Introduction
The PlayCity is a CPC expansion released on April 2014 with these features:
- 6 audio channels sound with programmable frequency (stereo line out and speaker mono mix in).
- 4 counter/timer channels for programmable interrupt (including NMI).
- IM2 vectorized interrupt support.
- Raster lines interrupt support.
- CRTC hardware CURSOR interrupt support.
- The LIGHT PEN/GUN connector.
Hardware Installation
- Power OFF your Amstrad / Schneider CPC.
- Attach the PlayCity board to one free MotherX4 slot.
- Turn ON your computer, and enjoy!
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 (vectors $BD13 and $BD16), in particular a peripheral that generates interrupts. Also used by FutureOS.
The PlayCity board use this feature to be sure that the CTC and YMZs circuits are properly reset before using them.
Light Pen/Gun connector
For making it easier to adapt Light Pens/Guns from other systems or new ones, we added a 4-pin connector to the board. The connections are labeled in the board as VCC (+5V), TR (Trigger), LS (Light Sensor) and GND.
Trigger (TR) = Request via output to port &FBFE. Light Sensor (LS) = CRTC Light Pen input.
Making the system compatible with Amstrad Magnum Phaser games, the most accuracy light gun system for CPC.
Counter/Timer Circuit
Information
The Z84C30 has four independently programmable counter/timer channels interfaced directly with the Z80 CPU. You can get full information in the CTC datasheet (File:Z80ctc.pdf).
The first thing to consider is how those channels are linked to the PlayCity board.
Channel 0 ($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 ($F881)
The channel 1 is typically used to generate raster interrupts. The input (TRG1) is linked to the CRTC CURSOR signal. The output (ZC/TO1) is connected to the NMI pin of the Z80.
Used as a counter, it's synchronized with the CRTC CURSOR signal, generating a smooth high priority rasters interrupt. Used as a timer (prescaler set to 256), its 15.625 kHz signal is scanline-synchronized. That means the time constant is the number of scanlines to wait before to send an NMI.
Channel 2/3 ($F882/$F883)
The channels 2 and 3 are dedicated to general purpose usages. Yes, it's for you! The input for the trigger (TRG2) is the 4 MHz system clock. The output (ZC/TO2) is linked to the trigger for the channel 3 (TRG3). They can be used as 2x 8-bit or 1x 16-bit counter/timer. They generate normal interrupts and allow using the Z80 vector interrupt (mode 2) too.
Coding examples
Each channel is programmed with two bytes; a third is necessary when interrupts are enabled. Once started, the CTC countdown automaticaly reloads its time constant and resume counting. Interrupt processing is simplified because only one vector need to be specified; the CTC internally generates an unique vector for each channel.
Before programming the CTC channels, you need to set the lower byte for the vectorized interrupts.
Vector interrupts
; Set Vector lower byte to 0 LD BC,$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 PlayCity 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.
YMZ294 Registers
They are exactly the same than the PSG, only remember there is not PSG I/O registers ($0E-$0F) in the YMZ. The registers are write only.
Coding Examples
YMZ Initialization
YMZ_SELECT, select a YMZ register. Use the port $F984 for the right channels and port $F988 for the left channels.
YMZ_WRITE, write a byte in the selected register. Use the port $F884 for the right channels and port $F888 for the left channels.
; Initialization of the YMZ registers LD A,$D .loop_init_ymz LD BC,YMZ_SELECT OUT (C),A ; Register LD BC,YMZ_WRITE CP 7 JR NZ,.send_zero LD A,$3F ; Noise and Tone disabled OUT (C),A ; Write in YMZ R7 LD A,6 JR .loop_init_ymz .send_zero OUT (C),0 ; Write 0 in the selected YMZ register DEC A JP P,.loop_init_ymz
CTC/YMZ Useful Values
| CTC | CTC Out (MHz) | YMZ (MHz) | Computer |
|---|---|---|---|
| $1 | 2,00 | 1,00 | = CPC |
| $2 | 3,00 | 1,50 | |
| $3 | 3,33 | 1,67 | |
| $4 | 3,50 | 1,75 | = ZX |
| $5 | 3,60 | 1,80 | ~ MSX |
| $6 | 3,67 | 1,83 | |
| $7 | 3,71 | 1,86 | |
| $8 | 3,75 | 1,88 | |
| $9 | 3,78 | 1,89 | |
| $A | 3,80 | 1,90 | |
| $B | 3,82 | 1,91 | |
| $C | 3,83 | 1,92 | |
| $D | 3,85 | 1,92 | |
| $E | 3,86 | 1,93 | |
| $F | 3,87 | 1,93 | |
| $0 | 3,98 | 1,99 | ~ ST |
| UNSET | 4,00 | 2,00 | = ST |
PlayCity coding tips
Interesting tips or "magic tricks" using the board should be documented here. If the code is long, you must put in other wiki page and link it here.
PlayCity detection
We are going to use the NMI interrupt generator to check if our program is running in a CPC with a PlayCity board.
; ---------------------------------------------------------------------------
; PlayCity check
; (c) 2013 SyX
; ---------------------------------------------------------------------------
; Constantes
CTC_TIM1 EQU $F881 ; Channel 1 (I: Cursor CRTC | O: NMI)
CTC_START_TIMER256 EQU %00110111
CTC_STOP_CHANNEL EQU %00000011
; ---------------------------------------------------------------------------
; NOTE: The lower ROM must be disabled before to run this code.
check_playcity
; Disable interrupts
DI
; Install NMI handler
LD A,$C3 ; JP $xxxx
LD HL,nmi_interrupt
LD ($0066),A
LD ($0067),HL
; Initialize playcity variable to 0
XOR A
LD (playcity),A
; Wait VBlank
LD B,$F5
.wait_vbl
IN A,(C)
RRA
JR NC,.wait_vbl
; Initialize CTC timer 1 (NMI generator)
LD HL,32 ; 32 scanlines
LD BC,CTC_TIM1
LD A,CTC_START_TIMER256
OUT (C),A ; Enable Timer
OUT (C),L ; Set new time constant
; Extra delay
LD IX,33 * 4 - 1 ; Wait 33 scanlines
CALL wait_scanlines_ix
LD A,(playcity)
OR A
JR NZ,.playcity_detected
; No PlayCity detected
.
.
.
.playcity_detected
.
.
.
; ---------------------------------------------------------------------------
nmi_interrupt
PUSH BC
PUSH AF
; Change playcity variable
LD A,$FF
LD (playcity),A
; Disable CTC timer 1 (NMI generator)
LD BC,CTC_TIM1
LD A,CTC_STOP_CHANNEL
OUT (C),A ; Disable Timer
POP AF
POP BC
EI
RETN
; ---------------------------------------------------------------------------
; Wait scanlines
; INPUT:
; IX: Scanlines to wait * 4 - 1
; ---------------------------------------------------------------------------
wait_scanlines_ix
DEFS 5,0 ; (5)
.loop_wait_scanlines_ix
DEFS 6 ; (6)
DEC IX ; (3)
LD A,IXH ; (2)
OR IXL ; (2)
JR NZ,.loop_wait_scanlines_ix ; (2/3)
; Total loop --> 16 * (IX - 1) + 15
RET ; (3)
; Total Routine --> 64 * SCANLINES
; ---------------------------------------------------------------------------
playcity
DEFS 1
PlayCity detection (alternative)
Faster and shorter, but untested or not working on all CPC.
;=========================
; PlayCity check
; If it's present, A=1
; Otherwise, A=0
;-------------------------
;
;=========================
macro PlaycityDetection
; Code for NMI management
ld a,#3c ; inc a
ld (#66),a
ld hl,#45ed ; (inverted) opcodes for retn
ld (#67),hl
; If a Playcity is present, the code generates a NMI then stops the counter.
ld bc,#F881 ; 3 NOPs CTC channel 1
ld hl,%00010111*256 + 2 ; 3 NOPs A NMI every 8 NOPs
out (c),h ; 4 NOPs
out (c),l ; 4 NOPs
ld a,0 ; 2 NOPs A is set to 0. If a CTC is present, it will be INCed by the INTerruption code
inc hl ; 2 NOPs L=3 => Stop CTC channel
nop ; 1 NOPs
out (c),l ; 4 NOPs <- If a Playcity is plugged, a NMI should be raised during this opcode
mend
Downloads
In File:Playcity examples.zip, you will find more examples with full sources of using the CTC, a customized arkos player that let you play songs using an external YMZ and the ReSeT party demo disk that includes a CPC version of the PT3 Turbo Sound player (6 channels song format).
Another example, in File:Test sfx.zip you will find a 3 channels SFX player, you can choose the sound chip to be used by the player.