Mike R / Mbed 2 deprecated Pinscape_Controller_V2

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 mechanical plunger, button inputs, and feedback device control.

In case you haven't heard of the idea 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 show the backglass artwork. Some cabs also include a third monitor to simulate the DMD (Dot Matrix Display) used for scoring on 1990s machines, or even an original plasma DMD. A computer (usually a Windows PC) 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 trim hardware.

It's possible to buy a pre-built virtual pinball machine, but it also makes a great 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 potentiometer (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 KL25Z can only run one firmware program at a time, so if you install the Pinscape firmware on your KL25Z, it will replace and erase your existing VirtuaPin proprietary firmware. If you do this, the only way to restore your VirtuaPin firmware is to physically ship the KL25Z back to VirtuaPin and ask them to re-flash it. They don't allow you to do this at home, and they don't even allow you to back up your firmware, since they want to protect their proprietary software from copying. For all of these reasons, if you want to run the Pinscape software, I strongly recommend that you buy a "blank" retail KL25Z to use with Pinscape. They only cost about $15 and are available at several online retailers, including Amazon, Mouser, and eBay. The blank retail boards don't come with any proprietary firmware pre-installed, so installing Pinscape won't delete anything that you paid extra for.

With those warnings in mind, if you're absolutely sure that you don't mind permanently erasing your VirtuaPin firmware, it is at least possible to use Pinscape as a replacement for the VirtuaPin firmware. Pinscape uses the same button wiring conventions as the VirtuaPin setup, so you can keep your buttons (although you'll have to update the GPIO pin mappings in the Config Tool to match your physical wiring). As of the June, 2021 firmware, the Vishay VCNL4010 plunger sensor that comes with the VirtuaPin v3 plunger kit is supported, so you can also keep your plunger, if you have that chip. (You should check to be sure that's the sensor chip you have before committing to this route, if keeping the plunger sensor is important to you. The older VirtuaPin plunger kits came with different IR sensors that the Pinscape software doesn't handle.)

--- a/TCD1103/TCD1103.h	Tue Dec 03 19:10:52 2019 +0000
+++ b/TCD1103/TCD1103.h	Fri Dec 27 20:14:23 2019 +0000
@@ -1,4 +1,4 @@
-// Toshiba TCD1103 linear CCD image sensor, 1x1500 pixels
+// Toshiba TCD1103 linear CCD image sensor, 1x1500 pixels.
 //
 // This sensor is conceptually similar to the TAOS TSL1410R (the original 
 // Pinscape sensor!).  Like the TSL1410R, it has a linear array of optical
@@ -7,32 +7,25 @@
 // electronic shutter, and a serial interface that clocks the pixels out
 // to the host in analog voltage level format.
 //
-// Mechanically, this sensor has an entirely different size scale vs the
-// TSL1410R.  The 1410R's sensor window is about the same size as a standard 
-// plunger's travel range (about 80mm), so the mechanical setup we use with
-// sensor is to situate the sensor adjacent to the plunger, with the pixel
-// window aligned with the plunger's axis of motion, so that the plunger
-// casts a shadow on the sensor at 1:1 scale.  The TCD1103, in contrast, is
-// a tiny little thing, with about an 8mm window.  That means that we have
-// to reduce the plunger shadow image by about 10X to fit the sensor, so an
-// optical lens is required.  This makes it more complicated to set up, but
-// it also adds the advantage of allowing us to focus the image, for a more
-// precise reading.  The shadow in the lens-less 1410R setup is usually about
-// four of five pixels wide, so we lose a lot of the sensor's native
-// precision to the poor optics - we only get about 1/50" resolution as a
-// result.  With a focusing lens, we could potentially get single-pixel
-// resolution, which would be about 1/500" resolution.  The reality will
-// be somewhat lower, depending on how hard we want to work at the optics,
-// but it should be possible to do much better than the unfocused 1410R.
+// The big physical difference between this sensor and the old TAOS sensors
+// is the size.  The TAOS sensors were (by some miracle) approximately the
+// same size as the plunger travel range, so we were able to take "contact"
+// images without any optics, by placing the plunger close to the sensor,
+// back-lighting it, and essentially taking a picture of its shadow.  The
+// Toshiba sensor, in contrast, has a pixel window that's only 8mm long, so
+// the contact image approach won't work.  Instead, we have to use a lens
+// to focus a reduced image (about 1:10 scale) on the sensor.  That makes
+// the physical setup more complex, but it has the great advantage that we
+// get a focused image.  The shadow was always fuzzy in  the old contact 
+// image approach, which reduced the effective resolution when determining 
+// the plunger position.  With a focused image, we can get single-pixel 
+// resolution.  With this Toshiba sensor's 1500 pixels, that's about 500 
+// dpi, which beats every other sensor we've come up with.
 //
-// The electronic interface to this sensor has some fairly tight timing
-// requirements, per the data sheet.  The sensor requires the host to 
-// provide a master clock that runs at 0.4 MHz to 4 MHz.  The data sheet's
-// timing diagrams imply that the master clock runs continuously, although
-// it's probably like the 1410R, where the clock is only needed when you 
-// want to run the shift register and can be stopped at other times.
+// The electronic interface to this sensor is similar to the TAOS, but it
+// has enough differences that we can't share the same code base.
 //
-// As with the 1410R, we'll have to use DMA for the ADC transfers in order
+// As with the 1410R, we have to use DMA for the ADC transfers in order
 // to keep up with the high data rate without overloading the KL25Z CPU.
 // With the 1410R, we're able to use the ADC itself as the clock source,
 // by running the ADC in continous mode and using its "sample ready" signal
@@ -42,14 +35,15 @@
 // sample completed.  This strategy won't work with the Toshiba sensor,
 // though, because the Toshiba sensor's timing sequence requires *two* clock
 // pulses per pixel.  I can't come up with a way to accomplish that with the
-// linked-DMA approach.  (I've tried!)
+// linked-DMA approach.  Instead, we'll have to generate a true clock signal
+// for the sensor, and drive the DMA conversions off of that clock.
 //
-// So instead, we'll have to generate a true clock signal for the sensor. 
-// The obvious way to do this (and the only way, as far as I can come up with)
-// is to use a TPM channel - that is, a PWM output.  TPM channels are designed
+// The obvious (and, as far as I can tell, only) way to generate the clock
+// signal with the KL25Z at the high frequency required is to use a TPM -
+// the KL25Z module that drives PWM outputs.  TPM channels are designed
 // precisely for this kind of work, so this is the right approach in terms of
 // suitability, but it has the downside that TPM units are an extremely scarce
-// resource on the KL25Z.  We only have three of them to work with.  Luckily
+// resource on the KL25Z.  We only have three of them to work with.  Luckily,
 // the rest of the Pinscape software only requires two of them: one for the
 // IR transmitter (which uses a TPM channel to generate the 41-48 kHz carrier
 // wave used by nearly all consumer IR remotes), and one for the TLC5940
@@ -65,7 +59,7 @@
 // generate a second clock signal at half the frequency of the master clock, 
 // and use that as the ADC trigger.  But as we just said, we only have three 
 // TPM units in the whole system, and two of them are already claimed for 
-// other uses, so we only have one unit to use here.  
+// other uses, so we only have one unit available for our use here.
 //
 // Fortunately, we can make do with one TPM unit, by taking advantage of a 
 // feature/quirk of the KL25Z ADC.  The quirk lets us take ADC samples at
@@ -83,6 +77,53 @@
 // ADC will always be ready again on the second tick.  So we'll get one ADC
 // sample for every two master clock ticks, exactly as we need.
 //
+// This is all possible because the ADC timing is deterministic, and runs on
+// the same clock as the TPM.  The KL25Z Subfamily Reference Manual explains
+// how to calculate the ADC conversion time for a given combination of mode
+// bits.  So we just have to pick an ADC mode, calculate its conversion time,
+// and then select a TPM period that's slightly more than 1/2 of the ADC
+// conversion time.
+//
+//
+// Pixel output signal
+//
+// The pixel output signal from this sensor is an analog voltage level.  It's
+// inverted from the brightness: higher brightness is represented by lower
+// voltage.  The dynamic range is only about 1V - dark pixels read at about 
+// 2V, and saturated pixels read at about 1V.  
+//
+//
+// Inverted logic signals
+//
+// The Toshiba data sheet recommends buffering the logic signal inputs from 
+// an MCU through a 74HC04 inverter, because the sensor's logic gates have
+// relatively high input capacitance that an MCU might not be able to drive 
+// fast enough directly to keep up with the sensor's timing requirements.  
+// SH in particular might be a problem because of its 150pF capacitance,
+// which implies about a 2us rise/fall time if driven directly by KL25Z
+// GPIOs, which is too slow.
+//
+// The software willo work with or without the logic inversion, in case anyone
+// wants to try implementing it with direct GPIO drive (not recommended) or 
+// with a non-inverting buffer in place of the 74HC04.  Simply instantiate the
+// class with the 'invertedLogicGates' template parameter set to false to use 
+// non-inverted logic.
+//
+//
+// How to connect to the KL25Z
+//
+// Follow the "typical drive circuit" presented in the Toshiba data sheet.
+// They leave some of the parts unspecified, so here are the specific values
+// we used for our reference implementation:
+//
+//   - 3.3V power supply
+//   - 74HC04N hex inverter for the logic gate inputs (fM, SH, ICG)
+//   - 0.1uF ceramic + 10uF electrolytic decoupling capacitors (GND to Vcc))
+//   - BC212A PNP transistor for the output drive (OS), with:
+//     - 150 ohm resistor on the base
+//     - 150 ohm resistor between collector and GND
+//     - 2.2K ohm resistor between emitter and Vcc
+//
 
 #include "config.h"
 #include "NewPwm.h"
@@ -91,12 +132,6 @@
 #include "DMAChannels.h"
 
 
-// Logic Gate Inverters:  if invertedLogicGates is true, it means that the
-// hardware is buffering all of the logic signals (fM, ICG, SH) through an
-// inverter.  The data sheet recommends using a 74HC04 between the host MCU
-// and the chip logic gates because of the high capacitive load on some of
-// the gates (particularly SH, 150pF).  
-
 template<bool invertedLogicGates> class TCD1103
 {
 public:
@@ -121,7 +156,7 @@
 
         // Calibrate the ADC for best accuracy
         os.calibrate();
-
+        
         // ADC sample conversion time.  This must be calculated based on the
         // combination of parameters selected for the os() initializer above.
         // See the KL25 Sub-Family Reference Manual, section 28.4.45, for the
@@ -151,14 +186,13 @@
         // results by using half of the ADC time plus a small buffer time.
         //
         fm.getUnit()->period(masterClockPeriod = ADC_TIME/2 + 0.1e-6f);
-        printf("TCD1103 master clock period = %g\r\n", masterClockPeriod);
         
         // Start the master clock running with a 50% duty cycle
         fm.write(0.5f);
 
-        // allocate our double pixel buffers
-        pix1 = new uint8_t[nPixSensor*2];
-        pix2 = pix1 + nPixSensor;
+        // Allocate our double pixel buffers.  
+        pix1 = new uint8_t[nPixAlo * 2];
+        pix2 = pix1 + nPixAlo;
         
         // put the first DMA transfer into the first buffer (pix1)
         pixDMA = 0;
@@ -202,7 +236,7 @@
     // call, at which point this buffer will be reused for the new capture.
     void getPix(uint8_t * &pix, uint32_t &t)
     {
-        // return the pixel array that ISN'T assigned to the DMA
+        // Return the pixel array that ISN'T assigned to the DMA.
         if (pixDMA)
         {
             // DMA owns pix2, so the stable array is pix1
@@ -215,15 +249,12 @@
             pix = pix2;
             t = t2;
         }
-
-        // debugging - print out the pixel transfer time stats periodically
-        static int n;
-        ++n;
-        if (n > 1000)
-        {
-            printf("TCD1103 scan last=%d, min=%d, max=%d (us)\r\n", dtPixXfer, minXferTime, maxXferTime);
-            n = 0;
-        }
+        
+        // The raw pixel array we transfer in from the sensor on the serial 
+        // connection consists of 32 dummy elements, followed by 1500 actual
+        // image pixels, followed by 14 dummy elements.  Skip the leading 32 
+        // dummy pixels when passing the buffer back to the client.
+        pix += 32;
     }
     
     // release the client's pixel buffer
@@ -242,8 +273,8 @@
 protected:
     // Start an image capture from the sensor.  Waits the previous
     // capture to finish if it's still running, then starts a new one
-    // and returns immediately.  The new capture proceeds autonomously 
-    // via the DMA hardware, so the caller can continue with other 
+    // and returns immediately.  The new capture proceeds asynchronously 
+    // via DMA hardware transfer, so the client can continue with other 
     // processing during the capture.
     void startTransfer()
     {
@@ -257,9 +288,12 @@
             clientOwnsStablePix = true;
         }
         
+        // figure our destination buffer
+        uint8_t *dst = pixDMA ? pix2 : pix1;
+        
         // Set up the active pixel array as the destination buffer for 
         // the ADC DMA channel. 
-        os_dma.destination(pixDMA ? pix2 : pix1, true);
+        os_dma.destination(dst, true);
         
         // Start the read cycle by sending the ICG/SH pulse sequence
         uint32_t tNewInt = gen_SH_ICG_pulse(true);
@@ -288,6 +322,9 @@
     // the DMA transfer completes.
     void transferDone()
     {
+        // stop the ADC triggering
+        os.stop();
+
         // add this sample to the timing statistics (for diagnostics and
         // performance measurement)
         uint32_t now = t.read_us();
@@ -298,22 +335,20 @@
         // collect debug statistics
         if (dt < minXferTime) minXferTime = dt;
         if (dt > maxXferTime) maxXferTime = dt;
-        
-        // check if there's still time left before we reach the minimum 
-        // requested integration period
+
+        // figure how long we've been integrating so far on this cycle 
         uint32_t dtInt = now - tInt;
-        if (dtInt < tIntMin)
-        {
-            // wait for the remaining interval before starting the next
-            // integration
-            integrationTimeout.attach_us(this, &TCD1103::startTransfer, tIntMin - dtInt);
-        }
-        else
-        {
-            // we've already reached the minimum integration time - start
-            // the next transfer immediately
-            startTransfer();
-        }
+        
+        // Figure the time to the start of the next transfer.  Wait for the
+        // remainder of the current integration period if we haven't yet
+        // reached the requested minimum, otherwise just start almost
+        // immediately.  (Not *actually* immediately: we don't want to start 
+        // the new transfer within this interrupt handler, because the DMA
+        // IRQ doesn't reliably clear if we start a new transfer immediately.)
+        uint32_t dtNext = dtInt < tIntMin ? tIntMin - dtInt : 1;
+        
+        // Schedule the next transfer
+        integrationTimeout.attach_us(this, &TCD1103::startTransfer, dtNext);
     }
 
     // Generate an SH/ICG pulse.  This transfers the pixel data from the live
@@ -478,10 +513,6 @@
         //   - Wait for the new TPM cycle
         //   - End the ICG pulse
         //
-
-        // The timing is so tight here that we want to be sure we're not
-        // interrupted by other tasks - disable interrupts.
-        __disable_irq();
         
         // disable the TPM->ADC trigger and abort the current conversion
         os.stop();
@@ -505,9 +536,9 @@
         // TPM cycle 1us from now.  (This step takes about 3 instructions.)
         os.resume();
         
-        // Okay, everything is queued up!  We just have to fire the starting
-        // pistol on the sensor at the right moment.  And that right moment 
-        // is the start of the next TPM cycle.  Wait for it...
+        // Eerything is queued up!  We just have to fire the starting gun
+        // on the sensor at the right moment.  And that right moment is the 
+        // start of the next TPM cycle.  Wait for it...
         fm.waitEndCycle();
         
         // And go!
@@ -516,9 +547,6 @@
         // note the start time of the transfer
         tXfer = t.read_us();
         
-        // done with the critical timing section
-        __enable_irq();
-        
         // return the timestamp of the end of the SH pulse - this is the start
         // of the new integration period that we just initiated
         return t_sh;
@@ -549,6 +577,13 @@
     // room for the dummy pixels during each read.
     static const int nPixSensor = 1546;
     
+    // Figure the number of pixels to allocate per pixel buffer.  Round
+    // up to the next 4-byte boundary, so that the buffers are both DWORD-
+    // aligned.  (This allows using DWORD pointers into the buffer to 
+    // operate on buffer pixels four at a time, such as in the negative 
+    // image inversion code in the generic PlungerSensorImage base class.)
+    static const int nPixAlo = (nPixSensor + 3) & ~3;
+    
     // pixel buffers - we keep two buffers so that we can transfer the
     // current sensor data into one buffer via DMA while we concurrently
     // process the last buffer