Seven Segment LED multiplexing

[mkstevo]

I recently had a need for creating a four digit, seven segment LED display. Whilst I found a few examples of single digit displays I couldn't find anything that used multiple segments. I wrote two versions and thought I'd share them. They are almost certainly a long way from perfect, but should be useable and may offer a starting point for something better.

[mkstevo]

This version uses the LED seven segment library, the segments of all five of the displays are connected in parallel, with the anodes being connected to separate ports on the PIC device through a series resistor. As the displays are multiplexed rapidly in sequence the brightness of the displays is reduced, to compensate, I fitted a low value of series resistor, just 47R. The brightness could be increased by increasing the constant 'DisplayTime' or removing any displays which might not be needed. If the DisplayTime is increased too much, the display may start to flicker objectionally.

'''A demonstration program for GCGB and GCB.
'''--------------------------------------------------------------------------------------------------------------------------------
'''This program will display a message using 5 x 7 Segment displays.
'''The 7 Segment displays are connected to PortC.0-6 for segments A-G respectively.
'''@author  EvanV plus works of HughC
'''@licence GPL
'''@version 1.0a
'''@date    31.01.2015
'''********************************************************************************

'''Multiplexing demonstration by mkstevo 28th May 2016

'Pic 16F1829 5 way 7 Segment LED Display

#Chip 16F1829, 8

'###########################################################################
#Config CP=On
'Read protected
'###########################################################################

; ----- Define Hardware settings
  ; All port settings, like in and out, are handled within the Seven Segment driver.

; ----- Constants
  ; You need to specify the port settings 

  #Define 7Seg_CommonAnode   'Pic   Disp
  #define DISP_SEG_A PORTc.3 '7     7
  #define DISP_SEG_B PORTc.4 '6     6
  #define DISP_SEG_C PORTc.5 '5     4
  #define DISP_SEG_D PORTa.4 '3     2
  #define DISP_SEG_E PORTa.5 '2     1
  #define DISP_SEG_F PORTc.6 '8     9
  #define DISP_SEG_G PORTc.7 '9     10

  #define Disp1Pwr PORTa.0   '19    3,8 use series 47R resistor
  #define Disp2Pwr PORTa.1   '18    3,8 use series 47R resistor
  #define Disp3Pwr PORTa.2   '17    3,8 use series 47R resistor
  #define Disp4Pwr PORTc.0   '16    3,8 use series 47R resistor
  #define Disp5Pwr PORTc.1   '15    3,8 use series 47R resistor
  #Define DispOff 0
  #Define DispOn  1
  #Define DisplayTime 3500 'uS increasing this improves brightness but may cause flicker

  #Define LoopTime 15
  Dir Disp1Pwr Out
  Dir Disp2Pwr Out
  Dir Disp3Pwr Out
  Dir Disp4Pwr Out  
  Dir Disp5Pwr Out

; ----- Variables
  Dim Message(30)                             'defines a string
  Message() = "Seven segment display V1a     “'End with five spaces 
  '            1234567890123456789012345678901234567890

  Dim MessageLen As Byte 
  Dim LoopCnt As Word
  Let MessageLen = 30 'Length of message + 1
  Let DispMessage = 1
  Let Disp1Pwr=DispOff
  Let Disp2Pwr=DispOff
  Let Disp3Pwr=DispOff
  Let Disp4Pwr=DispOff  
  Let Disp5Pwr=DispOff

; ----- Main body of program commences here

  Main:
    For LoopCnt = 0 to LoopTime
      Let Disp1Pwr=DispOff
      Let Disp2Pwr=DispOff
      Let Disp3Pwr=DispOff
      Let Disp4Pwr=DispOff
      Let Disp5Pwr=DispOn
      If DispMessage < MessageLen Then
         DisplayChar 1, Message(DispMessage)
      Else 
         DisplayChar 1," "
      End If 
      Wait DisplayTime uS
      Let Disp1Pwr=DispOff
      Let Disp2Pwr=DispOff
      Let Disp3Pwr=DispOff
      Let Disp4Pwr=DispOn
      Let Disp5Pwr=DispOff
      If DispMessage > 1 Then
         DisplayChar 1, Message(DispMessage-1)
      Else 
         DisplayChar 1," "    
      End If 
      Wait DisplayTime uS 
      Let Disp1Pwr=DispOff
      Let Disp2Pwr=DispOff
      Let Disp3Pwr=DispOn
      Let Disp4Pwr=DispOff
      Let Disp5Pwr=DispOff
      If DispMessage > 2 Then
         DisplayChar 1, Message(DispMessage-2)
      Else 
         DisplayChar 1," "    
      End If 
      Wait DisplayTime uS
      Let Disp1Pwr=DispOff
      Let Disp2Pwr=DispOn
      Let Disp3Pwr=DispOff
      Let Disp4Pwr=DispOff
      Let Disp5Pwr=DispOff
      If DispMessage > 3 Then
         DisplayChar 1, Message(DispMessage-3)
      Else 
         DisplayChar 1," "    
      End If 
      Wait DisplayTime uS
      Let Disp1Pwr=DispOn
      Let Disp2Pwr=DispOff
      Let Disp3Pwr=DispOff
      Let Disp4Pwr=DispOff
      Let Disp5Pwr=DispOff
      If DispMessage > 4 Then
         DisplayChar 1, Message(DispMessage-4)
      Else 
         DisplayChar 1," "    
      End If 
      Wait DisplayTime uS
    Next
    Let DispMessage = DispMessage + 1
    If DispMessage > MessageLen Then
       Let DispMessage = 1
    End If
  Goto Main

[mkstevo]

This version gives much better and brighter displays, but requires the use of one 74HC595 for each display. The displays are 'static' in as much as once the displays have been updated the displays continue to display the information on them, without needing continual refreshment as the multiplexed example above does. This was my primary reason for moving away from the multiplexed version. This example has the 'Enable' and 'Reset' pins of the 74HC595 connected to the PIC. If this isn't needed, these can be tied to 0V (Gnd for the Enable pin) and 5V (Supply for the Reset pin). The 'D_Clk' and 'D_Lat' (and if used 'D_Ena' and 'D_Reset') are all connected in parallel, the 'D_Data' (pin 14) of the right-most display 74HC595 is connected to the PIC pin 2, with pin 9 of this connecting to the next 74HC595 to the left pin 14 and so on down the line.

'Pic 16F1829 4 way 7 Segment LED Display Using 4x74HC595 Example
'Prints all numerals 0-9 then alphabet A-Z - mkstevo 28th May 2016
#Chip 16F1829, 8
'I've had this running at 32MHz perfectly well, with no delays in the ShiftOut Subroutine either

'###########################################################################
#Config CP=On
'Read protected
'###########################################################################

'This uses common Anode seven segment displays the Anode is
'connected to 5V with a 470R resistor, the decimal point
'is connected to the 74HC595 with a 470R resistor.

'The segments are connected to the 74HC595 with the display
'pin ‘a’ being connected to 74HC595 pin ‘a’, ‘b’ - ‘b’ etc.

'  aaa
' f    b
' f    b
'  ggg
' e   c
' e   c
'  ddd
'      p

'0=Element On
'pgfedcba
'11000000 '0
'11111001 '1
'10100100 '2
'10110000 '3
'10011001 '4
'10010010 '5
'10000010 '6
'11111000 '7
'10000000 '8
'10010000 '9
'

'To display a decimal point by a character, deduct 128 from value shown.

Table Numerals as Byte
    192   '0
    249   '1
    164   '2
    176   '3
    153   '4
    146   '5
    130   '6
    248   '7 
    128   '8 
    144   '9 
End Table

'To display a decimal point by a character, deduct 128 from value shown.

Table Alphabet As Byte
    136 'A
    131 'b
    167 'c
    161 'd
    134 'E
    142 'F
    194 'G
    139 'h
    207 'I
    241 'j
    137 'K
    199 'L
    200 'M
    171 'n
    163 'o
    140 'p
    152 'q
    175 'r
    146 'S
    135 't
    227 'u
    193 'V
    193 'W
    137 'X
    145 'y
    164 'Z
End Table

                                                            'Pic   74HC595 
  #define D_Data  PORTa.5    '2     14 (of right-most display,viewed from display front)
  #define D_Clk   PORTa.4    '3     11
                             '4 In Only
  #define D_Lat   PORTc.5    '5     12

  #define D_Ena   PORTc.4    '6     13 Low to Enable
  #Define D_Reset PORTc.3    '7     10 High to Reset

'Connect Pin 9 of right-most 74HC595 to pin 14 of next left 74HC595
'then 9 - 14 to the next left, 9 - 14 to the next left etc.

  Dir D_Reset Out
  Dir D_Ena   Out
  Dir D_Lat   Out
  Dir D_Clk   Out  
  Dir D_Data  Out

  Dim Dat As Byte
  Dim Digit As Byte
  Dim TableLen As Byte
  Let D_Reset = 0  'Hold in reset
  Let D_Ena   = 1  'Hold with enable OFF
  Let D_Lat   = 0
  Let D_Clk   = 0  
  Let D_Data  = 0

Main:
  PrintNumerals
  PrintAlphabet
Goto Main

Sub PrintAlphabet
  Let D_Reset = 1                 'Release reset
  Let D_Ena  = 0                  'Enable On
  ReadTable Alphabet, 0, TableLen
  For Dat = 1 To TableLen 
    ReadTable Alphabet, Dat, Digit 
    Let D_Lat  = 0                'Start output data latch  
    ShiftOut(Digit)
    Let D_Lat  = 1                'End output data latch
    Wait 500 mS
  Next Dat
  Let D_Reset = 0                 'Hold in reset
  Let D_Ena   = 1                 'Hold with enable OFF 
  Wait 1000 mS
End Sub

Sub PrintNumerals
  Let D_Reset = 1                 'Release reset
  Let D_Ena  = 0                  'Enable On
  ReadTable Numerals, 0, TableLen
  For Dat = 1 To TableLen
    ReadTable Numerals, Dat, Digit 
    Let D_Lat  = 0                'Start output data latch  
    ShiftOut(Digit)
    Let D_Lat  = 1                'End output data latch
    Wait 500 mS
  Next Dat
  Let D_Reset = 0                 'Hold in reset
  Let D_Ena   = 1                 'Hold with enable OFF 
  Wait 1000 mS
End Sub

Sub ShiftOut (DataOut as Byte)
    For Clocks = 1 to 8
      Let D_Clk = 0            'Shift register clocked
      Wait 10 uS               'These delays are only here to allow me to see them on the oscilloscope.
      Let D_Data = DataOut.7
      Let D_Clk = 1
      Wait 10 uS               'These delays are only here to allow me to see them on the oscilloscope.
      Rotate DataOut Left Simple
    Next Clocks                          
End Sub