Difference between revisions of "Analog Joysticks"

Revision as of 09:53, 4 April 2010

The CPC+ and GX4000 contain an analog joystick port (additionally to the normal Digital Joystick port).

Technical

The connector is similar as PC/Soundblaster joystick ports, for details on the pin-outs (and differences between CPC and PC joysticks), see:

• Analog Joystick Port Pin-Outs

The buttons are accessed same ways as for Digital Joysticks (ie. as part of the keyboard matrix).

The analog inputs are read from memory mapped ASIC registers:

 6808h  ADC0  Analogue Joystick 1, X-Axis (00h=Left=0 ohm, 3Fh=Right=138K ohm)
 6809h  ADC1  Analogue Joystick 1, Y-Axis (00h=Up  =0 ohm, 3Fh=Down =138K ohm)
 680Ah  ADC2  Analogue Joystick 2, X-Axis (00h=Left=0 ohm, 3Fh=Right=138K ohm)
 680Bh  ADC3  Analogue Joystick 2, Y-Axis (00h=Up  =0 ohm, 3Fh=Down =138K ohm)
 680Ch  ADC4  Unused, wired to +5V (twice the maximum of 2.5V)   (returns 3Fh)
 680Dh  ADC5  Unused, wired to GND (equivalent to 0 ohm)         (returns 00h)
 680Eh  ADC6  Unused, wired to +5V (twice the maximum of 2.5V)   (returns 3Fh)
 680Fh  ADC7  Unused, wired to GND (equivalent to 0 ohm)         (returns 00h)

To read that registers: Unlock the ASIC, and then map its register to memory at 4000h..7FFFh.

Values measured by Octoate:

 ADC input at 00h..01h  =  0 Ohm     (left/up)
 ADC input at 1Fh..20h  =  50 kOhm   (center) (that is, the center of ADC values, not of the resistor values)
 ADC input at 3Eh..3Fh  =  138 kOhm  (right/down)

Analog CPC+ Joysticks

Analogue joysticks confirmed to be compatible with the CPC+:

• Amstrad Analogue Joystick AJ-5
• Sinclair SPJ-1

Analog PC Joysticks

Analog PC Joysticks have a very similar 15pin connector. Theoretically, a few PC joysticks might be CPC+ compatible. However, in practice, this is rather unlikely - the potentiometers must have correct value, the button common pin must be correct, and the joystick may not contain additional electronics like auto-fire circuits, electrolyte capacitors, or digital-switch to analog-signal converters.

• Caution - compared with PCs, the CPC+/GX4000 do output reversed polarity on the joysticks power supply pins, so any special electronics in PC joysticks may get destroyed when connecting them to a CPC Plus/GX4000.

If you want to use a PC joystick: Remove any electronics other than the potentiometers and buttons. Ensure that buttons are wired to correct common pin.

Resistor Values - In PCs, the A/D conversion is done by software timings, meaning the PCs could handle joysticks with different potentiometer types (anything from 10 kiloohm to many megaohm should work). The CPC+ does the A/D conversion by hardware, this requires less CPU load, but it also means that it requires fixed potentiometer types (circe 138K ohm).
Analog PC joysticks are reportedly most commonly using 100K, 120K, or 150K ohm pots. These types should be more or less compatible with the CPC+ (100K/120K won't reach the ADC=3Fh max value, for example, resolution might be only ADC=00h..30h. 150K would be clipped to ADC=3Fh for anything above 138K ohm.
To handle those different pot types, CPC+ software should include calibration options. There are also some PC joysticks with incompatible pot types like 300K ohms - these won't work (since values above 138K will be clipped).

CPC+ Software with Analog Joystick Support

• Tennis Cup 2 ( Cartridge ) - allows to use Analog Joystick (as digital joystick replacement in two-player mode, selectable in options menu - the game merely emulates digital inputs, it doesn't actually support analog input)
• RP11 Arnold V diagnostic ROM cartridge - allows to display the eight ADC inputs in numeric form
• FutureOS - homebrew operating system (Plus version)