Update AY8913 peripheral 20 docs

Author: rejunity

docs/user_peripherals/20_AY8913.md

@@ -19,13 +19,27 @@ Peripheral index: 20
 
 ## What it does
 
-This is a replica of 8-bit classic **[AY-3-8913](https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910)** 3-voice programmable sound generator (PSG) chip from General Instruments. The AY-3-8913 is a smaller variant of AY-3-8910 or its analog YM2149.
+This is a replica of 8-bit classic **[AY-3-8910/AY-3-8912/AY-3-8913](https://en.wikipedia.org/wiki/General_Instrument_AY-3-8910)** 3-voice programmable sound generator (PSG) integrated into a modern RISC-V as a peripheral.
 
+The AY-3-891x family of programmable sound generators was introduced by General Instrument in 1978. Variants of the AY-3-891x were broadly used in:
+
+- home computers: Amstrad CPC, Atari ST, Oric-1, Sharp X1, MSX, ZX Spectrum 128/+2/+3
+- game consoles: Intellivision, Vectrex
+- and many arcade machines
+
+### Technical capabilities
+
+- **3 square wave** tone generators
+- A single **white noise** generator
+- A single **envelope** generator able to produce 10 different shapes
+- Chip is capable to produce a range of waves from a **30 Hz** to **125 kHz**, defined by **12-bit** registers.
+- **16** different volume levels on a logarithmic scale
 
 ## Register map
 
-The behavior of the AY-3-891x is defined by 14 registers. All registers are READ ONLY!
+The behavior of the AY-3-891x is defined by 14 registers. The register map matches the original AY-3-891x and should be able to play the original tunes without modifications.
 
+All registers are **read only** in this peripheral and start in **unknown** uninitialised state upon reset!
 
 | Address | Access | Bits used      | Function         | Description            |
 |---------|--------|----------------|------------------|------------------------|
@@ -44,10 +58,45 @@ The behavior of the AY-3-891x is defined by 14 registers. All registers are READ
 | 12      | R      | ```xxxxxxxx``` | ---//---         | 8-bit coarse frequency |   
 | 13      | R      | ```....xxxx``` | Envelope Shape   | 4-bit shape control |
 
+### Square wave tone generators
+
+Square waves are produced by counting down the 12-bit counters. Counter counts up from 0. Once the corresponsding register value is reached, counter is reset and
+the output bit of the channel is flipped producing square waves.
+
+### Noise generator
+
+Noise is produced with 17-bit [Linear-feedback Shift Register (LFSR)](https://en.wikipedia.org/wiki/Linear-feedback_shift_register) that flips the output bit pseudo randomly.
+The shift rate of the LFSR register is controller by the 5-bit counter.
+
+### Envelope
+
+The envelope shape is controlled with 4-bit register, but can take only 10 distinct patterns. The speed of the envelope is controlled with 16-bit counter. Only a single envelope is produced that can be shared by any combination of the channels.
+
+### Volume
+Each of the three AY-3-891x channels have dedicated DAC that converts 16 levels of volume to analog output. Volume levels are 3 dB apart in AY-3-891x.
+
+## IO pins
+
+There is a single PWM output signal that is connected to all the output pins uo_out[7:0].
+The input pins are not used.
+
 ## How to test
 
-TODO: Explain how to use your project
+All register state are in **unknown** uninitialised state upon reset. The very first thing you should do is to set them to zero!
+
+In order to play a standard pitch A440 note you have to enable Channel A tone in the mixer:
+
+	register[7] = 0b00_001_000
+
+Set volume loud:
+
+	register[8] = 0b0000_1111
+
+Set A440 tone using the formulas:
+
+	register[0] = (2_000_000 // (16 * 440)) & 0xFF
+	register[1] = (2_000_000 // (16 * 440)) >> 8
 
 ## External hardware
 
-Tiny Tapeout Audio PMOD
+(Tiny Tapeout Audio PMOD)[https://store.tinytapeout.com/products/Audio-Pmod-p716541601]