Topic       : The ATARI Compendium
Author      : Scott Sanders / JAY Software
Version     : 1.25 (20/6/2003)
Subject     : Documentation
Nodes       : 1117
Index Size  : 32614
HCP-Version : 6
Compiled on : Atari
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@lang       : en
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                            The MICROWIRE Interface


The STe and TT030 computers use the MICROWIRE interface to control
volume, mixing of the PSG and DMA output, and tone control. The original
ST is limited to amplitude control through the use of the appropriate PSG
register. The Falcon030 supports new XBIOS calls which allow volume and
mixing control.

The MICROWIRE interface is a write-only device accessed using two
hardware registers 0xFFFF8924 (mask) and 0xFFFF8922 (data). To write
a command to the MICROWIRE you must first place the value 0x07FF into the
mask register and then write the appropriate command to the data register.
The format for the data WORD is shown below:



Bits labeled 'x' will be ignored. Bits 9 and 10 should always be %10 to
correctly specify the device address which is a constant. Bits labeled 'c'
specify the command and bits labeled 'd' contain the appropriate data for
the command. The following table explains the valid MICROWIRE commands:

Command     'ccc'   'dddddd'

Set Master   011    Example Value   Result
Volume              %000000 -80dB   Attenuation
                    %010100 -40dB   Attenuation
                    %101000   0dB   Attenuation (Maximum)

Set Left     101    Example Value   Result
Channel             %000000 -40dB   Attenuation
Volume              %001010 -20dB   Attenuation
                    %010100   0dB   Attenuation (Maximum)

Set Right    100    Example Value   Result
Channel             %000000 -40dB   Attenuation
Volume              %001010 -20dB   Attenuation
                    %010100   0dB   Attenuation (Maximum)

Set Treble   010    Example Value   Result
                    %000000 -12dB   Attenuation
                    %000110   0dB   Attenuation
                    %001100 +12dB   Attenuation (Maximum)

Set Bass     001    Example Value   Result
                    %000000 -12dB   Attenuation
                    %000110   0dB   Attenuation
                    %001100 +12dB   Attenuation (Maximum)

Set PSG/DMA  000    Example Value   Result
Mix                 %000000 -12dB   Attenuation
                    %000001         Mix PSG sound output.
                    %000010         Don't Mix PSG sound output.

When configuring multiple settings at once, you should program a delay
between writes since the MICROWIRE takes at least 16sec to completely read
the data register. During a read the MICROWIRE rotates the mask register
one bit at a time. You will know a read operation has completed when the
mask register returns to 0x07FF. The following assembly segment
illustrates this by setting the left and right channel volumes to their
maximum values:

MWMASK          EQU     $FFFF8924
MWDATA          EQU     $FFFF8922

MASKVAL         EQU     $7FF
HIGHLVOL        EQU     $554
HIGHRVOL        EQU     $514

            .text

maxvol:
            move.w  MASKVAL,MWMASK      ; First write the mask and data values
            move.w  #HIGHLVOL,MWDATA
mwwrite:
            cmp.w   MASKVAL,MWMASK
            bne.s   mwwrite             ; loop until MWMASK reaches $7FF again
            move.w  #HIGHRVOL,MWDATA    ; ok, safe to write second value
            rts

            .end