From: ali@wgd562.uk.sb.com (Alastair Booker)


The AY-3-8910/8912 sound synthesizer IC.
========================================

Overview
--------

The AY-3-8910/8912 is a sound generator IC containing three tone generators
and a white noise generator. In addition to its sound producing capabilities,
the AY-3-8910 has two 8-bit I/O ports and the AY-3-8912 has one.

Pinouts (for the AY-3-8912 only)
--------------------------------
             __  __
SOUND C   1 |  \/  | 28  D0       Vcc is +5V.
PORT      2          27  D1       SOUND A, B and C can be tied together.
Vcc       3          26  D2       CLOCK can be some MHz.
SOUND B   4          25  D3
SOUND A   5          24  D4
GND       6          23  D5
PORT      7          22  D6
PORT      8          21  D7
PORT      9          20  BC1
PORT     10          19  BC2
PORT     11          18  BDIR
PORT     12          17  A8
PORT     13          16  /RESET
CLOCK    14 |______| 15  CLOCK



DA0-DA7 are multiplexed data and address lines, as used in the range of GI
processors that this IC was originally part of. The BC1 and BDIR pins are
used to control the IC as follows:

Function       BDIR   BC1
	
Inactive         0     0
Read             0     1
Write            1     0
Latch address    1     1

In addition to using BC1 and BDIR, A8 should be taken high to enable the
chip. IOA0-IOA7 are the input/output pins (the AY-3-8910 has an additional
set marked IOB0-IOB7). The IC does not contain an internal oscillator - the
clock input to the CLK pin should fall between 1MHz-2MHz, so a typical Z80
system's clock would need to be divided.

Registers
---------

The AY-3-8910/8912 contains 16 internal registers as follows:

Register        Function                        Range

 0              Channel A fine pitch            8-bit (0-255)
 1              Channel A course pitch          4-bit (0-15)
 2              Channel B fine pitch            8-bit (0-255)
 3              Channel B course pitch          4-bit (0-15)
 4              Channel C fine pitch            8-bit (0-255)
 5              Channel C course pitch          4-bit (0-15)
 6              Noise pitch                     5-bit (0-31)
 7              Mixer                           8-bit (see below)
 8              Channel A volume                4-bit (0-15, see below)
 9              Channel B volume                4-bit (0-15, see below)
10              Channel C volume                4-bit (0-15, see below)
11              Envelope fine duration          8-bit (0-255)
12              Envelope course duration        8-bit (0-255)
13              Envelope shape                  4-bit (0-15)
14              I/O port A                      8-bit (0-255)
15              I/O port B                      8-bit (0-255)

Notes:

- The AY-3-8912 does not contain register 15.
- The volume registers (8, 9 and 10) contain a 4-bit setting but if bit
  5 is set then that channel uses the envelope defined by register 13 and
  ignores its volume setting.
- The mixer (register 7) is made up of the following bits (low=enabled):

Bit: 7        6        5        4        3        2        1        0
   _         _
   I/O       I/O   Noise    Noise    Noise     Tone     Tone     Tone
     B        A        C        B        A        C        B        A

   The AY-3-8912 ignores bit 7 of this register.

Envelopes
---------

The AY-3-8910/8912 contains the following preset envelopes or waveforms (set
using control register 13). Note that these affect volume only and not the
pitch:

 0      \__________     single decay then off

 4      /|_________     single attack then off

 8      \|\|\|\|\|\     repeated decay

 9      \__________     single decay then off

10      \/\/\/\/\/\     repeated decay-attack
	      _________
11      \|              single decay then hold

12      /|/|/|/|/|/     repeated attack
	     __________
13      /               single attack then hold

14      /\/\/\/\/\/     repeated attack-decay

15      /|_________     single attack then off

Pitch values
------------

The course and fine pitch registers for each channel are used in the
following fashion (assuming channel A):

Registers 0 and 1 operate together to form channel A's final pitch. The eight
least significant bits are sent to register 0 and the four most significant
bits are sent to register 1. The output frequency is equal to the IC's
incoming clock frequency divided by 16 and then further divided by the number
written to the course and fine pitch registers, so the higher the number
written to these, the lower the pitch. For example, if a frequency of 1KHz
was required and the IC's clock frequency was 1MHz, a total division rate
of 1000 would be needed. The sound generator itself divides by 16 so the
course and fine pitch registers must provide a further division by 62.5 (due
to the fact that 1000/16 is 62.5). A division rate of 62 or 63 will be
accurate enough, since the registers can only store whole numbers. Therefore,
62 or 63 would be written to register 0 and 0 would be written to register
1.

The following frequency table may be useful in musical applications:

Note            Frequency (Hz)          Note            Frequency (Hz)

A               220                     D#              311.1
A#              233.3                   E               329.63
B               246.94                  F               349.23
middle C        261.63                  F#              370
C#              277.2                   G               392
D               293.66                  G#              415.3

Applications
------------

The AY-3-8910/8912 (and derivatives) has found its way into a variety of
home computers and games consoles including the following:
	. Sinclair ZX Spectrum 128/+2/+3
	. Amstrad CPC 464/664/6128
	. Mattel Intellivision
	. BBC Micro
	. Atari ST
	. Sega Master System

There are also a couple of emulators available at the time of writing:
	. "Z80" - the Spectrum emulator by Gerton Lunter for PCs and
	  compatibles which utilises the Adlib sound generator present
	  on most Soundblaster-compatible cards. This package fully emulates
	  the Spectrum 48/128 and is well worth looking at.
	. "DeliAY" - by Patrik Rak for the Commodore Amiga. This comes as
	  an add-on for "DeliTracker" (a music player) and emulates a Z80
	  and AY-3-8912 (in a Spectrum-like fashion) giving very accurate
	  results. A growing number of Spectrum tunes are being gathered
	  together to run under this player.

ZX Spectrum 128 specifics
-------------------------

The Spectrum's "control" and "data" output ports are as follows:
	- control = FFFD
	- data    = BFFD

So, to produce a simple sound from BASIC ...

	10 LET ayctrl = 65533
	20 LET aydata = 49149
	30 OUT ayctrl,7  : REM select the mixer register
	40 OUT aydata,62 : REM enable channel A only
	50 OUT ayctrl,1  : REM channel A course pitch
	60 OUT aydata,50 : REM set it
	70 OUT ayctrl,8  : REM channel A volume
	80 OUT aydata,15 : REM set it to maximum

... or from assembler ...

	ayctrl  EQU     $FFFD
	aydata  EQU     $BFFD

	start   ld      d,7             ; select the mixer register
		    ld      e,62            ; enable channel A only
			call    outer           ; send it to PSG
			ld      d,1             ; channel A course pitch
			ld      e,50            ; pitch value
			call    outer           ; send it to PSG
			ld      d,8             ; channel A volume
			ld      e,15            ; maximum
			call    outer           ; send it to PSG
			ret

	outer   ld      bc,ayctrl       ; select control port
			out     (c),d           ; send specified value
			ld      bc,aydata       ; select data port
			out     (c),e           ; send specified value
			ret

For an easy way to generate the required course and fine pitch values, try
the following program:

	10 INPUT "Note value ",n
	20 INPUT "Octave ",o
	30 LET f=INT(n/2^o)
	40 LET c=INT(f/256)
	50 PRINT "Fine = ";f;"  Course = ";c

The pitch values required by this program are as follows:

Note            Value                   Note            Value

C               3421                    F#              2419
C#              3228                    G               2283
D               3047                    G#              2155
D#              2876                    A               2034
E               2715                    A#              1920
F               2562                    B               1892

References
----------

Micro Interfacing Circuits - Book 2, by R. A. Penfold, ISBN 0-85934-106-2,
Spectrum 128 manual, (c) 1986 Sinclair Research Ltd,
Sinclair User magazine, 1986 (issue unknown).

This text was entered by Alastair Booker on 4th April 1995. Please feel free
to contact me regarding anything contained in this document. I have produced
a generic AY-3-8912 interface circuit for Z80-based systems and will make
this available to anyone who requires it.

email: booker_a%prodev.dnet@sb.com
or   : ali@tanagra.demon.co.uk

[ Alastair Booker             | Amiga 1200, Psion 3a, PC 486/50, ZX80  ]
[ ali@tanagra.demon.co.uk     | ZX81, Spectrum 48, Spectrum 128, QL    ]
[ booker_a%prodev.dnet@sb.com | Sinclair TV, Sinclair Multimeter       ]
[ em542@cleveland.freenet.edu | Sinclair Calculator... and other junk! ]