

    PUB XStep            ' Increments A by 1 and waits 1 second

        repeat               ' Infinite repeat loop

             DisplayTest        ' Go to DisplayTest code

                                       ' Return from PUB DisplayTest

           waitcnt(clkfreq + cnt)    ' wait - one second delay

             A++                       'increase A number by one--for next CASE

               if A > 16               'ensures count doesn't go past last Segment Port

                   A := 0              'if count > 16 (last CASE)

                                               'reset count to A

                                       ' Go to start of repeat loop



    PUB DisplayTest           'Name this object: DisplayTest

       case A                    'Test A

          0 :
                outa[SegmentA..SegmentG] := %1111110        'A = 0 display 0 (ZERO)

          1 :
                outa[SegmentA..SegmentG] := %0110000        'A = 1 display 1

          2 :
                outa[SegmentA..SegmentG] := %1101101        'A = 2 display 2

          3:
                outa[SegmentA..SegmentG] := %1111001        'A = 3 display 3

          4:
                outa[SegmentA..SegmentG] := %0110011        'A = 4 display 4

          5:
                outa[SegmentA..SegmentG] := %1011011        'A = 5 display 5

          6:
                outa[SegmentA..SegmentG] := %1011111        'A = 6 display 6

          7:
                outa[SegmentA..SegmentG] := %1110000        'A = 7 display 7

          8:
                outa[SegmentA..SegmentG] := %1111111        'A = 8 display 8

          9:
                outa[SegmentA..SegmentG] := %1111011        'A = 9 display 9

          10:
                outa[SegmentA..SegmentG] := %1001110        'A = 10 display C = Cold

          11:
                outa[SegmentA..SegmentG] := %1001111        'A = 11 display E = Empty

          12:
                outa[SegmentA..SegmentG] := %1000111        'A = 12 display F = Full

          13:
                outa[SegmentA..SegmentG] := %0110111        'A = 13 display H = Hot / High

          14:
                outa[SegmentA..SegmentG] := %0001110        'A = 14 display L = Low

          15:
                outa[SegmentA..SegmentG] := %0000000        'A = 15 display blank

          other :
                outa[SegmentA..SegmentG] := %1001001        'A = other than 0 to 15 display 3 bars



                        ' Return to Method that called this

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