RN-Digi Elemente einzeln abschalten

hi,
ich benutze die RN-Digi Schaltung zur Darstellung 1-4 stelliger Zahlen auf den 4 Siebensegmentanzeigen. Nun möchte ich bei beispielsweise bei 2- oder 3-stelligen Zahlen die vordersten Siebensegmentanzeigen abschalten. Ich bin aus dem Datenblatt des SAA1064 nicht ganz schlau geworden. Anscheinend kann man mit Controlbit 1 und 2 jeweils zwei Elemente lahmlegen. Aber wie kann ich einzelne ausschalten?
Hier noch mein bisher verwendeter Code und im Anhang das Datenblatt des SAA1064:

Code:

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Declare Sub Led_display_init()
Declare Sub Led_display(byval Ziffer As Byte , Byval Zahl As Byte)

$regfile = "m32def.dat"
$framesize = 32
$swstack = 32
$hwstack = 32

$crystal = 16000000                                        'Quarzfrequenz
$baud = 9600

Const Rn_digi_slave_write = &H70
Const Rn_digi_slave_read = &H71
Const Dezimalpunktziffer = 0      'Segment wo Dezimalpunkt leuchten soll (0=keinen)
Const Timervorgabe = 3036

Dim Segmente As Byte
Dim Z As Byte
Dim Beiner As Byte
Dim Bzehner As Byte
Dim Bhunderter As Byte
Dim Btausender As Byte
Dim Einer As Word
Dim Zehner As Word
Dim Hunderter As Word
Dim Tausender As Word
Dim A As Word
Dim Zaehlerirq0 As Long
Dim Impulseprosekunde As Long
Dim Impulseprominute As Long
Dim Umdrehungenprominute As Integer



Config Scl = Portc.0                                        'Ports fuer IIC-Bus
Config Sda = Portc.1
Config Int0 = Falling
Config Timer1 = Timer , Prescale = 256

Config Lcd = 16 * 2
Config Lcdpin = Pin , Db4 = Portb.0 , Db5 = Portb.1 , Db6 = Portb.2 , Db7 = Portb.3 , E = Portb.5 , Rs = Portb.4
Config Lcdbus = 4


Zaehlerirq0 = 0
On Int0 Irq0
Enable Int0

On Timer1 Timer_irq
Enable Timer1

Enable Interrupts





I2cinit
Led_display_init


Sound Portd.7 , 400 , 450                                  'BEEP
Sound Portd.7 , 400 , 250                                  'BEEP
Sound Portd.7 , 400 , 450                                  'BEEP

Cls
Locate 1 , 1
Lcd "Upm="
'Locate 2 , 1
'Lcd "Ips="

Bhunderter = 0
Btausender = 0
Beiner = 0
Bzehner = 0

Do

  Umdrehungenprominute = Impulseprosekunde * 26
  Umdrehungenprominute = Umdrehungenprominute / 15

  Locate 1 , 5
  Lcd Umdrehungenprominute
  'Locate 2 , 5
  'Lcd Impulseprosekunde


  A = Umdrehungenprominute

  Einer = A Mod 10
  A = A - Einer
  A = A / 10
  Zehner = A Mod 10
  A = A - Zehner
  A = A / 10
  Hunderter = A Mod 10
  A = A - Hunderter
  A = A / 10
  Tausender = A Mod 10

  If Btausender = Tausender Then
  Else
      Btausender = Tausender
      Led_display 2 , Btausender
  End If

  If Bhunderter = Hunderter Then
  Else
      Btausender = Tausender
      Led_display 1 , Bhunderter
  End If

  If Bzehner = Zehner Then
  Else
      Bzehner = Zehner
      Led_display 4 , Bzehner
  End If

  If Beiner = Einer Then
  Else
      Beiner = Einer
      Led_display 3 , Beiner
  End If

  Bhunderter = Hunderter
  Btausender = Tausender
  Beiner = Einer
  Bzehner = Zehner

Loop

End


Sub Led_display_init()
  I2cstart
  I2cwbyte Rn_digi_slave_write
  I2cwbyte 0                                                'Control Byte

  'Dynamic Alternative Mode und Helligkeit
  'Die oberen 3 Bits bestimmen die Helligkeit
  'Wenn es dunkler sein soll dann z.b. &B0110111
  I2cwbyte &B0110111                                        '&B1110111

  I2cstop

  'Alle Ziffern auf 0
  For Z = 1 To 4
      Led_display Z , 0
  Next Z

End Sub


Sub Led_display(ziffer An Byte , Zahl As Byte)
    I2cstart
    I2cwbyte Rn_digi_slave_write
    I2cwbyte Ziffer

    Select Case Zahl
        Case 0:
              Segmente = &H3F                              '00111111
        Case 1:
                Segmente = &H06                            '00000110
        Case 2:
                Segmente = &H5B                            '01011011
        Case 3:
                Segmente = &H4F
        Case 4:
                Segmente = &H66
        Case 5:
                Segmente = &H6D
        Case 6:
                Segmente = &H7D
        Case 7:
                Segmente = &H07
        Case 8:
                Segmente = &H7F
        Case 9:
                Segmente = &H67
        Case Else :
                Segmente = &H80                            'Ansonsten Dezimalpunkt

    End Select

    If Dezimalpunktziffer = Ziffer Then Segmente = Segmente Or &H80
    I2cwbyte Segmente
    I2cstop
End Sub


'Pro Impuls (Markierung auf Scheibe) ein Aufruf
Irq0:
 Incr Zaehlerirq0
Return


'Pro Sekunde ein Aufruf
Timer_irq:
  Timer1 = Timervorgabe
  Impulseprosekunde = Zaehlerirq0
  Zaehlerirq0 = 0
Return

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Gruß
Daniel