An I/O controller for virtual pinball machines: accelerometer nudge sensing, analog plunger input, button input encoding, LedWiz compatible output controls, and more.

Dependencies:   mbed FastIO FastPWM USBDevice

Fork of Pinscape_Controller by Mike R


This is Version 2 of the Pinscape Controller, an I/O controller for virtual pinball machines. (You can find the old version 1 software here.) Pinscape is software for the KL25Z that turns the board into a full-featured I/O controller for virtual pinball, with support for accelerometer-based nudging, a real plunger, button inputs, and feedback device control.

In case you haven't heard of the concept before, a "virtual pinball machine" is basically a video pinball simulator that's built into a real pinball machine body. A TV monitor goes in place of the pinball playfield, and a second TV goes in the backbox to serve as the "backglass" display. A third smaller monitor can serve as the "DMD" (the Dot Matrix Display used for scoring on newer machines), or you can even install a real pinball plasma DMD. A computer is hidden inside the cabinet, running pinball emulation software that displays a life-sized playfield on the main TV. The cabinet has all of the usual buttons, too, so it not only looks like the real thing, but plays like it too. That's a picture of my own machine to the right. On the outside, it's built exactly like a real arcade pinball machine, with the same overall dimensions and all of the standard pinball cabinet hardware.

A few small companies build and sell complete, finished virtual pinball machines, but I think it's more fun as a DIY project. If you have some basic wood-working skills and know your way around PCs, you can build one from scratch. The computer part is just an ordinary Windows PC, and all of the pinball emulation can be built out of free, open-source software. In that spirit, the Pinscape Controller is an open-source software/hardware project that offers a no-compromises, all-in-one control center for all of the unique input/output needs of a virtual pinball cabinet. If you've been thinking about building one of these, but you're not sure how to connect a plunger, flipper buttons, lights, nudge sensor, and whatever else you can think of, this project might be just what you're looking for.

You can find much more information about DIY Pin Cab building in general in the Virtual Cabinet Forum on Also visit my Pinscape Resources page for more about this project and other virtual pinball projects I'm working on.


  • Pinscape Release Builds: This page has download links for all of the Pinscape software. To get started, install and run the Pinscape Config Tool on your Windows computer. It will lead you through the steps for installing the Pinscape firmware on the KL25Z.
  • Config Tool Source Code. The complete C# source code for the config tool. You don't need this to run the tool, but it's available if you want to customize anything or see how it works inside.


The new Version 2 Build Guide is now complete! This new version aims to be a complete guide to building a virtual pinball machine, including not only the Pinscape elements but all of the basics, from sourcing parts to building all of the hardware.

You can also refer to the original Hardware Build Guide (PDF), but that's out of date now, since it refers to the old version 1 software, which was rather different (especially when it comes to configuration).

System Requirements

The new config tool requires a fairly up-to-date Microsoft .NET installation. If you use Windows Update to keep your system current, you should be fine. A modern version of Internet Explorer (IE) is required, even if you don't use it as your main browser, because the config tool uses some system components that Microsoft packages into the IE install set. I test with IE11, so that's known to work. IE8 doesn't work. IE9 and 10 are unknown at this point.

The Windows requirements are only for the config tool. The firmware doesn't care about anything on the Windows side, so if you can make do without the config tool, you can use almost any Windows setup.

Main Features

Plunger: The Pinscape Controller started out as a "mechanical plunger" controller: a device for attaching a real pinball plunger to the video game software so that you could launch the ball the natural way. This is still, of course, a central feature of the project. The software supports several types of sensors: a high-resolution optical sensor (which works by essentially taking pictures of the plunger as it moves); a slide potentionmeter (which determines the position via the changing electrical resistance in the pot); a quadrature sensor (which counts bars printed on a special guide rail that it moves along); and an IR distance sensor (which determines the position by sending pulses of light at the plunger and measuring the round-trip travel time). The Build Guide explains how to set up each type of sensor.

Nudging: The KL25Z (the little microcontroller that the software runs on) has a built-in accelerometer. The Pinscape software uses it to sense when you nudge the cabinet, and feeds the acceleration data to the pinball software on the PC. This turns physical nudges into virtual English on the ball. The accelerometer is quite sensitive and accurate, so we can measure the difference between little bumps and hard shoves, and everything in between. The result is natural and immersive.

Buttons: You can wire real pinball buttons to the KL25Z, and the software will translate the buttons into PC input. You have the option to map each button to a keyboard key or joystick button. You can wire up your flipper buttons, Magna Save buttons, Start button, coin slots, operator buttons, and whatever else you need.

Feedback devices: You can also attach "feedback devices" to the KL25Z. Feedback devices are things that create tactile, sound, and lighting effects in sync with the game action. The most popular PC pinball emulators know how to address a wide variety of these devices, and know how to match them to on-screen action in each virtual table. You just need an I/O controller that translates commands from the PC into electrical signals that turn the devices on and off. The Pinscape Controller can do that for you.

Expansion Boards

There are two main ways to run the Pinscape Controller: standalone, or using the "expansion boards".

In the basic standalone setup, you just need the KL25Z, plus whatever buttons, sensors, and feedback devices you want to attach to it. This mode lets you take advantage of everything the software can do, but for some features, you'll have to build some ad hoc external circuitry to interface external devices with the KL25Z. The Build Guide has detailed plans for exactly what you need to build.

The other option is the Pinscape Expansion Boards. The expansion boards are a companion project, which is also totally free and open-source, that provides Printed Circuit Board (PCB) layouts that are designed specifically to work with the Pinscape software. The PCB designs are in the widely used EAGLE format, which many PCB manufacturers can turn directly into physical boards for you. The expansion boards organize all of the external connections more neatly than on the standalone KL25Z, and they add all of the interface circuitry needed for all of the advanced software functions. The big thing they bring to the table is lots of high-power outputs. The boards provide a modular system that lets you add boards to add more outputs. If you opt for the basic core setup, you'll have enough outputs for all of the toys in a really well-equipped cabinet. If your ambitions go beyond merely well-equipped and run to the ridiculously extravagant, just add an extra board or two. The modular design also means that you can add to the system over time.

Expansion Board project page

Update notes

If you have a Pinscape V1 setup already installed, you should be able to switch to the new version pretty seamlessly. There are just a couple of things to be aware of.

First, the "configuration" procedure is completely different in the new version. Way better and way easier, but it's not what you're used to from V1. In V1, you had to edit the project source code and compile your own custom version of the program. No more! With V2, you simply install the standard, pre-compiled .bin file, and select options using the Pinscape Config Tool on Windows.

Second, if you're using the TSL1410R optical sensor for your plunger, there's a chance you'll need to boost your light source's brightness a little bit. The "shutter speed" is faster in this version, which means that it doesn't spend as much time collecting light per frame as before. The software actually does "auto exposure" adaptation on every frame, so the increased shutter speed really shouldn't bother it, but it does require a certain minimum level of contrast, which requires a certain minimal level of lighting. Check the plunger viewer in the setup tool if you have any problems; if the image looks totally dark, try increasing the light level to see if that helps.

New Features

V2 has numerous new features. Here are some of the highlights...

Dynamic configuration: as explained above, configuration is now handled through the Config Tool on Windows. It's no longer necessary to edit the source code or compile your own modified binary.

Improved plunger sensing: the software now reads the TSL1410R optical sensor about 15x faster than it did before. This allows reading the sensor at full resolution (400dpi), about 400 times per second. The faster frame rate makes a big difference in how accurately we can read the plunger position during the fast motion of a release, which allows for more precise position sensing and faster response. The differences aren't dramatic, since the sensing was already pretty good even with the slower V1 scan rate, but you might notice a little better precision in tricky skill shots.

Keyboard keys: button inputs can now be mapped to keyboard keys. The joystick button option is still available as well, of course. Keyboard keys have the advantage of being closer to universal for PC pinball software: some pinball software can be set up to take joystick input, but nearly all PC pinball emulators can take keyboard input, and nearly all of them use the same key mappings.

Local shift button: one physical button can be designed as the local shift button. This works like a Shift button on a keyboard, but with cabinet buttons. It allows each physical button on the cabinet to have two PC keys assigned, one normal and one shifted. Hold down the local shift button, then press another key, and the other key's shifted key mapping is sent to the PC. The shift button can have a regular key mapping of its own as well, so it can do double duty. The shift feature lets you access more functions without cluttering your cabinet with extra buttons. It's especially nice for less frequently used functions like adjusting the volume or activating night mode.

Night mode: the output controller has a new "night mode" option, which lets you turn off all of your noisy devices with a single button, switch, or PC command. You can designate individual ports as noisy or not. Night mode only disables the noisemakers, so you still get the benefit of your flashers, button lights, and other quiet devices. This lets you play late into the night without disturbing your housemates or neighbors.

Gamma correction: you can designate individual output ports for gamma correction. This adjusts the intensity level of an output to make it match the way the human eye perceives brightness, so that fades and color mixes look more natural in lighting devices. You can apply this to individual ports, so that it only affects ports that actually have lights of some kind attached.

IR Remote Control: the controller software can transmit and/or receive IR remote control commands if you attach appropriate parts (an IR LED to send, an IR sensor chip to receive). This can be used to turn on your TV(s) when the system powers on, if they don't turn on automatically, and for any other functions you can think of requiring IR send/receive capabilities. You can assign IR commands to cabinet buttons, so that pressing a button on your cabinet sends a remote control command from the attached IR LED, and you can have the controller generate virtual key presses on your PC in response to received IR commands. If you have the IR sensor attached, the system can use it to learn commands from your existing remotes.

Yet more USB fixes: I've been gradually finding and fixing USB bugs in the mbed library for months now. This version has all of the fixes of the last couple of releases, of course, plus some new ones. It also has a new "last resort" feature, since there always seems to be "just one more" USB bug. The last resort is that you can tell the device to automatically reboot itself if it loses the USB connection and can't restore it within a given time limit.

More Downloads

  • Custom VP builds: I created modified versions of Visual Pinball 9.9 and Physmod5 that you might want to use in combination with this controller. The modified versions have special handling for plunger calibration specific to the Pinscape Controller, as well as some enhancements to the nudge physics. If you're not using the plunger, you might still want it for the nudge improvements. The modified version also works with any other input controller, so you can get the enhanced nudging effects even if you're using a different plunger/nudge kit. The big change in the modified versions is a "filter" for accelerometer input that's designed to make the response to cabinet nudges more realistic. It also makes the response more subdued than in the standard VP, so it's not to everyone's taste. The downloads include both the updated executables and the source code changes, in case you want to merge the changes into your own custom version(s).

    Note! These features are now standard in the official VP releases, so you don't need my custom builds if you're using 9.9.1 or later and/or VP 10. I don't think there's any reason to use my versions instead of the latest official ones, and in fact I'd encourage you to use the official releases since they're more up to date, but I'm leaving my builds available just in case. In the official versions, look for the checkbox "Enable Nudge Filter" in the Keys preferences dialog. My custom versions don't include that checkbox; they just enable the filter unconditionally.
  • Output circuit shopping list: This is a saved shopping cart at with the parts needed to build one copy of the high-power output circuit for the LedWiz emulator feature, for use with the standalone KL25Z (that is, without the expansion boards). The quantities in the cart are for one output channel, so if you want N outputs, simply multiply the quantities by the N, with one exception: you only need one ULN2803 transistor array chip for each eight output circuits. If you're using the expansion boards, you won't need any of this, since the boards provide their own high-power outputs.
  • Cary Owens' optical sensor housing: A 3D-printable design for a housing/mounting bracket for the optical plunger sensor, designed by Cary Owens. This makes it easy to mount the sensor.
  • Lemming77's potentiometer mounting bracket and shooter rod connecter: Sketchup designs for 3D-printable parts for mounting a slide potentiometer as the plunger sensor. These were designed for a particular slide potentiometer that used to be available from an seller but is no longer listed. You can probably use this design as a starting point for other similar devices; just check the dimensions before committing the design to plastic.

Copyright and License

The Pinscape firmware is copyright 2014, 2021 by Michael J Roberts. It's released under an MIT open-source license. See License.

Warning to VirtuaPin Kit Owners

This software isn't designed as a replacement for the VirtuaPin plunger kit's firmware. If you bought the VirtuaPin kit, I recommend that you don't install this software. The VirtuaPin kit uses the same KL25Z microcontroller that Pinscape uses, but the rest of its hardware is different and incompatible. In particular, the Pinscape firmware doesn't include support for the IR proximity sensor used in the VirtuaPin plunger kit, so you won't be able to use your plunger device with the Pinscape firmware. In addition, the VirtuaPin setup uses a different set of GPIO pins for the button inputs from the Pinscape defaults, so if you do install the Pinscape firmware, you'll have to go into the Config Tool and reassign all of the buttons to match the VirtuaPin wiring.



File content as of revision 82:4f6209cb5c33:

// Fast Interrupt In for KL25Z
// This is a replacement for the mbed library InterruptIn class, which
// sets up GPIO ports for edge-sensitive interrupt handling.  This class
// provides the same API but has a shorter code path for responding to
// each interrupt.  In my tests, the mbed InterruptIn class has a maximum
// interrupt rate of about 112kHz; this class can increase that to about
// 181kHz.
// If speed is critical (and it is, because why else would you be using 
// this class?), you should elevate the GPIO interrupt priority in the
// hardware interrupt controller so that GPIO pin signals can preempt other
// interrupt handlers.  The mbed USB and timer handlers in particular spend
// relative long periods in interrupt context, so if these are at the same
// or higher priority than the GPIO interrupts, they'll become the limiting
// factor.  The mbed library leaves all interrupts set to maximum priority
// by default, so to elevate the GPIO interrupt priority, you have to lower 
// the priority of everything else.  Call FastInterruptIn::elevatePriority()
// to do this.
// Performance measurements:  I set up a test program using one KL25Z to
// send 50% duty cycle square wave signals to a second KL25Z (using a PWM
// output on the sender), and measured the maximum interrupt frequency
// where the receiver could correctly count every edge, repeating the test
// with FastInterruptIn and the mbed InterruptIn.  I tested with handlers
// for both edges and handlers for single edges (just rise() or just fall()).
// The Hz rates reflect the maximum *interrupt* frequency, which is twice
// the PWM frequency when testing with handlers for both rise + fall in
// effect.  In all cases, the user callbacks were minimal code paths that
// just incremented counters, and all tests ran with PTA/PTD at elevated
// IRQ priority.  The time per interrupt values shown are the inverse of
// the maximum frequency; these reflect the time between interrupts at
// the corresponding frequency.  Since each frequency is the maximum at
// which that class can handle every interrupt without losing any, the
// time between interrupts tells us how long the CPU takes to fully process
// one interrupt and return to the base state where it's able to handle the
// next one.  This time is the sum of the initial CPU interrupt latency
// (the time it takes from an edge signal occuring on a pin to the CPU
// executing the first instruction of the IRQ vector), the time spent in
// the InterruptIn or FastInterruptIn code, the time spent in the user
// callback, and the time for the CPU to return from the interrupt to
// normal context.  For the test program, the user callback is about 4
// instructions, so perhaps 6 clocks or 360ns.  Other people have measured
// the M0+ initial interrupt latency at about 450ns, and the return time
// is probably similar.  So we have about 1.2us in fixed overhead and user
// callback time, hence the rest is the time spent in the library code.
//   mbed InterruptIn:
//     max rate 112kHz
//     -> 8.9us per interrupt 
//        less 1.2us fixed overhead = 7.7us in library code
//   FastInterruptIn:
//     max rate 181kHz
//     -> 5.5us per interrupt
//        less 1.2us fixed overhead = 3.3us in library code
// Limitations:
// 1. KL25Z ONLY.  This is a bare-metal KL25Z class.
// 2. Globally incompatible with InterruptIn.  Both classes take over the
// IRQ vectors for the GPIO interrupts globally, so they can't be mixed
// in the same system.  If you use this class anywhere in a program, it
// has to be used exclusively throughout the whole program - don't use
// the mbed InterruptIn anywhere in a program that uses this class.
// 3. API differences.  The API is very similar to InterruptIn's API,
// but we don't support the method-based rise/fall callback attachers.  We
// instead use static function pointers (void functions with 'void *'
// context arguments).  It's easy to write static methods for these that 
// dispatch to regular member functions, so the functionality is the same; 
// it's just a little different syntax.  The simpler (in the sense of
// more primitive) callback interface saves a little memory and is
// slightly faster than the method attachers, since it doesn't require
// any variation checks at interrupt time.
// Theory of operation
// How the mbed code works
// On every interrupt event, the mbed library's GPIO interrupt handler
// searches for a port with an active interrupt.  Each PORTx_IRQn vector
// handles 32 ports, so each handler has to search this space of 32 ports
// for an active interrupt.  The mbed code approaches this problem by
// searching for a '1' bit in the ISFR (interrupt status flags register),
// which is effectively a 32-bit vector of bits indicating which ports have
// active interrupts.  This search could be done quickly if the hardware
// had a "count leading zeroes" instruction, which actually does exist in
// the ARM instruction set, but alas not in the M0+ subset.  So the mbed
// code has to search for the bit by other means.  It accomplishes this by
// way of a binary search.  By my estimate, this takes about 110 clocks or
// 7us.  The routine has some other slight overhead dispatching to the
// user callback once one is selected via the bit search, but the bulk of
// the time is spent in the bit search.  The mbed code could be made more
// efficient by using a better 'count leading zeroes' algorithm; there are 
// readily available implementations that run in about 15 clocks on M0+.
// How this code works
// FastInterruptIn takes a different approach that bypasses the bit vector
// search.  We instead search the installed handlers.  We work on the 
// assumption that the total number of interrupt handlers in the system is 
// small compared with the number of ports.  So instead of searching the 
// entire ISFR bit vector, we only check the ports with installed handlers.
// The mbed code takes essentially constant time to run.  It doesn't have
// any dependencies (that I can see) on the number of active InterruptIn
// pins.  In contrast, FastInterruptIn's run time is linear in the number
// of active pins: adding more pins will increase the run time.  This is
// a tradeoff, obviously.  It's very much the right tradeoff for the Pinscape 
// system, because we have very few interrupt pins overall.  I suspect it's
// the right tradeoff for most systems, too, since most embedded systems
// have a small fixed set of peripherals they're talking to.
// We have a few other small optimizations to maximize our sustainable
// interrupt frequency.  The most important is probably that we read the
// port pin state immediately on entry to the IRQ vector handler.  Since
// we get the same interrupt on a rising or falling edge, we have to read
// the pin state to determine which type of transition triggered the
// interrupt.  This is inherently problematic because the pin state could 
// have changed between the time the interrupt occurred and the time we 
// got around to reading the state - the likelihood of this increases as
// the interrupt source frequency increases.  The soonest we can possibly
// read the state is at entry to the IRQ vector handler, so we do that.
// Even that isn't perfectly instantaneous, due to the unavoidable 450ns
// or so latency in the hardware before the vector code starts executing;
// it would be better if the hardware read the state at the moment the
// interrupt was triggered, but there's nothing we can do about that.
// In contrast, the mbed code waits until after deciding which interrupt
// is active to read the port, so its reading is about 7us delayed vs our
// 500ns delay.  That further reduces the mbed code's ability to keep up
// with fast interrupt sources when both rise and fall handlers are needed.


#include "mbed.h"
#include "gpio_api.h"

struct fiiCallback
    fiiCallback() { func = 0; }
    void (*func)(void *);
    void *context;
    inline void call() { func(context); }

class FastInterruptIn
    // Globally elevate the PTA and PTD interrupt priorities.  Since the
    // mbed default is to start with all IRQs at maximum priority, we
    // LOWER the priority of all IRQs to the minimum, then raise the PTA
    // and PTD interrupts to maximum priority.  
    // The reason we set all priorities to minimum (except for PTA and PTD) 
    // rather than some medium priority is that this is the most flexible
    // default.  It really should have been the mbed default, in my opinion,
    // since (1) it doesn't matter what the setting is if they're all the
    // same, so an mbed default of 3 would have been equivalent to an mbed
    // default of 0 (the current one) for all programs that don't make any
    // changes anyway, and (2) the most likely use case for programs that
    // do need to differentiate IRQ priorities is that they need one or two
    // items to respond MORE quickly.  It seems extremely unlikely that
    // anyone would need only one or two to be especially slow, which is
    // effectively the case the mbed default is optimized for.
    // This should be called (if desired at all) once at startup.  The 
    // effect is global and permanent (unless later changes are made by
    // someone else), so there's no need to call this again when setting
    // up new handlers or changing existing handlers.  Callers are free to 
    // further adjust priorities as needed (e.g., elevate the priority of
    // some other IRQ), but that should be done after calling this, since we
    // change ALL IRQ priorities with prejudice.
    static void elevatePriority()
        // Set all IRQ priorities to minimum.  M0+ has priority levels
        // 0 (highest) to 3 (lowest).  (Note that the hardware uses the
        // high-order two bits of the low byte, so the hardware priority
        // levels are 0x00 [highest], 0x40, 0x80, 0xC0 [lowest]).  The
        // mbed NVIC macros, in contrast, abstract this to use the LOW
        // two bits, for levels 0, 1, 2, 3.)
        for (int irq = 0 ; irq < 32 ; ++irq)
            NVIC_SetPriority(IRQn(irq), 0x3);
        // set the PTA and PTD IRQs to highest priority
        NVIC_SetPriority(PORTA_IRQn, 0x00);
        NVIC_SetPriority(PORTD_IRQn, 0x00);
    // set up a FastInterruptIn handler on a given pin
    FastInterruptIn(PinName pin)
        // start with the null callback
        callcb = &FastInterruptIn::callNone;
        // initialize the pin as a GPIO Digital In port
        gpio_t gpio;
        gpio_init_in(&gpio, pin);

        // get the port registers
        PDIR = gpio.reg_in;
        pinMask = gpio.mask;
        portno = uint8_t(pin >> PORT_SHIFT);
        pinno = uint8_t((pin & 0x7F) >> 2);
        // set up for the selected port
        IRQn_Type irqn;
        void (*vector)();
        switch (portno)
        case PortA:
            irqn = PORTA_IRQn;
            vector = &PortA_ISR;
            PDIR = &FPTA->PDIR;
        case PortD:
            irqn = PORTD_IRQn;
            vector = &PortD_ISR;
            PDIR = &FPTD->PDIR;
            error("FastInterruptIn: invalid pin specified; "
                "only PTAxx and PTDxx pins are interrupt-capable");
        // set the vector
        NVIC_SetVector(irqn, uint32_t(vector));
    // read the current pin status - returns 1 or 0
    int read() const { return (fastread() >> pinno) & 0x01; }
    // Fast read - returns the pin's port bit, which is '0' or '1' shifted
    // left by the port number (e.g., PTA7 or PTD7 return (1<<7) or (0<<7)).
    // This is slightly faster than read() because it doesn't normalize the
    // result to a literal '0' or '1' value.  When the value is only needed
    // for an 'if' test or the like, zero/nonzero is generally good enough,
    // so you can save a tiny bit of time by skiping the shift.
    uint32_t fastread() const { return *PDIR & pinMask; }
    // set a rising edge handler
    void rise(void (*func)(void *), void *context = 0)
        setHandler(&cbRise, PCR_IRQC_RISING, func, context);
    // set a falling edge handler
    void fall(void (*func)(void *), void *context = 0)
        setHandler(&cbFall, PCR_IRQC_FALLING, func, context);
    // Set the pull mode.  Note that the KL25Z only supports PullUp
    // and PullNone modes.  We'll ignore other modes.
    void mode(PinMode pull)
        volatile uint32_t *PCR = &(portno == PortA ? PORTA : PORTD)->PCR[pinno];
        switch (pull)
        case PullNone:
            *PCR &= ~PORT_PCR_PE_MASK;
        case PullUp:
            *PCR |= PORT_PCR_PE_MASK;

    // set a handler - the mode is PCR_IRQC_RISING or PCR_IRQC_FALLING
    void setHandler(
        fiiCallback *cb, uint32_t mode, void (*func)(void *), void *context)
        // get the PCR (port control register) for the pin
        volatile uint32_t *PCR = &(portno == PortA ? PORTA : PORTD)->PCR[pinno];

        // disable interrupts while messing with shared statics

        // set the callback
        cb->func = func;
        cb->context = context;
        // enable or disable the mode in the PCR
        if (func != 0)
            // Handler function is non-null, so we're setting a handler.
            // Enable the mode in the PCR.  Note that we merely need to
            // OR the new mode bits into the existing mode bits, since
            // disabled is 0 and BOTH is equal to RISING|FALLING.
            *PCR |= mode;
            // if we're not already in the active list, add us
            // Handler function is null, so we're clearing the handler.
            // Disable the mode bits in the PCR.  If the old mode was
            // the same as the mode we're disabling, switch to NONE.
            // If the old mode was BOTH, switch to the mode we're NOT
            // disabling.  Otherwise make no change.
            int cur = *PCR & PORT_PCR_IRQC_MASK;
            if (cur == PCR_IRQC_BOTH)
                *PCR &= ~PORT_PCR_IRQC_MASK;
                *PCR |= (mode == PCR_IRQC_FALLING ? PCR_IRQC_RISING : PCR_IRQC_FALLING);
            else if (cur == mode)
                *PCR &= ~PORT_PCR_IRQC_MASK;
            // if we're disabled, remove us from the list
        // set the appropriate callback mode
        if (cbRise.func != 0 && cbFall.func != 0)
            // They want to be called on both Rise and Fall events. 
            // The hardware triggers the same interrupt on both, so we
            // need to distinguish which is which by checking the current
            // pin status when the interrupt occurs.
            callcb = &FastInterruptIn::callBoth;
        else if (cbRise.func != 0)
            // they only want Rise events
            callcb = &FastInterruptIn::callRise;
        else if (cbFall.func != 0)
            // they only want Fall events
            callcb = &FastInterruptIn::callFall;
            // no events are registered
            callcb = &FastInterruptIn::callNone;
        // done messing with statics
    // add me to the active list for my port
    void listAdd()
        // figure the list head
        FastInterruptIn **headp = (portno == PortA) ? &headPortA : &headPortD;
        // search the list to see if I'm already there
        FastInterruptIn **nxtp = headp;
        for ( ; *nxtp != 0 && *nxtp != this ; nxtp = &(*nxtp)->nxt) ;
        // if we reached the last entry without finding me, add me
        if (*nxtp == 0)
            *nxtp = this;
            this->nxt = 0;
    // remove me from the active list for my port
    void listRemove()
        // figure the list head
        FastInterruptIn **headp = (portno == PortA) ? &headPortA : &headPortD;
        // find me in the list
        FastInterruptIn **nxtp = headp;
        for ( ; *nxtp != 0 && *nxtp != this ; nxtp = &(*nxtp)->nxt) ;
        // if we found me, unlink me
        if (*nxtp == this)
            *nxtp = this->nxt;
            this->nxt = 0;
    // next link in active list for our port
    FastInterruptIn *nxt;
    // pin mask - this is 1<<pinno, used for selecting or setting the port's
    // bit in the port-wide bit vector registers (IFSR, PDIR, etc)
    uint32_t pinMask;
    // Internal interrupt dispatcher.  This is set to one of 
    // &callNone, &callRise, &callFall, or &callBoth, according 
    // to which type of handler(s) we have registered.
    void (*callcb)(FastInterruptIn *, uint32_t pinstate);
    // PDIR (data read) register
    volatile uint32_t *PDIR;
    // port and pin number
    uint8_t portno;
    uint8_t pinno;

    // user interrupt handler callbacks
    fiiCallback cbRise;
    fiiCallback cbFall;
    static void callNone(FastInterruptIn *f, uint32_t pinstate) { }
    static void callRise(FastInterruptIn *f, uint32_t pinstate) { f->; }
    static void callFall(FastInterruptIn *f, uint32_t pinstate) { f->; }
    static void callBoth(FastInterruptIn *f, uint32_t pinstate)
        if (pinstate)
    // Head of active interrupt handler lists.  When a handler is
    // active, we link it into this static list.  At interrupt time,
    // we search the list for an active interrupt.
    static FastInterruptIn *headPortA;
    static FastInterruptIn *headPortD;

    // PCR_IRQC modes
    static const uint32_t PCR_IRQC_DISABLED = PORT_PCR_IRQC(0);
    static const uint32_t PCR_IRQC_RISING = PORT_PCR_IRQC(9);
    static const uint32_t PCR_IRQC_FALLING = PORT_PCR_IRQC(10);
    static const uint32_t PCR_IRQC_BOTH = PORT_PCR_IRQC(11);
    // IRQ handlers.  We set up a separate handler for each port to call
    // the common handler with the port-specific parameters.  
    // We read the current pin input status immediately on entering the 
    // handler, so that we have the pin reading as soon as possible after 
    // the interrupt.  In cases where we're handling both rising and falling
    // edges, the only way to tell which type of edge triggered the interrupt
    // is to look at the pin status, since the same interrupt is generated
    // in either case.  For a high-frequency signal source, the pin state
    // might change again very soon after the edge that triggered the
    // interrupt, so we can get the wrong state if we wait too long to read
    // the pin.  The soonest we can read the pin is at entry to our handler,
    // which isn't even perfectly instantaneous, since the hardware has some
    // latency (reportedly about 400ns) responding to an interrupt.  
    static void PortA_ISR() { ISR(&PORTA->ISFR, headPortA, FPTA->PDIR); }
    static void PortD_ISR() { ISR(&PORTD->ISFR, headPortD, FPTD->PDIR); }
    inline static void ISR(volatile uint32_t *pifsr, FastInterruptIn *f, uint32_t pdir)
        // search the list for an active entry
        uint32_t ifsr = *pifsr;
        for ( ; f != 0 ; f = f->nxt)
            // check if this entry's pin is in interrupt state
            if ((ifsr & f->pinMask) != 0)
                // clear the interrupt flag by writing '1' to the bit
                *pifsr = f->pinMask;
                // call the appropriate user callback
                f->callcb(f, pdir & f->pinMask);
                // Stop searching.  If another pin has an active interrupt,
                // or this pin already has another pending interrupt, the
                // hardware will immediately call us again as soon as we
                // return, and we'll find the new interrupt on that new call.
                // This should be more efficient on average than checking all 
                // pins even after finding an active one, since in most cases 
                // there will only be one interrupt to handle at a time.