PlayCity

Introduction

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By choosing the PlayCity board, you have acquired a high-quality product which will set new standard features for Amstrad and Schneider CPC 464/664/6128 range of computers.

Its most important features are:

• Raster programmable interrupts with CURSOR and NMI support
• Six stereo audio channels with programmable frequency
• General purpose counters and timers

Hardware Installation

• Power OFF your Amstrad / Schneider CPC.
• Attach the PlayCity board to one free MotherX4 slog.
• Turn ON your computer, and enjoy! (if a grey screen appear, turn off your computer and check the connexions)

Peripheral Soft Reset

All expansion peripherals should be reset when an output is performed to I/O port $F8FF.

Used by the standard BIOS functions MC_BOOT_PROGRAM and MC_START_PROGRAM (vector BD13h and BD16h), in particular a peripheral that generates interrupts.

The PlayCity board use this feature to be sure that the CTC and YMZs circuits are properly reseted before using them.

Counter/Timer Circuit

Informations

The Z84C30 has four independently programmable counter/timer channels witch interface directly with the Z80 CPU. The first thing to consider is how those channels are linked to the CTC-AY board.

Channel 0 ($F880)

The channel 0 is exclusively used to generate the frequency of the two embedded soundchips (YMZ294). The input for the trigger (TRG0) is the 4 MHz system clock of the CPC. The output signal (ZC/TO0) is sent to the clock pins of the soundchips.

Channel 1 ($F881)

The channel 1 is typically used to generate raster interrupts. The input (TRG1) is linked to the CRTC CURSOR signal. The output (ZC/TO1) is connected to the NMI pin of the Z80.

Used as a counter, it's synchronized with the CRTC CURSOR signal, generating a smooth high priority rasters interrupt. Used as a timer (prescaler set to 256), it's 15.625 kHz signal is scanline-synchronized. That means the time constant is the number of scanlines to wait before to send an NMI.

Channel 2/3 ($F882/$F883)

The channels 2 and 3 are dedicated to general purpose usages. Yes, it's for you! The input for the trigger (TRG2) is the 4 MHz system clock. The output (ZC/TO0) is linked to the trigger for the channel 3 (TRG3). They can be used as 2x 8-bit or 1x 16-bit counter/timer. They generate normal interrupts and allow to use the Z80 vector interrupt (mode 2) too.

Coding examples

Each channel is programmed with two bytes; a third is necessary when interrupts are enabled. Once started, the CTC countdown automaticaly reloads its time constant and resume counting. Interrupt processing is simplified because only one vector need to be specified; the CTC internally generates an unique vector for each channel.

Before programming the CTC channels, you need to set the lower byte for the vectorized interrupts.

Vector interrupts

; Set Vector lower byte to 0
LD BC,$F880
OUT (C),0

Channel 0 example

; Set both YMZ294 clocks to sound like the CPC AY-3-8912
LD BC,$F880
LD HL,$7F01
OUT (C),H ; $7F = Clock generator
OUT (C),L  ; $01 = CPC AY

Channel 2 example

; Start CTC channel 2 in timer mode (prescalar 256 and set new time constant)
LD H,%10110111 ; Timer mode and preescalar 256
LD L,1 ; Time constant
LD BC,$F882
OUT (C),H  ; Enable Timer
OUT (C),L ; Set new time constant

; Stop CTC channel 2
LD BC,$F882
LD A,%00000011
OUT (C),A ; Disable Timer 2

Channel 2/3 example

; Start CTC channel 2 and 3 in 16-bit counter mode
LD BC,$F882
LD HL,32768 ; Timer constant
LD A,%11110111 ; Counter mode
OUT (C),A ; Enable Timer 2
OUT (C),L ; Set new time constant (lower byte)
INC BC
OUT (C),A ; Enable Timer 3
OUT (C),H ; Set new time constant (high byte)

; Stop CTC channel 2 and 3 (16 bit mode)
LD BC,$F882
LD A,%00000011
OUT (C),A ; Disable Timer 2
INC BC
OUT (C),A ; Disable Timer 3

Don't forget to use RETN for NMI handlers as well as RETI for normal interrupt handlers. In other case, the Z80 CPU will not aknowledge the next interrupt properly.

Audio Channels

Informations

The CTC-AY board is populated with two AY compatible soundchips, adding 6 stereo channels. The YMZ294 eliminate the I/O port and improve the CPU interface through /CS, /WR control signals and a 8-bit data bus. Each sound chip can be directly programmed using two dedicated I/O ports for registers and data. No initialization code is required.

By default, the YMZs are clocked at 4 MHz but run internaly at 2 MHz. So, they will sound like an Atari ST, while you don't change the frequency by reprograming the CTC Channel 0.