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A Simple Video Display Processor for the Micro-Professor MPF-1

Things you will need

Hardware

  • Arduino UNO
  • 1x GAL20V8 PLD IC
  • 1x 470 Ohm resistor
  • 1x 1K Ohm resistor
  • Programmer for the GAL20V8. The popular TL866 works well

Software

  • The Arduino IDE
  • WinCupl (only if you want to change the GAL20V8 pin assignments)
  • MiniPro Programmer software (in the case of the TL866)

The circuit

The MPF-1 P1 expansion port pinouts

Programming the GAL20V8

  • I have included the .JED file that you can write to the PLD for use with the circuit above.
  • You can use WinCupl if you want to change the pinouts or use a different PLD.

Loading the Arduino sketch

  • I have included my Arduino directory, you will have to copy the contents to your own sketchbook directory.
  • I have also included a patched copy of the TVOut library with some added features (ie. cursor position, invert text, mirror text ...).

Usage

You have probably noticed that only 4 bits of the 8-bit bus on the Z80 are connected to the PLD and from the PLD to the Arduino.

This means we will have to send the byte nibble for nibble at a time.

We also need to warn the Arduino VDP that we are about to send a byte and we do this by first sending 0xF and only then sending the two nibbles (first high, then low).

The following subroutine will do the needed work to write a byte to the Arduino VDP ...

VDP     PUSH    BC              ; start of VDP subroutine ... copy BC to the stack for save keeping
        LD      (VDPB), A       ; store the byte to display in memory
        LD      C, 0xC0         ; C contains the IO address for the VDP
        LD      A, 0xFF         ; load 0xFF into the accumulator
        OUT     (C), A          ; Write the 0xFF to the VDP indicating that the high nibble of the byte will follow 
        LD      B, 50           ; load 50 into the B register for a delay loop
VDPD1   DJNZ    VDPD1           ; do the delay loop so that the VDP microcontroller has some time to do its thing
        LD      A, (VDPB)       ; load the byte we want to load back into the accumulator 
        RRA                     ; rotate right ... we want to get to the high nibble
        RRA                     ; rotate some more
        RRA                     ; and again
        RRA                     ; yay we made it
        OUT     (C), A          ; write the high nibble to the VDP
        LD      B, 50           ; again with the delay loop ... that microcontroller is running at 16mhz!
VDPD2   DJNZ    VDPD2           ; loopy loop
        LD      A, (VDPB)       ; load the original byte we want to display back into the A register
        LD      B, $0F          ; bitmask to get rid of the high nibble leaving only the lower
        AND     B               ; apply the bitmask to the accumulator
        OUT     (C), A          ; write the low nibble to the VDP
        LD      B, 200          ; final delay loop ... if we weren't so lazy we could have implementedd an ACK signal
VDPD3   DJNZ    VDPD3           ; last loopy loop
        POP     BC              ; recover BC
        RET                     ; return from subroutine

See the complete demo listing here ...

Escape Code Examples

Cursor Position

  • Move the cursor to row 5 column 10
0x1B, 0x1, 10, 5

Change the font

0x1B, 0x2, 0x1  ; font4x6
0x1B, 0x2, 0x2  ; font6x8
0x1B, 0x2, 0x3  ; font8x8
0x1B, 0x2, 0x4  ; font8x8ext
0x1B, 0x2, 0x5  ; game sprites https://github.com/damianvila/font-bescii

Draw a line

  • Draw a line from 5,5 (x,y) and ending at 20, 20
0x1B, 0x3, 5, 5, 20, 20

Draw a rectangle

  • Draw a rectangle at position 5,5 (x,y) with a width of 20 pixels and height of 10 pixels.
0x1B, 0x4, 5, 5, 20, 10

Draw a filled rectangle

  • Draw a filled rectangle at position 5,5 (x,y) with a width of 20 pixels and height of 10 pixels.
0x1B, 0x5, 5, 5, 20, 10

Draw a circle

  • Draw a circle at position 5,5 (x,y) with a radius of 10.
0x1B, 0x6, 5, 5, 10

Draw a filled circle

  • Draw a filled circle at position 5,5 (x,y) with a radius of 10.
0x1B, 0x7, 5, 5, 10

Invert the text colour (black on white)

0x1B, 0x8, 0x2

Invert the text colour (default)

0x1B, 0x8, 0x1

Enable the cursor

0x1B, 0x9, 0x2

Disable the cursor (default)

0x1B, 0x9, 0x1

Enable mirroring of text characters (handy for custom fonts containing sprites)

0x1B, 0x10, 0x2

Disable mirroring of text characters (default)

0x1B, 0x10, 0x1

Credits / Resources / Tools