Reading a Sega 6-button pad

Part 2: some code

In my earlier post I mentioned that I was going to handle reading the Sega 6-button joypad in software, using an interrupt routine, and use programmable logic to handle masquerading as a CD32 pad.

The six inputs from the Sega pad (up, down, left, right, button1, button2) are mapped to PSOC pins P2.0 to P2.5.  Since these are adjacent, we can read and write to them easily as a single entity.  We create a “pins” component with six inputs, named Sega_Inputs, and PSOC creator automatically generate headers and stubs so that we can read their status in main.c simply by calling Sega_Inputs_Read().  Similarly, we create an output pin, Sega_Select on pin P2.6, which we can write in the code using Sega_Select_Write().

The code currently looks like this:

#include <project.h>
#include <stdio.h>

static int Sega_SixButton;
static int Sega_DirBC;
static int Sega_AStart;
static int Sega_XYZMode;
static int CD32_Buttons;
static int CD32_Directions;

char buf[64];

CY_ISR(timerISR)
{
    static int framectr=255;
    static int sixbutton;
    FrameTimer_ReadStatusRegister();

    switch(framectr)
    {
        case 8:
            // Read regular button statuses here, immediately prior to first low pulse
            Sega_DirBC=Sega_Inputs_Read();
            sixbutton=1; // Assume we're dealing with a six-button controller unless proved otherwise
            break;
        case 7:
            // Read regular button statuses here, on trailing edge of first low pulse
            Sega_AStart=Sega_Inputs_Read();
            break;

            // Nothing exciting happens during the second low pulse
            // (fall through)
        case 6:
        case 5:
            // Third low pulse
        case 4:
            break;

        case 3:
            // If we have a six-button pad, then the direction lines will be low here
            if(Sega_Inputs_Read() & 0xf)
                sixbutton=0;    // Not a six-button pad
            break;
        case 2:
            // After the trailing edge of the third low pulse, XYZ and mode are readable.
            Sega_XYZMode=Sega_Inputs_Read();
            break;

        case 1:
            // Finally, after the last low pulse, the four direction lines should all read high.
            if((~Sega_Inputs_Read()) & 0xf)
                sixbutton=0;    // Not a six-button pad            
            Sega_SixButton=sixbutton;

            // Now send the button status to the CD32 Shifter.
            CD32_Directions=Sega_DirBC&0xf;
            CD32_Buttons=(Sega_DirBC<<2)&0xc0;
            CD32_Buttons|=Sega_AStart&0x30;
            if(sixbutton)
                CD32_Buttons|=Sega_XYZMode&0xf;
            else
                CD32_Buttons|=0xf;
            CD32Shifter_1_Data_Reg=CD32_Buttons;
            CD32Shifter_1_Fire_Reg=(Sega_DirBC&0x10) ? 0x00 : 0xff;
            break;

        default:
            break;
    }

    // Send four low pulses
    if(framectr<9)
        Sega_Select_Write(framectr&1);

    --framectr;
    if(framectr==0)
        framectr=63;
}

int main()
{
    Isr_Timer_StartEx(timerISR);
    FrameTimer_Start();
    CyGlobalIntEnable;
    CySysPmSleep();
    return(0);
}

/* [] END OF FILE */

I will probably want to remap the buttons somewhat once I have the CD32Shifter side of the project working, but reading the Sega pad seems to be working well.

I’ve started a GitHub repository to hold the project files, which can be found here.

 

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