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

/media/uploads/mjr/pinscape_no_background_small_L7Miwr6.jpg

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 vpforums.org. Also visit my Pinscape Resources page for more about this project and other virtual pinball projects I'm working on.

Downloads

  • 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.

Documentation

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 mouser.com 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 Aliexpress.com 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.

NewPwm/NewPwm.h

Committer:
mjr
Date:
2017-03-23
Revision:
79:682ae3171a08
Parent:
77:0b96f6867312
Child:
93:177832c29041

File content as of revision 79:682ae3171a08:

// New PWM 
//
// This is a replacement for the mbed PwmOut class.  It's both stripped
// down and beefed up.  It's stripped down to just the functionality we 
// need in the Pinscape code, and to a purely KL25Z implementation, which
// allows for a smaller memory footprint per instance.  It's beefed up to
// correct a number of problems in the mbed implementation.  
//
// Note that this class isn't quite API-compatible with the mbed version.
// We make the channel/TPM unit structure explicit, and we put the period()
// method (to change the PWM cycle time) on the unit object rather than the
// channel.  We do this to emphasize in the API that the period is a property
// of the unit (which contains multiple channels) rather than the channel.
// The mbed library is misleading when it pretends that the period is a
// property of the channel, since this confusingly suggests that a channel's
// period can be set independently.  It can't; the period can only be set for
// the whole group of channels controlled by a unit.
//
// Improvements over the mbed version:
//
// 1. We provide an alternative, non-glitching version of write().  The mbed
// version of write(), and our default version with the same name, causes a 
// glitch on every write by resetting the TPM counter, which cuts the cycle
// short and causes a momentary drop in brightness (from the short cycle) 
// that's visible if an LED is connected.  This is particularly noticeable 
// when doing a series of rapid writes, such as when fading a light on or off.
//
// We offer a version of write() that doesn't reset the counter, avoiding the 
// glitch.  This version skips the counter reset that the default version does.
//
// But this must be used with caution, because there's a whole separate
// problem if you don't reset the counter, which is why the mbed library
// does this by default.  The KL25Z hardware only allows the value register
// to be written once per PWM cycle; if it's written more than once, the
// second and subsequent writes are simply ignored, so those updates will
// be forever lost.  The counter reset, in addition to casuing the glitch,
// resets the cycle and thus avoids the one-write-per-cycle limitation.
// Callers using the non-glitchy version must take care to time writes so
// that there's only one per PWM period.  Or, alternatively, they can just
// be sure to repeat updates periodically to ensure that the last update is
// eventually applied.
//
// 2. We optimize the TPM clock pre-scaler to maximize the precision of the
// output period, to get as close as possible to the requested period.  The
// base mbed code uses a fixed pre-scaler setting with a fixed 750kHz update
// frequency, which means the period can be set in 1.333us increments.  The
// hardware is capable of increments as small as .02us.  The tradeoff is that
// the higher precision modes with smaller increments only allow for limited
// total period lengths, since the cycle counter is 16 bits: the maximum
// period at a given clock increment is 65535 times the increment.  So the
// mbed default of 1.333us increments allows for periods of up to 87ms with
// 1.333us precision, whereas the maximum precision of .02us increments only
// allows for a maximum period of 1.36ms.
//
// To deal with this tradeoff, we choose the scaling factor each time the
// period is changed, using the highest precision (smallest time increment,
// or lowest pre-scaling clock divider) available for the requested period.
// 
// Similar variable pre-scaling functionality is available with the FastPWM
// class.
//
// 3. We properly handle the shared clock in the TPM units.  The mbed library
// doesn't, nor does FastPWM.
//
// The period/frequency of a PWM channel on the KL25Z is a function of the
// TPM unit containing the channel, NOT of the channel itself.  A channel's
// frequency CANNOT be set independently; it can only set for the entire 
// group of channels controlled through the same TPM unit as the target
// channel.
//
// The mbed library and FastPWM library pretend that the period can be set
// per channel.  This is misleading and bug-prone, since an application that
// takes the API at its word and sets a channel's frequency on the fly won't
// necessarily realize that it just changed the frequency for all of the other
// channels on the same TPM.  What's more, the change in TPM period will
// effectively change the duty cycle for all channels attached to the PWM,
// since it'll update the counter modulus, so all channels on the same TPM
// have to have their duty cycles reset after any frequency change.
//
// This implementation changes the API design to better reflect reality.  We
// expose a separate object representing the TPM unit for a channel, and we
// put the period update function on the TPM unit object rather than on the
// channel.  We also automatically update the duty cycle variable for all
// channels on a TPM when updating the frequency, to maintain the original
// duty cycle (or as close as possible, after rounding error).
//
// Applications that need to control the duty cycle on more than one channel
// must take care to ensure that the separately controlled channels are on 
// separate TPM units.  The KL25Z offers three physical TPM units, so there
// can be up to three independently controlled periods.  The KL25Z has 10
// channels in total (6 on unit 0, 2 on unit 1, 2 on unit 2), so the remaining
// 7 channels have to share their periods with their TPM unit-mates.


#ifndef _NEWPWMOUT_H_
#define _NEWPWMOUT_H_

#include <mbed.h>
#include <pinmap.h>
#include <PeripheralPins.h>
#include <clk_freqs.h>

// TPM Unit.  This corresponds to one TPM unit in the hardware.  Each
// unit controls 6 channels; a channel corresponds to one output pin.
// A unit contains the clock input, pre-scaler, counter, and counter 
// modulus; these are shared among all 6 channels in the unit, and
// together determine the cycle time (period) of all channels in the
// unit.  The period of a single channel can't be set independently;
// a channel takes its period from its unit.
//
// Since the KL25Z hardware has a fixed set of 3 TPM units, we have
// a fixed array of 3 of these objects.
class NewPwmUnit
{
public:
    NewPwmUnit()
    {
        // figure our unit number from the singleton array position
        int tpm_n = this - unit;
        
        // start with all channels disabled
        activeChannels = 0;
        
        // get our TPM unit hardware register base
        tpm = (TPM_Type *)(TPM0_BASE + 0x1000*tpm_n);
        
        // Determine which clock input we're using.  Save the clock
        // frequency for later use when setting the PWM period, and 
        // set up the SIM control register for the appropriate clock
        // input.  This setting is global, so we really only need to
        // do it once for all three units, but it'll be the same every
        // time so it won't hurt (except for a little redundancy) to
        // do it again on each unit constructor.
        if (mcgpllfll_frequency()) {
            SIM->SOPT2 |= SIM_SOPT2_TPMSRC(1); // Clock source: MCGFLLCLK or MCGPLLCLK
            sysClock = mcgpllfll_frequency();
        } else {
            SIM->SOPT2 |= SIM_SOPT2_TPMSRC(2); // Clock source: ExtOsc
            sysClock = extosc_frequency();
        }
    }
    
    // enable a channel
    void enableChannel(int ch)
    {
        // if this is the first channel we're enabling, enable the
        // unit clock gate
        if (activeChannels == 0)
        {
            // enable the clock gate on the TPM unit
            int tpm_n = this - unit;
            SIM->SCGC6 |= 1 << (SIM_SCGC6_TPM0_SHIFT + tpm_n);
            
            // set a default period of 20us
            period(20.0e-3f);
        }
        
        // add the channel bit to our collection
        activeChannels |= (1 << ch);
    }
    
    // Set the period for the unit.  This updates all channels associated
    // with the unit so that their duty cycle is scaled properly to the
    // period counter.
    void period(float seconds)
    {        
        // First check to see if we actually need to change anything.  If
        // the requested period already matches the current period, there's
        // nothing to do.  This will avoid unnecessarily resetting any
        // running cycles, which could cause visible flicker.
        uint32_t freq = sysClock >> (tpm->SC & TPM_SC_PS_MASK);
        uint32_t oldMod = tpm->MOD;
        uint32_t newMod = uint32_t(seconds*freq) - 1;
        if (newMod == oldMod && (tpm->SC & TPM_SC_CMOD_MASK) == TPM_SC_CMOD(1))
            return;
    
        // Figure the minimum pre-scaler needed to allow this period.  The
        // unit counter is 16 bits, so the maximum cycle length is 65535
        // ticks.  One tick is the system clock tick time multiplied by
        // the pre-scaler.  The scaler comes in powers of two from 1 to 128.
        
        // start at scaler=0 -> divide by 1
        int ps = 0;
        freq = sysClock;
        
        // at this rate, the maximum period is 65535 ticks of the system clock
        float pmax = 65535.0f/sysClock;
        
        // Now figure how much we have to divide the system clock: each
        // scaler step divides by another factor of 2, which doubles the
        // maximum period.  Keep going while the maximum period is below
        // the desired period, but stop if we reach the maximum per-scale
        // value of divide-by-128.
        while (ps < 7 && pmax < seconds)
        {
            ++ps;
            pmax *= 2.0f;
            freq /= 2;
        }

        // Before writing the prescaler bits, we have to disable the
        // clock (CMOD) bits in the status & control register.  These
        // bits might take a while to update, so spin until they clear.
        while ((tpm->SC & 0x1F) != 0)
            tpm->SC &= ~0x1F;

        // Reset the CnV (trigger value) for all active channels to
        // maintain each channel's current duty cycle.
        for (int i = 0 ; i < 6 ; ++i)
        {
            // if this channel is active, reset it
            if ((activeChannels & (1 << i)) != 0)
            {
                // figure the old duty cycle, based on the current
                // channel value and the old modulus
                uint32_t oldCnV = tpm->CONTROLS[i].CnV;
                float dc = float(oldCnV)/float(oldMod + 1);
                if (dc > 1.0f) dc = 1.0f;
                
                // figure the new value that maintains the same duty
                // cycle with the new modulus
                uint32_t newCnV = uint32_t(dc*(newMod + 1));
                
                // if it changed, write the new value
                if (newCnV != oldCnV)
                    tpm->CONTROLS[i].CnV = newCnV;
            }
        }

        // reset the unit counter register
        tpm->CNT = 0;
        
        // set the new clock period
        tpm->MOD = newMod = uint32_t(seconds*freq) - 1;
        
        // set the new pre-scaler bits and set clock mode 01 (enabled, 
        // increments on every LPTPM clock)
        tpm->SC = TPM_SC_CMOD(1) | TPM_SC_PS(ps);
    }
    
    // wait for the end of the current cycle
    void waitEndCycle()
    {
        // clear the overflow flag
        tpm->SC |= TPM_SC_TOF_MASK;
        
        // The flag will be set at the next overflow
        while (!(tpm->SC & TPM_SC_TOF_MASK)) ;
    }
    
    // hardware register base
    TPM_Type *tpm;
    
    // Channels that are active in this unit, as a bit mask:
    // 1<<n is our channel n.
    uint8_t activeChannels;
    
    // fixed array of unit singletons
    static NewPwmUnit unit[3];
    
    // system clock frequency
    static uint32_t sysClock;
};


class NewPwmOut
{
public:
    NewPwmOut(PinName pin)
    {
        // determine the TPM unit number and channel
        PWMName pwm = (PWMName)pinmap_peripheral(pin, PinMap_PWM);
        MBED_ASSERT(pwm != (PWMName)NC);
        unsigned int port = (unsigned int)pin >> PORT_SHIFT;
        
        // decode the port ID into the TPM unit and channel number
        tpm_n = (pwm >> TPM_SHIFT);
        ch_n  = (pwm & 0xFF);
        
        // enable the clock gate on the port (PTx)
        SIM->SCGC5 |= 1 << (SIM_SCGC5_PORTA_SHIFT + port);
        
        // enable the channel on the TPM unit
        NewPwmUnit::unit[tpm_n].enableChannel(ch_n);

        // set the channel control register:
        //   CHIE                = 0    = interrupts disabled
        //   MSB:MBA:ELSB:ELSA   = 1010 = edge-aligned PWM
        //   DMA                 = 0    = DMA off
        TPM_Type *tpm = getUnit()->tpm;
        tpm->CONTROLS[ch_n].CnSC = (TPM_CnSC_MSB_MASK | TPM_CnSC_ELSB_MASK);
                
        // wire the pinout
        pinmap_pinout(pin, PinMap_PWM);
    }
    
    float read()
    {
        TPM_Type *tpm = getUnit()->tpm;
        float v = float(tpm->CONTROLS[ch_n].CnV)/float(tpm->MOD + 1);
        return v > 1.0f ? 1.0f : v;
    }
    
    void write(float val)
    {
        // do the glitch-free write
        glitchFreeWrite(val);
        
        // Reset the counter.  This is a workaround for a hardware problem
        // on the KL25Z, namely that the CnV register can only be written
        // once per PWM cycle.  Any subsequent attempt to write it in the
        // same cycle will be lost.  Resetting the counter forces the end
        // of the cycle and makes the register writable again.  This isn't
        // an ideal workaround because it causes visible brightness glitching
        // if the caller writes new values repeatedly, such as when fading
        // lights in or out.
        TPM_Type *tpm = getUnit()->tpm;
        tpm->CNT = 0;    
    }

    // Write a new value without forcing the current PWM cycle to end.
    // This results in glitch-free writing during fades or other series
    // of rapid writes, BUT with the giant caveat that the caller MUST NOT
    // write another value before the current PWM cycle ends.  Doing so
    // will cause the later write to be lost.  Callers using this must 
    // take care, using mechanisms of their own, to limit writes to once
    // per PWM cycle.
    void glitchFreeWrite(float val)
    {
        // limit to 0..1 range
        val = (val < 0.0f ? 0.0f : val > 1.0f ? 1.0f : val);
    
        // Write the duty cycle register.  The argument value is a duty
        // cycle on a normalized 0..1 scale; for the hardware, we need to
        // renormalize to the 0..MOD scale, where MOD is the cycle length 
        // in clock counts.  
        TPM_Type *tpm = getUnit()->tpm;
        tpm->CONTROLS[ch_n].CnV = (uint32_t)((float)(tpm->MOD + 1) * val);
    }
    
    // Wait for the end of a cycle
    void waitEndCycle() { getUnit()->waitEndCycle(); }
    
    // Get my TPM unit object.  This can be used to change the period.
    inline NewPwmUnit *getUnit() { return &NewPwmUnit::unit[tpm_n]; }
    
protected:
    // TPM unit number and channel number
    uint8_t tpm_n;
    uint8_t ch_n;
};

#endif