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First version; what I currently believe about the Mockingboard based on a schematic.

Thomas Harte 2018-06-15 15:45:34 -04:00
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The-Mockingboard.md Normal file

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The Mockingboards are a series of audio expansions for the Apple II, the most common of which provides stereo sound from a combination of two 6522s, connected to two AY-3-8910s.
== AY-3-8910s ==
Each AY is connected to its 6522:
* the 6522 port A connected to the AY's data pins;
* bit 0 of the 6522's port B connected to the AY's BC1;
* bit 1 of the 6522's port B connected to the AY's BCDIR;
* BC2 connected to +5v; and
* bit 2 of the 6522's port B connected to the AY's reset.
The three channels from each AY are mixed together with equal volume.
== 6522s ==
The 6522s are exposed to the processor's bus:
* the four register select lines are connected to address lines 0 to 3; and
* the reset and data lines connected as expected.
The 6522 for the left channel:
* is selected when address line 7 is low; and
* its interrupt output is connected to the processor's IRQ input.
The 6522 for the right channel:
* is selected when address line 7 is high; and
* its interrupt output is connected to the processor's NMI input.
Both also require the board's IOSEL to be active — they appear only in the ordinary IO area for their board.
== Clock Rate ==
Both the 6522s and the AYs are clocked according to the Apple card's phase 1 clock input. So they are clocked at the same rate as the CPU.