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An electronic fuel injection program written in c++ to experiment with 1 us resolution on an Arduino, or STM32 Cortex processors.

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# CoreEFI
An electronic fuel injection program written in c++ to see if sequential fuel injection with 1 us resolution could be achieved on an Arduino 16 MHZ Mega or 84 MHZ Arduino Due. This has also been run on a STM32F4 407 Cortex processor, but not fully implemented.  This has never been tested with any actual engine, so can't be considered much more than a proof of concept at this point.

Above shows an example of the event schedule, the Java ui code is not yet released.

Some distinguishing features:

<ol>
<li>Utilizes variable length timers to minimize event scheduling overhead.</li>
<li>Low impact metrics track performance rates and time spent in critical sections.</li>
<li>Complete console interface allows viewing stats on all major components.</li>
<li>All console output is in a json style which makes it very easy to parse.</li>
<li>Table driven architecture makes it easy to add new variables, tables, etc.</li>
<li>External xml config file used to generate the parameter lists and table data</li>
<li>Cylinders only limited by the number of events that can be processed at max RPM.</li>
<li>Contains all tables/functions from a Ford EEC-IV computer using a MAF system.</li>
<li>Most viable platform implemented right now is the Arduino Due at 84 MHZ</li>
<li>Will compile and run on a 16 MHZ Arduino Mega using about 4k of ram and 40k of code.</li>
<li>Compiles with gcc-arm-none for the STM32F4 series of ARM processors</li>
<li>Can compile and run on the Linux command-line for testing/simulation.</li>
</ol>

Initial development started on an Arduino UNO, but switched to a Mega because 2k of ram is just not enough!  Then switched to the Due because the Mega timers combined with the 16 MHZ clock rate was pathetic.

The original system I used for my model also runs at 16 MHZ, so looking forward to seeing how accurate it really is.  I would hope most of it is written in assembly and with much better timer resolution.  Still I would like to think that I can make the Arduino Mega perform better than this 25 year old efi computer.

<h2>Architecture</h2>

The code is a combination of C/C++.  Having not worked with a variety of hardware platforms, there were doubts about how portable C++ would be.  Programming wise, there is zero dynamic allocations, absolutely no use of virtual methods/vtables, polymorphism, operator overloading, or even much inheritance, so there shouldn't be any surprises.  Given the sketchy startup code with the stm32 environment there isn't any reliance on static constructors, which wasn't missed anyhow.  Most all methods are declared inline if they are critical path or only called once.  Some classes are completely hidden with a C api, and the only reason some became full blown classes in a header file was for inlining methods or polyinstantiation.

Here are some of the most major implementation details:

<h3>Table Driven</h3>

All of the variables that may be logged or used to calculate other values exist in a single enumeration with a table of meta data for their min/max and scale data.  All access is done through convenience routines creating a parameters database instead of spreading the fields across various data structures.  Meta-data is verified for all access to make sure the correct types are used.

The strategy queries values and returns a value for computed parameters.  This is very easy to extend by adding new parameters to the enumeration, the meta-data, and any calculations.  Any parameter can be extended to be a lookup table, function, or translation.

Caching and write-through cache flushing can be easily implemented within the parameter list.  Some of this is partially enabled, but it defeats the performance testing given many of the inputs don't change during simulation, so not as far as it should be.

All of the parameters exist in a Java program and the enumeration and meta data are exported to an xml file.  The same program is used to generate the source code for the parameters and all of the data storage and various indexes.  The data structures were designed for a low memory footprint since it was originally implemented on an Arduino UNO.

The table driven approach allowed for many features to be provided with a reduced code footprint.  Even storing much of the data in program memory.  This means anything defined in the parameter list can be read/written using the console.

There is some additional overhead since the parameters are encapsulated in a simple database for the rest of the application to use.  There is additional overhead since it does range checking, type checking and storage conversions.  It is best to not access the parameters database from within an ISR.

Another little feature is that constants are created for the maximum cylinders and encoder teeth, and the actual values used can be changed at runtime .. for testing only!

<h3>Simplified Decoder/Event Math</h3>

The decoder handling is actually the most critical piece since if there is drift/error in the decoder, the accuracy in scheduling no longer matters.  Simplified processing means the ISR can finish in less time, reducing overall error.

In some other implementations it appears they convert from a time basis to degrees, and appear to calculate the absolute degrees often.  A large amount of effort goes into trying to calculate the current RPM and absolute angle.  The events are always calculated in degrees and then converted to time at the last.

Current RPM is something that will result in each pulse getting shorter/longer as the engine accelerates/decelerates.  The RPM at the beginning of a cycle will be different than the RPM at the end, so if RPM is calculated only once per revolution, the error will compound until the RPM is calculated again.  RPM exact degree calculations require division which can make these more expensive time-wise.

The current approach uses a cyclic buffer to store the pulses and for each event, subtract the last pulse time and add the current pulse time to a running total.  The new total represents the time in the last cycle covering 720 degrees.  Subtract the current time from absolute TDC time and you always know where you are at relative to the cycle time.

A correction factor may need to be added to project current RPM. Maybe keeping the relative difference between the preceding n pulses to calculate a percentage change to the cycle time.  Not implementing anything for projecting RPM until I work with real data.  This may add a division operation .. or maybe not.

The benefits of the current approach is there is 0 division and 0 floating point calculations required in the collection of pulses or in the translation from event degrees to absolute time.  So 0 division/floating point calculations in the ISR's.

A downside is that the accuracy of the TDC pulse is very important since it is used as the basis to convert degrees to absolute time.  If the TDC pulse was late, the next pulse would be early, so this should actually reduce error instead of accumulate it so it may all cancel out.  Jitter in the encoder/decoder would be difficult to compensate for.

<h3>Single Event Scheduler</h3>

The event handler is designed to operate in a separate thread either from a coprocessor, or driven from an interrupt.

Each event has two parts, an absolute start which specifies degrees from 0 to 720, and then a duration part in ticks.  So for a fuel/spark on each cylinder, there are actually 4 different events.  When a schedule is computed the leading events are stored with an angle only, the follow-up event contains the same angle and the duration.  These values are stored in an unsigned 2 byte int, so 0 to 720 is stored as 0 to 65535.  This simplifies phase issues where events wrap around by leveraging the natural wrapping when working with unsigned values.

A strategy calculates the 4 events for each cylinder.  An estimated event time is calculated for sorting, but the final time is computed one or more times before it actually occurs.  For example, the strategy decides the spark should happen 20 deg BTDC and the dwell should be 5 ms and the injector should open at 360 BTDC and stay open for 2 ms.  That information is stored in the events and can be used to calculate the absolute time relative to the latest RPM.

This allows the strategy to be completely asynchronous with the events, which is required because the strategy is compute intensive and may not be able to calculate each cycle and second the RPM is changing through the cycle, so computed times need to adjust accordingly.  For example, 20,000 RPM means the crankshaft makes a rotation in 3ms, and so a cycle will take 6 ms.  The compute time for a complete strategy on an Arduino Mega is about 6 ms, but because the cpu is saturated with event processing, it may take twice as long.  The strategy calculation time does not scale up/down with the number of cylinders.

<h3>Scheduler</h3>

The main loop uses a non-preemptive scheduler that prioritizes based on the run interval.  This is how the encoder/events execute on Linux.  There is a mode that runs the system with perfect 1 tick resolution which is used to test for a zero error environment.

<h2>Strategy</h2>

The initial strategy is based on extracted eeprom data from a Ford EEC-IV computer and then implemented using a best guess approach, in other words, it is incomplete and maybe not even accurate.  For example, their dwell numbers didn't make any sense at all, so used a 5-6 us dwell because that is what a ls1 coil requires.  The injector durations look a little long, and the lamda calculation is not incorporated yet, so no attempt at closed-loop operation.

The positive side is that this will flush itself out pretty quick with some empirical knowledge, and the table driven architecture makes this very easy to change/extend.  The main concern at this point was whether the values looked somewhat believable, and there was something computed in every case where it needed to be to get an accurate performance profile.

<h2>Accuracy</h2>

So how accurate does it have to be?  Of the 4 events per cylinder, the timing of the injector open, spark and dwell can probably tolerate quite a bit.  At 8000 RPM, a 0.1 degree accuracy can be maintained with 2.08us of jitter .  For pulse width 2us is 0.05% per ms of duration, 40us for 1%.  I found a ms2 reference of 0.9 deg at 7,500 and older code revs being 3x as bad.  That was on a 40 MHZ processor, but the newer ms3 uses the xgate co-processor, and should be much more consistent.

At 8000 RPM, 0.5 degree accuracy requires the jitter to be 10.42us or less.  That would be against 7500 us/rev, or 15000 us/cycle.  The issue isn't whether any given processor can run an efi system at idle, the issue is what is the maximum RPM given the number of events per cylinder?  Full sequential requires 32 events for 8 cylinders, cut that in half for semi-sequential/wasted spark.  I am not sure if semi would make much difference at high RPM, especially once the duty cycle reaches 100%.

Half the events will cut ISR contention in half.  It may be that a batch mode is required during cranking, then run full sequential up to a certain RPM, then switch to semi-sequential after that.  Wasted spark can also cut the coil events in half, so an 8 cylinder would require 16 injector events, and only 8 coil events.

There is a bmw v8 that can spin 20k RPM, and 0.1 degrees of error would equate to 0.8 us, but for your more standard 6k redline, you could have 2.8us of jitter and any accuracy beyond that is insignificant relative to other errors within the rest of the system.

<h3>The Problem</h3>

The issue is that with a single processor and multiple interrupts, there will always be the potential for contention.  And the maximum run-time of one ISR will set the maximum error in the event scheduler ISR.

For example on a 84 MHZ due, the encoder/decoder simulated timer interrupt takes between 9-32 us with 6 us required for the ISR overhead, that means the scheduler ISR can be 40 us late.  A correction factor can be added to try to wake early and then sleep, but that extends the ISR time, hurting the decoder ISR.

The sleep method works well to reduce errors at lower RPM where there is lower contention, but hurts higher RPM where it really matters and has an adverse affect on other ISR routines like the decoder timing.  Corrupt the decoder timing, and all other accuracy attempts are meaningless.

The best solution so far is to have them run on a separate thread with a coprocessor or multi-core cpu or just get a processor that is so fast that everything runs under 1 us.

<h2>Platforms</h2>

This is called CoreEFI because the focus was on the main components of any efi system, how they interacted, and the final accuracy of the results.  Significant time has been spent trying to reduce error, and measure error accurately within the software without skewing the results.  Uncertainty principal applies here.  There could be more errors induced by the hardware when measured on a scope, but hopefully the worse case scenario is already known from inside the cpu.

There are only a handful of methods required to port this between platforms, and the entire system can be simulated by running the ISR routines direct from the main scheduler.  Once it is running from the scheduler, move the encoder/event calls into timer routines and it will start showing the error rates.

Simply clone this project and execute the Makefile to run on Linux or mac. To run on an Arduino Mega/Due, just load the src.ino file.  Should work on cygwin, but that would require me to actually run Windows, and that will take some persuasion.

Once running, just type ? to get some options.  u 0 5000, will set the decoder rpm, e, d, i, t are also used often.

<h3>Linux/Mac</h3>

Values are initialized to support a running state, but any value can be overwritten using the prompt.  The events process and an encoder simulator are run from the main loop scheduler instead of from interrupts.  This mode is primarily for rapid prototyping, and profiling or using valgrind.  The linux process is also used to test the Java interface.  Passing any argument at all to the program switches it to run with the perfect 1 tick timer for testing.

Have to say .. the ST libraries suck, the worse I have ever seen.  Incomplete examples, bugs, complex state machines with no docs to explain them.  My USB implementation is still crashing periodically because they don't cleanup an TxState, and not the previously known TxState error.  I have 10x+ the number of hours as it took to get the Due working and it is still buggy.  The issues have been numerous and hard to figure out, e.g. the latest is the printf formating for %f totally fails, had to use a .ld file from their old libraries to make it work right, and this kind of bs has been at every turn.

That said, the performance is solid, at 168mhz was seeing a solid 10x+ increase over the mega in intial tests which makes most performance issues go away.  The best part is that where even the due had a best case of 6 us overhead in the timer isr, the 407 has 0-2 us which makes me think either I am getting incorrect results or the due should do better.

I still do not believe the smt32f4 numbers, the gpio is not enabled yet, and there may be a problem with one of the timers or who knows what?  It looks promising, and would be nice if they hold.

Did some basic benchmarking, and the results were impressive.  The 407 has a fpu, but it only works for floating point, not double precision.  The double precision is about 10x faster than the due, but the floating point sqrt is 140x!  A basic 4 byte integer op is 33.5x.  The unusual part which is part of both tests is that the call to my custom clock ticks function the the 407 is 30x faster than the due.  I need to do a similar implementation on the due to see.  Also I think the 407 ticks are a little fast which is a disadvantage, I need to measure both on a scope to really know how they compare but that can't account for a double digit change.

So makde sure all double precision numbers were switch to floats.  There were 4 numbers I compared before and after, the due times were reduced by about 25% for the strategy calc, but the 407 was reduced to 25% of the old number.  Unreal!  It does appear tha the 7 us jitter on the 407 is going to stay constant unless I can optimize out the 4 hal function calls, so the results were unchanged.  The due late percentages dropped by a full 5% at 22k with updated numbers below.

<b>Here are some stats from the program for 8 cylinders, 32 events, 12 tooth cam encoder (5/19/15):</b>

All the different counters say that it is doing the work and the many cross-checks i have appear to line up, but this sounds too good to be true .. but if it is, it says something about avr timer implementations.  The above is running the same as the previous tests.

Here is a test with 16 cylinders (5/19/15)..

All the work may have been worth it, will update this if the numbers take a nose-dive.  Need to still release a 407 based project to build this.  Also pretty sure that my clock reference is running .01% too fast.

<h3>Arduino Due (84 MHZ)</h3>

Note that the numbers below have already been reduced by 5-6% for each of the 8 cylinder tests below because of some more optimisations, and pretty sure they will get another small drop, and then I will update them.

A downside of the Due is that there is not any non-volatile memory available on the main board!

The Due has 32bit timers so there is no requirement to ever use the pre-scalars in this application.  Also the timer interrupt overhead was optimized down to 6 us.

Still the decoder/encoder took 9-30 us, and the scheduler took closer to 40 us. Still not great numbers, but far better than the 16 MHZ Mega.  These times will improve using an external interrupt for the decoder by reducing the time a little more.

<b>Here are some stats from the program for 8 cylinders, 32 events, 12 tooth cam encoder (5/21/15):</b>

Note that this is with running an encoder simulator that runs the decoder.  The encoder has some round-off error and some jitter, thats why the rpm is not a whole number.  An external interrupt should have a little less overhead and should help overall.

This is with a 12 tooth decoder, there will be more contention with a 36 or 60 tooth decoder, especially if crank vs cam driven.

Removing some of the metrics that measure errors would probably help some and they are easy to disable, but then all measurements have to be done externally.

The error rates look like they would be acceptable for a standard v8 that doesn't rev past 8k rpm's.

2800 rpm's appears to be the cross-over where there is an occasional late every few seconds, anything below that is 100% on time.

Somewhere around 8k, events get queued and there is a big jump in errors.  The example at 7800 was a best case, and just shows that even at a high rpm events don't always get queueud.  There is a number not shown, which is the % late within 2 us, and the number does not jump as much as late%.

At 22k rpm the durations hit 100% duty cycle so the events get queued.  5+ degrees sounds unusable at this rpm. Queued ISR's is what causes the late time to increase, a single revolution happens in under 3 ms so not much time to fit everything in.

<b>Here is a 4 cylinder, 16 events, 12 tooth cam encoder (5/21/15):</b>

<pre>
rpm=    3888	-late=  -12	+late=  -1	-deg=   -0.28	+deg=   -0.02	late%=  0.00
rpm=    7882	-late=  -15	+late=  5	-deg=   -0.71	+deg=   0.24	late%=  0.13
rpm=    16786	-late=  -14	+late=  9	-deg=   -1.41	+deg=   0.91	late%=  24.44	latex=  9.93
rpm=    22412	-late=  -15	+late=  47	-deg=   -2.01	+deg=   6.31	late%=  33.75	latex=  8.06
</pre>

<h3>Arduino Mega (16 MHZ)</h3>

On the AVR based Arduino within the timer ISR you only get one read on the clock, any successive reads return the same value .. although the latest IDE appears to be working different, or maybe because I am enabling interrupts.  Using a regularly timed interrupt had to be at a very high frequency to maintain any accuracy and quickly saturates the cpu so a variable length timer was used to try to have one wakeup per event.

The problem is that if you don't know the time at the end of the ISR, you don't know how long to sleep.  Based on being early/late a correction factor is subtracted to try to assure that in all cases the timer wakes up early, never late.  Also for jumps beyond 4096 us a prescaler is required so an intermediate sleep is used to come within 4k of the target time.  To compensate for being early, a delay was used, which extended the ISR time and created additional contention.

Bottom line, simulating an 8 cylinder engine with 4 events per cylinder, open/close the injector and charge/fire the coil I was able to get 1 us resolution nearly 100% of the time at idle, high 98+ at about 2k RPM, and then it dropped dramatically after that.  That may translate to 1 cylinder at 16,000 RPM, but probably other limitations will make it far lower.  This was using the early wakeup/sleep method.  The variable timer would use as much lead-in as it could get, .5ms, or even a full ms because the worst case was so bad with the delays.

Using some timer test code that only would wakeup and immediately sleep, the timer overhead appeared to be around 40 us.  When running multiple timers, successive timers were adversely affected and the lead times required to stay on time would grow to 100-200us without doing any real work.

Given the mega and due have the same form factor and not that much difference in price, there is no reason to continue with a mega.

<h2>Other Alternatives</h2>

There are at least 4 other systems out there that actually have run engines, but are already heavily invested in their own approaches.  This is a quick synopsis of what I have seen so far:

<ul>
<li>diyefi/freeems - I think the oldest project and the most vehicle installs.  Uses the freescale processor, but not using xgate, so the implementation is using all of the output compare interrupts to drive the timing.  Only 6 total and one needs to be used for the decoder, so it limits the number of cylinders that can be run independently.  The code appears very convoluted and difficult to read.  Being the oldest, it is already tied to it's own legacy and not pushing the state of the art.  A positive is that they have a board design with units available to purchase.  The community is still active.</li>

<li>open5xxxecu.org - On the same or similar cpu as diyefi.  There is lots of posts in their forums, but new posts are far and few between and it does not look like their is hardware available.  The code appears to be using some embedded rtos on a freescale processor but not sure if it is the same as diyefi with xgate or not.</li>

<li>libreems - this is just a branch from diyefi which fractured the efforts there.  There is some advancement with using the xgate co-processor, but the community is also pretty quiet.  They are also on the same cpu as diyefi.</li>

<li>rusefi.org - the newest of the projects, appears to have a lot of momentum and using the same stm32f4 407 processor.  Hardware is available for sale and already on a second revision.  They are using chibios for a rtos and still not sure if that is a positive or a negative.  I see they also wrote their own display software and have a simulation mode so we were on parallel paths.  They compare themselves as the iphone to the newton, being the Cortex ARM solution vs the freescale processors used by all of the above and the megasquirt software.  I am still skeptical that 1us can be achieved in a uni-processor approach.</li>

</ul>

<h2>Summary</h2>

Knowing that 1 us resolution is attainable even sometimes on a 16 MHZ Arduino tells me that the best case timing can be attained nearly anywhere, it is really the worse case that matters.  Any time interrupts get queued waiting for another to finish, it is going to be late in processing.  Interrupt contention will always create situations where one ISR is waiting for another and that drives your worse case.

The freescale processor with the xgate co-processor running at 40 MHZ can probably attain a higher accuracy, more of the time, than the 168 MHZ ARM if implemented correctly because it should have 0 interrupt contention.  Also frees up the ISR for the decoder which is the most important.  I may investigate this further once the 407 is more fully tested.

Of the platforms above, I don't think the 16 MHZ AVR is viable except for maybe low cylinder counts, and limited RPM.  The 84 MHZ Arduino Due looks like a viable platform for a v8.  The st arm is double the clock rate of the due, so should be able to cut all the error rates in half, so maybe running this on the rusefi hardware will be the final platform.

The 25 year old target engine is not close to running yet in the 50 year old car, so it will be awhile before I can test it with real sensors.

This started as a simple fan controller and the desire to be able to tap into all the instrumentation in the EFI computer instead of creating a parallel system of sensors.  There are other desired functions like running a TFT to provide cockpit feedback and controlling fans, EPAS, and other modern features without having to add lots of buttons/gauges to a very old car.

The plan is to eventually integrate this into the existing system passively and then later replace the EEC-IV entirely.

Next Steps:

<ul>
<li>Fix the bugs, there is a phase related one that sometimes effects error calculations<li>
<li>Run numbers with an externally triggered decoder simulator<li>
<li>Verify all numbers externally with an oscilloscope, could be better or worse<li>
<li>See if the Arduino forums has any suggestion on improving the Mega timers</li>
<li>Further development with the 407/due with a tft display</li>
<li>Flush out the strategy computations and incorporate fan controller</li>
<li>Release the Java program as open source, actually the libraries..</li>
<li>Integrate this with an actual engine to see where I have over simplified things</li>
<li>Try a freescale dev board with an xgate scheduler .. maybe</li>
</ul>

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