MPU6050 FIFO Buffer Management Study

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Posted 13 October 2019

I have been attempting to use the Invensense MPU6050 6-axis IMU for some time now in both my two and four-wheel robots for improved wall-following ability. By measuring the relative heading change during turns, I could get the robot to accurately acquire and maintain a specified distance from the currently tracked wall.  I say ‘attempting’, as I have experienced somewhat mixed results in getting reliable results from the IMU.  A large part of the problem, as I described in this post, wasn’t the IMU at all, but rather the sensitivity of the Arduino I2C bus to RFI/EMI caused by the motor drivers.  However, even after solving this problem, my programs would still occasionally ‘lose synch’ with the IMU’s FIFO buffer and start returning garbage for heading values – not good!

In addition to the I2C issue, there are several factors that make the MPU6050 harder do work with:

  • Invensense, the company that makes the MPU6050 chip, appears to have been purchased by TDK, and their technical support forums don’t appear to be supported any longer.
  • Apparently there are significant pieces of the MPU6050 firmware that aren’t freely available as human-readable code, so much of the MPU6050 magic is just that – magic.  In particular, the details of how the MPU6050 handles its internal FIFO are (at least to me) completely unknown, except by reverse-engineering.
  • While there is a lot of MPU6050-related information and code floating around out there in the i-verse, that is as much a trial as a blessing; it has been very difficult for me to wade through everything and to try to sort out the wheat from the chaff.
  • Jeff Rowberg’s wonderful I2CDevLib contains support for the MPU6050, with examples.  While it is fairly easy to get started using Jeff’s examples, it was difficult for me to understand how the examples work so I would know how to modify them for my application without running off into the bushes.
  • Almost all the example code out there assumes an interrupt based IMU management scheme.  For my wall-following robot application, the interrupt scheme was overkill and then some, so I wanted to use a polling arrangement, which is very poorly documented. Eventually I developed a working polling algorithm (described here) , which I now use in my robots.
  • Invensense (now TDK) has released several updates to the MPU6050 firmware, and it is difficult (at least for me) to figure out what the differences are and whether or not those differences are worthwhile for my application.  There is some information in the header files provided by Jeff Rowberg, but if there is any sort of formal change history, I haven’t found it.

Despite all this, the MPU6050 is a wonderful device, and it’s EVERYWHERE – you can get MPU6050 modules from Adafruit, Sparkfun, DFRobots, and GY-521 Chinese knockoffs from eBay.  The GY-521 modules have some reputed issues with quality control, but at about $1-2 per module, it’s hard to go wrong.

In my continued attempts to understand how the MPU6050 works, and the details of some of the latest example code provided on Jeff Rowberg’s Github site, I posted an issue related to a particular part of the example code that defied my ability to understand, as shown below:

This code obviously works, but I get a headache everytime I try to figure out how it makes sense.  One of the replies I got from this post was from Homer Creutz, who I knew to be one of the very best experts on all things MPU6050.  The gist of Homer’s reply was “yeah, it looks kinda clumsy, but it does work”. But then Homer went on to suggest an alternative using the modulus (%) operator that piqued my interest, as I had used this technique in my four-wheel robot code.  In subsequent email conversations Homer went WAY out of his way to thoroughly answer my stupid questions about the MPU6050, especially about the issue with FIFO overflow.  He sent me a link to this video explaining how circular buffers work, and the following graphic illustration (slightly edited for clarity) of the problem of MPU6050 overflow and multiple-byte packets

The combination of the video and the above simple graphic finally allowed me to understand why properly managing FIFO overflow is so critical for successful MPU6050 implementations. Ironically, FIFO overflow management is more of an issue in my low-speed, high mass, low-dynamics environment than at the other end of the scale.  In high-dynamics applications, FIFO overflow is almost never an issue because the application sucks data out of the FIFO as fast as it is put in, in order to provide the best possible stabilization and control.  However, in low-dynamics applications like my wall-following robots, there is no need for IMU information more than a few times per second, meaning that the FIFO will almost certainly overflow if it isn’t proactively drained even if the information isn’t really needed.

Homer also sent me a couple of untested alternatives for FIFO management to replace the ‘while() within an if()’ algorithm, so I decided to test them and report the results back to Homer. After all, Homer was going WAY out of his way to answer my ignorant questions, so the least I could do was to be his lab tech.  So, I started with Jeff Rowberg’s MPU6050_DMP6_using_DMP_V6.12 example (the one with the ‘while() within an if()’ snippet) and modified it to deliberately cause FIFO overflows. After a bit of debugging and some very slight changes to Homer’s code, I got the following output from a run using a 100 mSec delay at the start of loop():

As can be seen from the above, about 20 interrupt cycles are skipped in each loop() iteration, causing the 1024-byte FIFO contents to expand by either 280 or 252 bytes each time, until it overflowed and was reset.  The example code handled FIFO overflows properly resetting the FIFO each time so that the retrieved yaw values continued to make sense.

The next step was to replace the example code with Homer’s proposed setup using the modulus operator for FIFO management. The first section below shows the example code loop() function before Homer’s modifications:

And the following shows the same loop() function after implementing Homer’s suggested code:

This resulted in the following output:

Showing that FIFO overflow was indeed handled properly.  The FIFO overflowed after the 3rd time through the loop, (the returned count was capped at the 1024-byte physical length of the FIFO), and Homer’s code correctly removed the 16 corrupted bytes at the beginning of the FIFO, plus one more 28-byte packet.  The subsequent mpu.getFIFOBytes() call retrieved an entire valid packet, which was then process to produce a valid yaw value.  Of course, since only one extra packet was removed, the FIFO overflowed again when the next 336 bytes were loaded during the 100 mSec delay at the start of the next loop() iteration.

When the code snippet to retrieve all available packets

was uncommented from the above program, I got an almost perfect output as shown below:

There were 4 bad values starting at 15250 mSec, as shown below

I’m not real sure what happened here.  A FIFO count of 308 is nowhere near the overflow condition, and it is an even multiple of 28 (the packet size), so everything should have gone swimmingly.  However the displayed value of 2.04 degrees at time 8859 mSec is clearly incorrect, as I was manually (and slowly) rotating the MPU at the time.

Another issue with all of this is the FIFO count associated with the number of interrupts shown in the output.  22 interrupts should produce 22*28 = 616 bytes, but mpu.GetFIFOCount() only returns 308 – exactly half the expected value.  So, either the packet size is actually 14 bytes (not very likely, as mpu.GetPacketSize() returns 28) or the IMU is only loading half a packet on each interrupt.  I added digitalWrite() statements to the ISR so I could directly monitor interrupt occurrences with my O’scope, and the interrupts happened exactly as expected, at approximately 4.58 mSec intervals (about 220Hz).  The 100 mSec delay at the top of loop() should produce approximately 100/4.58 = ~22 interrupts each iteration, and that is what is reported in the output.  So, why the x2 error in the reported FIFO count?

I ran another test, and this one responded perfectly for as long as I let it run (about 22 seconds). During the run I manually rotated the robot (and its attached IMU) back and forth, as shown in the following Excel plot

14 October 2019 Update:

There is clearly something not-quite-right with the way the MPU6050 reports the current length of the FIFO contents.  Here are the results from a recent run:

Aside from the fact that 22 x 28 = 616 not the reported 308, there is also the problem that after one more interrupt (23 vs 22), the reported FIFO content length didn’t go up by 14 (half the expected 28, but…) but instead by 12 bytes – what the heck?  This clearly implies that MPU interrupts aren’t entirely synchronous with actually filling the FIFO – like some sort of race condition?  In other words, the number that is reported by mpu.getFIFOCount() isn’t always an integer multiple of the packet size!  This is contrary to Homer’s assumption that the only way for mpu.getFIFOCount() to retrieve a non-integral multiple was for the FIFO to overflow. This clearly is not happening here, but I’m still getting non-integer multiple results.  Here’s another snippet from the same run:

In the above snippet, it can be seen that a 22 interrupt interval can sometimes result in 316 bytes being reported rather than the expected (ignoring for the moment the issue of a 2x error), and the ‘success’ of removing 36 bytes still resulted in a ‘bad’ yaw value computation (-179.17) 308.  In the very next loop iteration, 22 interrupts resulted in 328 bytes being reported. This time removing the excess allowed a valid yaw computation (-30.46).  So, a 22 interrupt loop interval can result in 308 (the ‘normal’ result), 316, or 328.

15 October 2019 Update:

I changed the loop() delay time from 100 mSec to 200 mSec, and (with no other changes) re-ran the test, with the following output:

The above results showed the expected 42-43 interrupt count between loop() iterations, and the expected (ignoring for the moment the 2x error factor) FIFO contents byte count of 588-616.  However, there were a couple of anomalous occurrences on two consecutive loop() iterations at 19094 and 19312 mSec.  The first one reported a FIFO contents count of 600 instead of 616 and (even after error correction) a clearly erroneous yaw value of -179.20, and the second one reported 604 bytes and (after error correction) an apparently valid yaw value of 61.54.

After an email exchange with Homer, I tried replacing this line

with this one:

In the following section of Homer’s algorithm

After this change, I re-ran the test at the 100 mSec loop() iteration delay setting with and without the above code change.  In both cases, I still got errors trapped,  as shown below (The test conditions for each run below are noted at the top of the text file)

So then, also at Homer’s suggestion, I instrumented the code to get the FIFO count rapidly several times after an error detection to see if the first mpu.getFIFOCount() occurred while data was actually being loaded into the FIFO and therefore got an erroneous count. So, I changed Homer’s code correction section to the following:

and re-ran the 100 mSec loop() iteration delay test.  What I got was this:

Wow!  the FIFO count changed!  The first mpu.getFIFOCount() at the top of the detection section got 334, and the next 3 calls all got 336!  So the first mpu.getFIFOCount() call must have hit the mpu 26/28 of the way through the packet load!

So, the MPU6050 packet load scheme isn’t atomic and there is, in fact, some sort of a race condition.  I think we have all been assuming that the MPU6050 loads the FIFO with a complete packet and then triggers an interrupt, while it appears that it is actually happening the other way around; the interrupt is triggered and then the packet is loaded into the FIFO. Most of the time this doesn’t cause a problem, but if you ‘get lucky’, the register associated with the mpu.getFIFOCount() call is read before the entire packet is loaded

16 October 2019 Update:

Homer asked me to change the code to determine exactly how long it takes to “clear the error”, which I take to mean “how long would a program have to wait for the MPU6050 to finish loading the rest of the packet into the FIFO”.  Homer sent me some sample code, which I modified slightly to produce the output Homer was looking for, as shown below:

When I ran this code, I got the following output:

 

21 October 2019 Update:

After several more email exchanges with Homer, he believes that he has now come up with pretty ‘bullet-proof’ way of handling MPU6050 errors, with the following subroutine

The idea behind this subroutine is to ensure that any overflow condition is detected and managed properly. The routine is completely independent of interrupts, so it can be used in a program using interrupts or polling.

Homer also sent me some test results using the program, with a variable loop delay time designed to be just below and then just above the delay required to overflow the buffer. This demonstrated that his subroutine properly handles FIFO overflow conditions, and returns valid packet data whenever possible.

In the above output, the first column is the loop delay in mSec, then the ‘Flag’ value returned by the subroutine, and then the ypr (yaw,pitch, roll) values extracted from the buffer filled by the subroutine.  As is shown, loop delay values above about 177 mSec start returning ‘2’ Flag values, indicating the routine detected (and recovered from) an overflow condition.

I replicated this experiment on my end, but discovered that for my installation, I had to use a loop delay almost exactly twice the value used by Homer (360-370 vs 177-178). The implication is that either my MPU6050 is loading the FIFO at one-half the rate of Homer’s unit, or my IMU has a buffer size twice the one being used by Homer.  Curioser and Curioser ;-).

Here’s my code and results:

Summary of results to date:

 

  • Homer has clearly created an effective algorithm for detecting and recovering from FIFO overflow events, and the subroutine that implements his algorithm can be used in an interrupt-driven or polling configuration.  I personally like the polling arrangement because it requires one less connecting wire, and removes the need for an ISR.
  • Both Homer and I have demonstrated that the algorithm works as designed, but the loop delay required to just trigger FIFO overflows in my configuration is almost exactly twice the delay needed for Homer’s. This needs to be explained.
  • There is still the problem of a factor of 2 error between the expected return from mpu.getFIFOCount() and the number calculated by multiplying the number of interrupts times the expected packet length. In my configuration using an interrupt-driven arrangement, a 22-interrupt loop delay resulted in a FIFO count of 308.  But, 22 x the expected packet size of 28 yields 616, not 308!  This also needs to be explained.

23 October 2019 Update:

To further investigate the ‘factor of 2 error’ problem, I went back and re-ran the initial experiment that produced the problem, just to verify that it was still there.  Here’s the entire program to recreate the results, and a partial printout of the results:

Significant points to note from the above output:

  • The time (in mSec) between adjacent output lines is about 111 mSece on average (110.5762 mSec according to Excel), and the reported number of interrupts is either 20 or 21 in almost every case (average is 20.0833333 according to Excel).  O’Scope observations confirm this is the case, as the output from the interrupt monitoring pin shows almost exactly 5 mSec between interrupts and an almost exactly 200 Hz interrupt frequency.  So, an interrupt count of 20 or 21 is reasonable, and cannot be the reason for the ‘factor of 2’ error in the FIFO buffer count.  However, there is an apparent ‘off by 2’ problem with the interrupt count, as the reported FIFO counts are consistent with interrupt counts of 22 & 23 rather than 20 or 21 as shown
  • Every so often a 22 interrupt span produces a FIFO count of 336 instead of 308 – a 28 byte difference.  In fact, over a run time of about 6.5 minutes (388,671 mSec), this phenomenon occurred 264 times, about 0.07% of the time.

The inference I draw from the above two points is that the MPU6050 chip isn’t actually loading an entire 28-byte packet during each interrupt cycle, but is in fact loading only 14 bytes each time.  With an artificially imposed 100 mSec (about 22 interrupt cycles) loop delay, the MPU loads 22 * 14 = 308 bytes.

An alternative explanation is that the MPU6050 loads complete packets into the FIFO every other interrupt. Under this scenario, it takes 11 cycles (22 interrupts) to load 11*28 = 308 bytes, and 12 cycles (24 interrupts) to load 12 * 28 = 336 bytes.

Another (and maybe even more reasonable) possibility is that the MPU6050 loads packet data into the FIFO one byte at a time, at an average rate of 14 bytes per 5 mSec, or 2.8 KBS. To the outside world, this might not be noticeable unless the application was trying to retrieve packets at the full 200 Hz.  At rates of 100Hz or less, the MPU would still have loaded at least one complete packet every time one was requested.

29 October 2019 Update:

As someone once said as both a benediction and a curse – “May you live in interesting times”.  In our ongoing investigation into the depths of MPU6050 behavior, we now have solved one mystery solved, only to encounter another one.

As I have noted several times above, there appears to be a factor of 2 mismatch between the number of interrupts counted from ISR activations and the number of bytes reported by mpu.getFIFOCount().  Either the interrupt count is off, or the FIFO count is off, and there doesn’t seem to be any other explanation.  Well, now I have determined that there is apparently a third option – that the MPU6050 only loads a packet into the FIFO on every other interrupt! This doesn’t make a whole lot of sense to me, but I now have what I believe to be irrefutable proof that this is exactly what is happening.

I set up a very simple program to get the FIFO byte count as rapidly as possible.  In order to avoid slowing the system down, I stored the results in a 1000-entry array during the retrieval process, and then printed out all 1000 entries at the end. Then I plotted the results in Excel as shown in the following figure:

The stairsteps in the above plot are (almost) uniformly the expected packet size of 28 bytes;  the MPU6050 is clearly loading entire 28-byte packets into the FIFO each time, contrary to one of the possibilities I had considered to explain the factor of 2 inconsistency between the interrupt count and the FIFO count.

However, when I started looking at the time interval between FIFO loads, I got the following plot:

As the above plot clearly shows, the MPU6050 loads a new packet into the FIFO every 10 mSec or so (I think it is exactly 10 mSec, with the differences explained by the lack of resolution in the time axis).  But wait – the MPU6050 produces an interrupt on the Arduino interrupt pin every 5 mSec – not every 10 mSec!  So, the mystery of the ‘factor of 2’ error is now solved.  The MPU6050 loads a new packet into the FIFO every other interrupt – not every interrupt as expected. So, the interrupt count is too high by a factor of 2 when compared to the actual FIFO count.

Unfortunately (or fortunately depending on one’s point of view), that simply begs the question – why is the MPU6050 producing interrupts on a 5 mSec schedule when it only changes the FIFO count on a 10 mSec schedule?  Who knows?

Changing the subject slightly, I got another ‘overflow proof’ version of GetCurrentFIFOPacket() from Homer Creutz to try.  I set up a small program to test it.   Since the idea was that GetCurrentFIFOPacket() would return a valid packet no matter how long it had been since the last time it was called, I set up a program to iterate through a sequence of delay times from 100 mSec to 500 mSec. For each loop delay setting I called GetCurrentFIFOPacket() multiple times and printed out the extracted yaw value and the return status from the function.

As the plot below shows, everything works swimmingly up to a loop delay of 350 mSec. Unfortunately, the wheels came off with a 400 mSec loop delay, and the packet values were all invalid after that – bummer!

 

06 November 2019 Update:

Holy cow!  Homer and I started this marathon project back in mid October, and we don’t seem to be any closer to nirvana than we were before.  What I can say though, is that I have learned a lot more about the MPU6050 and Jeff Rowberg’s driver software ;-).

One of the major issues we encountered with the polling method (vs interrupt-driven) is that, without the synchronization with MPU6050’s internal processes provided by the interrupt model, we can’t count on (no pun intended) the FIFO count returned by mpu.getFIFOCount() being accurate.  Depending on the timing of the call, the return value could be wildly inaccurate.  However, we discovered that two back-to-back calls to mpu.getFIFOCount() always resulted in an accurate count, although there was still a very small probability that a 3rd call would be required.  So, I created a small routine called ‘getPollingFIFOCount()’ to wrap this construct, as follows:

This function simply loops until two adjacent calls to mpu.getFIFOCount() return the same value. As always, however, there is a backup counter to force the function to exit if it gets hung up for any reason.  In the calling program I have a line like:

to set the backup loop counter value.

Another major issue with the MPU6050 is that it can overflow its packet FIFO buffer, and there doesn’t appear to be any way to prevent this, other than removing packets from the FIFO at the same rate or higher than they are loaded.  This may not be a problem for ‘high dynamics’ applications like quadcopters or balancing robots that need continuous attitude information, but for ‘slow dynamics’ applications like my wall-following robot where yaw information is only needed a few times per second, overflow becomes a practical certainty.  In an interrupt -driven environment, it might be reasonable to simply retrieve and then discard DMP packets as fast as they arrive, and then only process the latest packet when the application needs an update.  However, for a polling strategy, doing this may or may not work depending on what else is going on. So, for polling we need a way of ensuring we can retrieve a valid packet from the DMP FIFO, whether or not the FIFO has overflowed. Doing so would be trivial if the FIFO length was an integral multiple of packetSize, but it isn’t – yuk!!  So, now when the FIFO overflows, the first packet in the FIFO is guaranteed to be corrupted. The good news is, the last complete packet (i.e. the most recent information)  in the FIFO is always valid, but getting to that last good packet is non-trivial.  To summarize:

  • FIFO overflow is almost certain to happen in a ‘low dynamics’ polling-based program
  • When FIFO overflow occurs, the first packet is always corrupted, but the last one is still valid
  • The last (valid) packet isn’t the last [packetSize] bytes, due to the non-modular size of the FIFO
  • The MPU6050 DMP may start loading another packet at any time, but there will always be 10 mSec or so between packet loads
  • Any last-valid-packet retrieval algorithm must work for any loop delay time.

So, the ‘last-valid-packet-retrieval’ algorithm is something like this:

  1. Get the current packet count, using the ‘getPollingFIFOCount()’ routine above
  2. Extract [packetSize] chunks until there are less than 2* [packetSize]  bytes left. This ensures there is exactly one valid packet remaining in the FIFO.
  3. Extract one more [packetSize] chunk and return it as the result.

Note that implementation of this algorithm will require several ‘while’ loops, so there must also be provision for forcibly terminating all such loops in case some edge condition causes the normal exit condition to never be reached.  Homer and I have been creating and testing versions of this function for the last couple of weeks, without quite getting there yet.  Either they don’t handle all the edge conditions, or they are too slow as the loop delays get larger.

While I was testing my most recent concoction, I ran into a third major problem with the MPU6050 API.  I wanted to be able to remove up to 35 [packetSize] chunks (980 bytes) in one go, and I expected the mpu.getFIFOBytes() API call to manage the required chunking for me.  When I tried this trick, the getFIFOBytes() function crashed repeatedly.  Eventually I figured out that the reason it was crashing wa that it’s ‘length’ parameter is declared as a ‘uint8_t’ object and of course it couldn’t handle a value greater than 255 without choking.  I thought that was a little odd, but that maybe changing the declaration from ‘uint8_t’ to ‘uint16_t’ in MPU6050.h/cpp would solve the problem.  Nope – It turns out that, due to the way that the Arduino I2C bus operates, there is a limitation on how many bytes can be transferred across the bus in one operation, and this limit is currently set at 32 bytes.  As a result of this fundamental limitation, all the I2CDev functions that use the I2C bus also have the same underlying limitation and all of them have their length parameters declared as ‘uint8_t’. This reminds me of the old pre-scientific myth about the earth resting on the back of a turtle.  When the myth was challenged, the defender says “its no use – it’s turtles all the way down”.  In our case “it’s no use – it’s uint8_t all the way down!”.

So, I had to figure out a way around this problem, so I decided to create yet another wrapper function, this one cleverly called ‘getManyFIFOBytes()’. The idea for this would be to pull one [packetSize] chunk off the FIFO at a time using the normal ‘mpu.getFIFOBytes()’ call and place the result in a destination buffer large enough to hold the entire result.  Since it has been a (long, long) while since I last played with pointer gymnastics, I decided to write a short test program to figure out a reasonable technique.  Here is the program, and some results:

As the output shows, the ‘getManyFIFOBytes(uint16_t* buffer, uint16_t len)’ function can take an arbitrary ‘uint16_t’ length parameter and fill the destination buffer with [packetSize] (28 bytes in this case) chunks, followed by the non-modular remainder.  Although this test was done with a simulated receive buffer and simulated packet contents, I believe it will work using the actual contents of the MPU6050 FIFO and repeated calls to ‘mpu.getFIFOBytes()’ to retrieve the ‘chunks’ and any non-modular remainder bytes.

Having convinced myself that my two helper functions actually did what I wanted, I revised my latest MPU6050 test program (MPU6050_Overflow8.ino) to test the whole thing out. The test program steps through a series of loop delays from 50 to 550 msec, and takes 25 yaw measurements at each loop delay setting.  The Excel plot and a snippet of the output log from the program is shown below

  • There were no invalid packets in the entire run, so the attempt to avoid invalid packet retrieval was a success.
  • The actual loop delay per measurement pass varied quite a bit from the desired loop delay setting.  For instance the average measured loop delay for the 25 yaw measurement passes at the 200 mSec loop delay setting was actually 274.08 mSec, almost 50% higher than desired.  At the 150 mSec loop delay setting, the average loop delay was 223.88 mSec, and at the 100 mSec setting it was 133.24 mSec.  So, if the application needs 5 measurements/sec, the allowable loop delay between passes needs to be between 100 and 150 mSec.

12 November 2019 Update:

Based on some comments and data from Homer’s experiments, it appears that a FIFO reset can be done in less than 10 mSec.  This means that a packet retrieval algorithm based on a mpu.resetFIFO() call will miss at most one 10 mSec FIFO load interval, which is insignificant compared to the typical polling interval (200 mSec in my case).  So I decided to try a ‘brute force’ approach to ‘GetCurrentFIFOPacket()’ as follows:

When I first tested this algorithm, I ran into an occasional glitch where the FIFO would somehow fail to reset, resulting in a FIFO count > 28. So I added an outer loop to allow multiple shots at getting a clean FIFO reset.

Shown below are some Excel plots from some long runs

 

The first plot above shows a long run (over 17 minutes) of valid yaw data (the perturbations in the yaw plot are due to occasional manual rotations of the sensor to verify that the sensor was still responding).  The interesting thing about this plot is the yellow curve, showing the ‘outer loop’ count. The only way this value can be greater than 1 is if the first mpu.resetFIFO() call fails to actually clear the FIFO, which appears to happen an average of about once per minute, or about once every 6,000 10 mSec MPU6050 FIFO cycles.

The second plot is a closeup of the first 171.42 seconds of the overall plot, showing the detail of the FIFO clear failures, occurring about once per minute.

So, it is clear that the MPU6050 has some significant behavioral quirks, especially when used in a non-interrupt-driven environment.  That being said, I believe the ‘brute force’ algorithm shown above is a reliable way of interfacing with the MPU6050 in a polling environment, and obviates the need for a separate interrupt line, and the associated ISR software.

This will probably be the last update on this subject, as I now think Homer and I have pretty much beat the MPU6050 FIFO issue to death.

Stay tuned!

Frank

 

 

 

 

 

 

 

 

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12 thoughts on “MPU6050 FIFO Buffer Management Study”

  2. eddy HODAK

    Frank, what an impressive write-up of the even more impressive detective work that you and Homer Creutz have done. I do hope that many Arduino users find this and read it as the methods you use are so very thorough. As a retired experimental physicist, I felt totally at home reading your “research notebook” of work done.

    I am attempting to use the MPU6050 also and thought that using the onboard DMP would permit very fast computational modes. However, the FIFO overflow issue that I am also experiencing seems to require significant slowing (using delay()) to avoid. It seems that this obviates the desire/opportunity to process faster, yes/no? For sure, with your code and my implementation on PlatformIO on VSC (requires a repositioning of the subroutines ahead of the setup() and loop() routines) will be my next challenge. However, I am likely to first work directly with your code on Arduino IDE to make sure all is OK and that I have all the libraries installed.

    One real discovery: I was completely unaware of Print_Ex library which you use to real advantage. I will get this sorted also. If nothing else, this discovery was worth it. Actually, just reading your blog with its completeness made my day. Well done, mate.

    Reply ↓
    1. paynterf Post author

      Thanks for the kind words – I think experimental physicists and engineers are cut from the same cloth; both have to be very careful about jumping to conclusions and seeing only the data that matches preconceptions ;-).

      Regarding the MPU6050 speed – I typically run mine at >=5Hz, and I know others run them much faster than that. I do recommend you work directly with my code first to establish that all-important experimental baseline. Once you have some confidence with the baseline, then feel free to expand on the code in your preferred direction

      Reply ↓
  3. Jack Hohner

    I have studied your code extensively. Good job on such a thorough trouble shooting. I have been wrestling the fifo overflow problem for a few years. My device, which is in production, is working good. However, I still have the annoying overflow, which causes almost no glitches. My data output spike is averages about once a minute, which I noticed in your last report was about what you were seeing after your last attempt to resolve the problem. My hope is that you have since developed or uncovered an effective solution to the overflow problem? Thanks again for your extensive efforts and methodology.

    Reply ↓
  4. Jack Hohner

    A follow up to my last post. I have been trying to remove those once a minute spikes with a simple “if” statement. If current ay > ay+ constant, then ay equals last ay. My spikes are typically a factor of 10 to 50 times the value of the current ay output. So my constant can be any value above 2 or 3. I’ve tried many. I think my problem is my accel output is happening faster than my if statement. Although I would think it would be in succession. Thoughts are welcome.

    Reply ↓
    1. paynterf Post author

      Jack,

      I haven’t done any more work on this since this post, but I have been using the following ‘GetCurrentFIFOPacket()’ routine since that time (over 3 years ago) without any problems. Of course, YMMV 😉

      uint8_t GetCurrentFIFOPacket(uint8_t* data, uint8_t length, uint16_t max_loops)
      {
      mpu.resetFIFO();
      delay(1);
      //int countloop = 0;

      fifoCount = mpu.getFIFOCount();
      GetPacketLoopCount = 0;

      //myTeePrint.printf("In GetCurrentFIFOPacket: before loop fifoC = %d\t", fifoCount);
      while (fifoCount < packetSize && GetPacketLoopCount < max_loops) { GetPacketLoopCount++; fifoCount = mpu.getFIFOCount(); delay(2); } //myTeePrint.printf("In GetCurrentFIFOPacket: after loop fifoC = %d, loop count = %d\n", fifoCount, GetPacketLoopCount); if (GetPacketLoopCount >= max_loops)
      {
      return 0;
      }

      //if we get to here, there should be exactly one packet in the FIFO
      mpu.getFIFOBytes(data, packetSize);
      return 1;
      }

      Reply ↓
  5. Jack Hohner

    Paynter;
    Thanks so much for getting back to me. I will play with your solution and see if I can finally get rid of that pesky fifo overflow.
    I did have some success with clipping those spikes with my if statement. It turns out I had to add a 10 ms delay for it to successfully update my roll variable before it got to my condition statements that fire my LED’s. However, a bandaid fix. I would certainly prefer solving the fifo overflow problem.

    Reply ↓
  6. Gary

    Great story! Love the way you’ve documented your journey – warts and all. Your posts are very insightful, and has inspired me to offer some suggestions on how to improve stability and reliability from your readings….. From previous experience, I can say that you really don’t want to wait for a buffer overflow to occur, before you react. Even reacting 1-2 frames before an overflow mightn’t be enough…… if you’re still interested in moving this journey forward, I’d be interested to hear what the results are if you react 3-4-5 frames before a potential overflow, so you have enough CPU time to send your results over the Serial UART, and process the next batch of FIFO entries.
    Happy to contribute towards this as I find it really interesting.

    Reply ↓
  7. Julian

    Thanks for this great study. Currently I’m trying to implement the results of this study to a microPython lib for the MPU6050. During the readout of FIFObuffer I sometimes get invalid data without an FIFO overflow (might caused by switching of the order of data storage – due to this I think it might be a problem with readout the fifo_count – race condistion bewteen readout and internal storage of the data in FIFO).
    That’s why I want to test implementing the FIFO_CountPolling, but I don’t unterstand your routine:

    int GetPollingFIFOCount(int max_tries)
    {
    long int startUsec = micros();
    int ctr = 0;
    int lastFIFOCount = mpu.getFIFOCount();
    int fifoC = mpu.getFIFOCount();
    while (fifoC != lastFIFOCount && ctr < max_tries)
    {
    ctr++;
    lastFIFOCount = fifoC;
    }
    long int endUsec = micros();
    return fifoC;
    }

    In the while-loop you don't readout a new FIFO-Count with mpu.getFIFOCount(), so the loop ends after one iterations, because in the loop you write fifoC in lastFIFOCount so the loop condition is False after one iterations.

    Maybe it is my fault, but I don't get it.

    It would be great if somebody has a explanation or solution for that! Thanks and kind regards

    Reply ↓
    1. paynterf Post author

      Yep – I agree that loop is probably defective. However, I don’t think it is used in the final configuration. Here is the ‘GetCurrentFIFOPacket() function as used throughout my various robot programs, and AFAICT, it works all the time every time. YMMV 🙂

      uint8_t GetCurrentFIFOPacket(uint8_t* data, uint8_t length, uint16_t max_loops)
      {
      mpu.resetFIFO();
      delay(1);
      //int countloop = 0;

      fifoCount = mpu.getFIFOCount();
      GetPacketLoopCount = 0;

      //gl_pSerPort->printf(“In GetCurrentFIFOPacket: before loop fifoC = %d\t”, fifoCount);
      while (fifoCount < packetSize && GetPacketLoopCount < max_loops) { GetPacketLoopCount++; fifoCount = mpu.getFIFOCount(); delay(2); } //gl_pSerPort->printf(“In GetCurrentFIFOPacket: after loop fifoC = %d, loop count = %d\n”, fifoCount, GetPacketLoopCount);

      if (GetPacketLoopCount >= max_loops)
      {
      return 0;
      }

      //if we get to here, there should be exactly one packet in the FIFO
      mpu.getFIFOBytes(data, packetSize);
      return 1;
      }

      Reply ↓
    1. paynterf Post author

      Anthony,

      Don’t know what to say – I’ve been using an MP6050 in my robot for several years now with no problem at all. It’s hard to troubleshoot your problem with just such a short note, but you may be under the impression that overflows never happen. They *do* happen, but the process I described in my post handles that condition properly. Hope this helps.

      Frank

      Reply ↓

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