PRT_LINE x1k10                                ͻ
                                     MC68901     
                                 bit assignments 
                                ͼ
                                 Motorola Corporation 68901
                                 Multi-Fonction Peripheral
 Ŀ
  General Purpose input output      b7=monochrom monitor                 (NOT)
  Interrupt Port                    b6=auxiliary ring indicator          (NOT)
                                    b5=FDC/HDC                           (NOT)
                                    b4=MC6850_IKBD/MC6850_MIDI           (NOT)
                                    b3=blitter busy                      (NOT)
                                    b2=auxiliary clear to send           (NOT)
                                    b1=auxiliary data carrier detected   (NOT)
                                    b0=printer busy                      (NOT)
 
         Ŀ
          Active Edge Register   b7-b0=(see GPIP)              (0:low  1:high)
         
      Ŀ
       Data Direction Register   b7-b0=(see GPIP)          (0:input  1:output)
      
  Ŀ                                 Priority  &
   Interrupt Enable Register A                                  Vector [b3-b0]
                                    b7=monochrom monitor                 1111
                                    b6=auxiliary ring indicator          1110
                                    b5=timer A                           1101
                                    b4=auxiliary receive register full   1100
                                    b3=auxiliary receive error           1011
                                    b2=auxiliary transmit register empty 1010
                                    b1=auxiliary transmit error          1001
                                    b0=timer B                           1000
  Ĵ
   Interrupt Enable Register B      b7=FDC/HDC                           0111
                                    b6=MC6850_IKBD/MC6850_MIDI           0110
                                    b5=timer C                           0101
                                    b4=timer D                           0100
                                    b3=blitter busy                      0011
                                    b2=auxiliary clear to send           0010
                                    b1=auxiliary data carrier detected   0001
                                    b0=printer busy                      0000
                                                         (0:disable  1:enable)
  
Ŀ
Interrupt Pending Register A/B   b7-b0=(see IERA/B)      (0:nothing 1:pending)

Ŀ
Inter. In Service Register A/B   b7-b0=(see IERA/B)   (0:nothing 1:in service)

  Ŀ
   Interrupt Mask Register A/B   b7-b0=(see IERA/B)    (0:masked 1:authorized)
  
              Ŀ
               Vector Register   b7-b4=[b7-b4]=non auto-vector high
                                    b3=software interrupt mode
                                       0:automatic end of interrupt
                                       1:software  end of interrupt
              
   Ŀ
    Timer A/B Control Register      b4=reset
                                       0:no
                                       1:yes
                                 b3-b0=mode
                                       0000:stop
                                       0001:delay  fr./4
                                       0010:delay  fr./10
                                       0011:delay  fr./16
                                       0100:delay  fr./50
                                       0101:delay  fr./64
                                       0110:delay  fr./100
                                       0111:delay  fr./200
                                                  (frequency=2.4576 MHz)
                                       1000:event count
                                            (timer A input=printer busy  )
                                            (timer B input=display enable)
                                       1001:measure fr./4
                                       1010:measure fr./10
                                       1011:measure fr./16
                                       1100:measure fr./50
                                       1101:measure fr./64
                                       1110:measure fr./100
                                       1111:measure fr./200
   
   Ŀ
    Timer C+D Control Register   b6-b4=timer C mode (see timer A/B b2-b0)
                                 b2-b0=timer D mode (see timer A/B b2-b0)
                                       (serial clock=timer D output)
   
  Ŀ
   Timer A/B/C/D Data Register   b7-b0=data
  
Ŀ
Synchronous Character Register   b7-b0=character

Ŀ
 Universal                          b7=serial frequency
 synchronous asynchronous              0:frequency
 receiver transmitter                  1:frequency/16
 Control Register                b6-b5=character size
                                       00:8 bits
                                       01:7 bits
                                       10:6 bits
                                       11:5 bits
                                 b4-b3=frame
                                          transmission    start-bit  stop-bit
                                       00:synchronous          0         0
                                       01:asynchronous         1         1
                                       10:asynchronous         1         1.5
                                       11:asynchronous         1         2
                                    b2=parity
                                       0:no
                                       1:yes
                                    b1=parity type
                                       0:odd
                                       1:even

     Ŀ
      Receiver Status Register      b7=buffer  full               (0:no 1:yes)
                                    b6=overrun error              (0:no 1:yes)
                                    b5=parity  error              (0:no 1:yes)
                                    b4=frame   error (async.)     (0:no 1:yes)
                                    b3=found (sync.) / break     (async.)
                                       0:found         0:no detecded
                                       1:search        1:detected
                                    b2=match (sync.) / character (async.)
                                       0:no            0:nothing
                                       1:yes           1:in progress
                                    b1=synchronous strip
                                       0:disable
                                       1:enable
                                    b0=receiver
                                       0:disable
                                       1:enable
     
  Ŀ
   Transmitter Status Register      b7=buffer   empty             (0:no 1:yes)
                                    b6=underrun error             (0:no 1:yes)
                                    b5=auto turnaround            (0:no 1:yes)
                                    b4=end of transmission        (0:no 1:yes)
                                    b3=break (async.)             (0:no 1:yes)
                                 b2-b1=low and high bits
                                       00:high impedance
                                       01:low
                                       10:high
                                       11:high (loop back)
                                    b0=transmitter
                                       0:disable
                                       1:enable
  
          Ŀ
           USART Data Register   b7-b0=data
          
