Tag Archives: PIC32MX220

Ultrasonic Anemometer Part 23: First successful measurements

In my last post I was happy to report that I managed to get the USB interface to work. This interface has since proved to be extremely valuable in software development and testing. While the device is taking measurements you can look at the results (or intermediate results) at your PC in real time. You can even log large amounts of data to a .csv file and inspect the results in Excel.

20160514_StandaloneAnemometer_049

But let’s look at the developements in a bit more detail. Last time the device was sending pulses and capturing some of the resulting zero-crossings but not much more than that. And in a not very structured way.

Measuring amplitude

Now it’s not only capturing the times when the zero-crossings occur but also measures the amplitude of the half-waves in between. In order to do that an interrupt is generated at every zero-crossing capture. Inside that interrupt service routine time of the zero-crossing is saved (as previously) and the ADC is started. In order to measure the amplitude at its maximum (or minimum in case of a negative half-wave), the ADC first samples the input for a 6.25 microseconds (which corresponds to a quarter-wave at 40kHz). I’ve chosen the sampling period slightly shorter to compensate for the time from when the zero-crossing occured unil the ADC is started. After the sampling period the internal sample-and-hold amplifier switches from sample to hold and the actual analog-to-digital conversion takes place.

The PIC’s datasheet advertises a maximum sampling rate of 1 million samples per second. Beware, though, that this requires separate Vref+ and Vref- connections and doesn’t work in differential mode anyway. In my configuration without separate analog references and differential measurements the maximum sampling rate drops to 400k samples per second. And the reported conversion time doesn’t include the sampling time yet. I wasn’t aware of that but luckily the PIC’s ADC is still more than fast enough for what I’m doing here.

20160428_StandaloneAnemometer_028

Finding the absolute phase

Capturing the zero-crossings is relatively easy and the results are very precise. But there’s a challenge: there are hundreds of zero crossings for each burst of pulses sent. How do you know which zero-crossings to use?

The Arduino-based anemometer used an analog envelope detector to solve this problem. After averaging plenty of samples this works most of the time. But even after many attempts to tweek the analog circuit the envelope detector approach was never as reliable as I wanted it to be.

My standalone anemometer has an entirely different approach. Being able to digitally measure the amplitude of each half-wave we can tackle the challenge in software. We have a series of measurements and we have an idea of that the signal we are looking for looks like. Looking the scope screenshot below you could easily and reliably determine where the maximum amplitude occurs. That’s what we need to teach the software to do.

ZCD_Capture

One could, of course, just loop through the all the amplitudes and find the maximum. But there are a lot of similar amplitudes and so any noise would make the result unreliable.

I did a bit of reading on digital signal processing (DSP) and realized that this is a classic DSP problem. The solution is a matched filter. You take the signal you are looking for (also known as kernel or template) and multiply it sample by sample with the captured signal and sum up the results. Do that for every possible (or reasonable) overlap of signal and kernel and find the position where the result reaches its maximum.

If you’re new to the subject (like I am), Steven W. Smith’s Digital Signal Processing is a great place to start. It’s even available online for free at dspguide.com. The method is also referred as convolution or correlation depending on who you ask. This wikipedia article gives a nice introduction to the subject.

Implementing the matched filter

I’ve parameterized the software to capture 34 zero crossings as well as the n=33 amplitudes in between them. The kernel consists of k=17 data points with a peak in the middle.

There are n-k+1=17 possibilities to entirely overlap the kernel with the signal. We can skip all the odd numbers where a negative kernel value would be multiplied with a positive sample value and vice versa. This leaves us with 9 possible positions to chose from.

For each position we need to do 17 multiplications and 16 additions. And we need to do that for every single measurement, i.e. 512 times per second. That sounds like a lot of work and it probably is. Luckily, since this is a very common DSP task, microcontrollers like the PIC32 have a special instruction for this. It’s called MAC – Multiply Accumulate and executes in a single clock cycle. The result is just amazing. The corresponding ISR takes less than 30 microseconds, including some housekeeping tasks as can be seen in the screenshot below.

Once the maximum amplitude is found, the 16 zero-crossings around it (i.e. the 8 before and the 8 after) are summed up and the sum is saved as the result of that measurement.

ConvolutionTime

Averaging measurements

The goal is to report wind speed and temperature four times per second. Since there are 512 measurements per second we can use up to 128 individual measurements  (32 in each direction) to do our calculations.

So we have plenty of data to identify and exclude outliers and do some averaging of the remaining samples. Robust statistics is the key word here, click here for the corresponding wikipedia article.

For now I’m doing something reasonably simple: First the 32 results per direction are sorted. The 12 lowest and 12 highest results are ignored and only the 8 results in the middle are summed up.

This might seem wasteful but it makes the average very robust against outliers and still results in 16 x 8 = 128 zero-crossings being averaged. The resolution of each zero-crossing is equal to 1 / 48MHz or about 20.83 nanoseconds. Summing up 128 of them gives us a resolution of far below a nanosecond. Beware however that resolution doesn not imply accuracy.

As a last step, the resulting sum is multiplied with 125 and then divided by 768 to make the unit 1 nanosecond. So every 250 milliseconds we finally have 4 time-of-flight measurements with a resolution of 1 nanosecond. That’s what we will use to calculate the winds speed as well as wind direction and temperature.

This sorting and averaging step is a bit more costly in terms of computation time. It takes around 600 microseconds to complete (see below). But unlike the convolution, it only has to be performed 4 times per second so this is more than fine.

CalculatingAverageToF

Calculating wind speed

We are finally in a position to calculate the actual wind speed. There are various ways of doing this. For now I’ve just done the simple approach of taking the difference in time-of-flight, assuming room temperature and solving for wind speed. This means solving a quadratic equation for which I have resorted to floating point math and using <math.h> for the square root function.

I don’t have my wind tunnel any more so doing any testing was difficult. But one thing was soon obvious: at zero wind speed, the time-of-flight measurements match extremely well and are extrordinary stable from measurement to measurement. Also, looking at the intermediate results (i.e. the 128 single measurements before averaging) shows that there are basically no outliers present. I could randomly pick a measurement and still get a very reasonable value.

Something seems to be systematically wrong with the first of the 128 measurements but I haven’t had time to look into this. Otherwise, the results are impressively stable. And I’m only using a relatively primitive kernel for the matched filter.

USB data logging

As I’ve mentioned at the beginning, the USB interface lets me do some serious data logging even at this early stage of development. Here’s a list of what I can do by sending a character to the device from a USB terminal.

  • ‘Z’: Get all the 34 zero-crossings of a single measurement
  • ‘A’: Get all 33 amplitudes of a single measurement
  • ‘F’: Get a full single measurement, i.e. 34 zero-crossings plus 33 amplitudes
  • ‘T’: Get all the 4 x 32 = 128 time-of-flights for a measurement series
  • ‘R’: Get the 4 averaged time-of-flights as well as wind speed and temperature

Some versions of these commands also let you specify the direction you’re interested in as well as how many sets of data you want to receive. This makes it easy to log large amounts of data that you can use to test possible algorithms on your PC before you implement them on the PIC.

20160428_StandaloneAnemometer_025

Next steps

The next step is to get the I2C digipot working so I can control the amplification in software. My idea is to aim for a maximum amplitude of around 3 volts independent of wind speed and so on.

There’s also plenty of work to do to improve the algorithm that calculates wind speed and temperature. And then I also have to implement the I2C and SPI interfaces that let the anemometer communicate to other embedded devices like an Arduino or Raspberry Pi.

Having used floating point math and <math.h> I’m also running out of flash memory. I’m currently using 93% of flash (32kB total) and 52% of ram (8kB total). There will be a slightly revised board (Rev B) that uses a PIC32MX250 which is identical to the MX220 used here but has four times as much flash and ram.

That’s it for today. On the overview page you find the software at the stage of developement just described as well as some examples of logged data (all at zero wind speed) as .csv files.

The next post on this project is here: Part 24.

Ultrasonic Anemometer Part 22: USB up and running

20160514_StandaloneAnemometer_047

Last time I showed you the nice new hardware of the new standalone ultrasonic anemometer. But at that time I had hardly any software written for it so I couldn’t do much with its 32 bit microcontroller. So the last two or three weeks I spend lots of time writing code that I’d like to share with you today.

20160428_StandaloneAnemometer_039

As I mentioned last time I worried most about getting the USB working properly. All the bit-fiddling with timers, PWM modules, input capture modules and the like can be time consuming and at time even frustrating. But I have all the confidence that I finally get what I aimed for. It might take a few nights digging through data sheets but in the end it will almost definitely work.

Now with USB I don’t quite have that level of confidence. The USB specification is quite overwhelming and there are almost countless registers to properly set and USB descriptors to specify. Without some sample code I’m pretty much lost I must admit.

Outdated Application Notes

So I was happy to see that Microchip has published a number of application notes and accompanying software. A handful of .h and .c files that you can include in your project, change some settings to suit your particular application and you’re ready to go. At least that’s what I thought.

20160514_StandaloneAnemometer_048

All those application notes were published around 2008 and the code they are referring to is nowhere to be found nowadays. Aparently, Microchip has been quite pushy trying to get their PIC32 customers migrate to their new library or rather framework called Harmony. MPLAB no longer even includes the the PLIB peripheral library everyone has been using for decades. Microchip has depreciated it and while you can still download it they make very clear that Harmony is the library of the future.

On the road with MPLAB Harmony

So faced with few other alternatives I turned to the aforementioned Harmony library. It’s many hundred megabytes to download and takes up almost two gigabytes of disk space. It integrates nicely into MPLAB X and so I created a first project. You can graphically configure the clock and pin settings so I did that. Clicked on ‘create code’ and was nothing less than shocked by the code I got.  Around a dozend source files scattered around in a dozend deeply nested folders and subfolders. And that was not yet it. Those files referenced dozends more files that came as part of harmony. It took me quite a while to just more or less figure out what the project was doing. And it didn’t do anything yet except setting some clock and port settings. Just a nightmare.

20160514_StandaloneAnemometer_045

But I felt I had few other options so I continued with harmony anyway. And after a few hours I had an almost working USB device. It enumerated just fine but I couldn’t send any data from or to it. It took me more than a week to finally get it working. In the end it was just a single character: a 1 instead of a 2 in one of the configuration files. But until then I had spent a lot of time with that harmony code and was forced to read through a lot of its documentation which is worth thousands uppon thousands of pages of PDFs.

So USB was finally working and I had acquired some rudimentary understanding of MPLAB Harmony. I still hated the whole framework but I thought that I now understand it well enough to change the code to suit my taste. So I spent another few nights trying to do that until it dawned on me that this is leading nowhere. Fortunately, by that time I had read and learned some more about Harmony and was willing to give it another try working with it as opposed to against it. So for the last week or so I was doing that.

And while I still don’t love everyting about it I now feel comfortable working with that library. I’ve tried to follow its conventions and recommendations even where they didn’t make too much sense to me. I think this will make it much easier for others to work with my code and chances are that the conventions and recommendations will start to make more sense as I get more familiar with the framework. And its nice to know that I can integrate another module or migrate to another microcontroller without having to re-invent everything.

20160428_StandaloneAnemometer_037

That was a long introduction but after all that pain the last few weeks I just had to write that down. Now to business.

USB

The anemometer inplements the USB HID (human interface device) class. While mainly aimed at devices such as mice and keyboards, this USB device sees a lot of use in applications that have nothing to do with human interaction. You can transfer up to 64 kbytes of data (64 bytes every millisecond) in each direction with guaranteed latency and that’s plenty for many applications. That data can be anything and in any format you like. And the good thing is that you don’t need to write your own drivers. You can free-ride on the device drivers that came with the operating system just like you don’t need to install a driver when you attach a new keyboard to your computer.

I’ve written a bit of code on top of the Harmony library to provide the main application with a simple to use interface. It nicely encapsulates the the USB logic and data and adds a bit of buffering functionality on top of that. It’s just two files: app_usb.h and app_usb.c in the app_usb subfolder. You can basically include them in your project and they handle all the USB stuff for you.

Once initialized, the necessary code runns inside an interrupt service routine (ISR) so the main application has nothing to worry about. All the USB data including the data buffers are private to the app_usb.c file. The app_usb.h defines just 8 simple, non-blocking functions:

  • void app_usb_init(void);
  • bool app_usb_isReady(void);
  • uint8_t* app_usb_getReceiveBuffer(void);
  • uint8_t app_usb_getReceiveBufferCount(void);
  • uint8_t* app_usb_getTransmitBuffer(void);
  • uint8_t app_usb_getTransmitBufferCount(void);
  • void app_usb_freeReceiveBuffer(void);
  • void app_usb_sendTransmitBuffer(void);

At system initialization you call app_usb_init(). But you can’t expect to have an USB connection. You need to check app_usb_isReady() every time. You never know if there is a USB cable plugged in and even if you had a connection that cable might be unplugged at any time. The module handles all that hot plugging/unplugging gracefully.

20160428_StandaloneAnemometer_038

The module implements a circular buffer for both received packages (aka frames) and packages to be sent. Each package is 64 bytes in size and the ring buffers can be set at compile time but must be a power of 2. Currently the receive buffer consists of 4 frames and the transmit buffer of 8 frames.

The app_usb_getReceiveBufferCount() informs the caller how many new USB frames have been received if any. app_usb_getReceiveBuffer() then returns a handle to the first received frame. FIFO – First In First Out. Once the frame is no longer needed the application calls app_usb_freeReceiveBuffer() and the buffer can be re-used.

Sending data works similarly simple. app_usb_getTransmitBufferCount() returns the number of transmit buffers currently in use. If this  number is smaller than then number of buffers then more data transmissions can be scheduled. Get a handle to then next buffer by calling app_usb_getTransmitBuffer(), write the data to be sent to the buffer and then call app_usb_sendTransmitBuffer().

Taking measurements

This way the main application can focus on more interesting tasks such as measuring wind speed and direction. The first task for doing so is sending some pulses.

SendReceiveOverview

If you’ve alredy followed this project for a while you are probably familiar with the kind of screenshot above. Bursts of a PWM signal at 40kHz (SIGNAL, red) are sent trough ultrasonic transducers. Note that the signal is now active-low (i.e. inverted) since the mosfet drivers used to drive the transducers are themselfs inverting.

As before, the DIRECTION  and AXIS signal determine which transducer is transmitting and receving. The SCLK and MOSI signals are just used for debugging purposes for now. They indicate when an ISR is running which helps a lot since in this example ALL the code is running inside an ISR.

TimeZero

As you can see above, the AXIS and DIRECTION signals are set a few microseconds before we start sending pulses. The first edge of the burst occurs when the 32 bit-timer (consisting of the 16-bit timers TMR2 and TMR3) is cleared. This is the point in time all our measurements are referenced to. The timer runs at the full CPU clock of 48Mhz so the resolution is about 20.8 nanoseconds.

SendReceivePWM

Output Compare module 2 (OC2) handles the PWM pulses. Currently each burst consists of 12 pulses. At 48MHz a full wave equals 1200 clock cycles as opposed to 400 on the Arduino previously used. Note that the frequency of the pulses is precisely 40kHz as expected and that very little time is spend in ISRs despite the unoptimized C code.

SendReceiveCloseUpWithComments

The timer overflows precisely 512 times per second so each measurement takes a little less than 2 milliseconds. Output compare module 1 (OC1) can be used to generate an interrupt at any time during that interval. Currently it does three things: It takes care of the AXIS and DIRECTION signals which are set just shortly before the timer overflows. It also triggers the measurements of the zero-crossings and cancels them if they don’t complete withing reasonable time.

ZCD_Capture

The input capture module 4 (IC4) captures the zero-crossings of the received and amplified signal. The board contains a high speed comparator to detect these events so the microcontroller only needs to measure the precise time whn they occur. The IC4 module has an internal buffer so we don’t need to generate an interrupt after every capture. I’ve currently set things up so that an interrupt occurs after 4 values have been captured up to a total of 32 measurements.

Commicating the results over USB

In order to find the wind speed and direction we need to somehow identify the peak in the received signal which is where the ADC comes in. But I think this is enough for now and I instead show you how the measurements taken so far can be sent to a PC via USB.

The board doesn’t have any display or anything to communicate with a human. There’s not even a status LED we could blink. That would have been nice but there are just not enough I/O pins unfortunately. So connecting the board to a PC via USB is a straight-forward way of communicating with this device.  Actually, USB will be the main testing and debugging tool during development. The in-circuit debugger isn’t very useful in this application since the measurements have to happen in real time.  Setting breakpoints will just mess things up.That’s why I insisted to get USB working first.

I’ve downloaded a simple terminal called YAT (yet another termial) that allows me to easily talk to the anemometer board. The anemometer looks at the first character in any message sent from the host and then decides what to do. So far I’ve implemented:

  • ‘T’: Return the current value of the 32-bit timer
  • ‘Z’: Return axis (0/1), direction(0/1), measurement count (0-31) values as well as the results of the first 9 zero-crossing measurements
  • Anything else: Echo back the received string

Here’s what my chat with the Standalone Ultrasonic Anemomete looked like.

YAT_Capture

Of course, the measurement is far from complete at this stage but I think this is a nice foundation to build uppon. The code is quite clean for this early stage of development and being able to communicate with the device in real-time over USB is really a great advantage.

The software can be downloaded on the project over view page and as always I appreciate any feedback.

The next post on this project can be found here.

Ultrasonic Anemometer Part 21: Standalone Anemometer Hardware

Last time I went through the design of my new standalone anemometer. Now it’s time to build this thing and see if it works as planned.

20160428_StandaloneAnemometer_034

After I fried a couple of chips on my driver circuit testing board due to a wrong chip in the power supply I was a bit more careful this time and built up the board step by step.

20160426_StandaloneAnemometer_016

Only after I confirmed that the power supply was ok I dared to solder some more.

20160426_StandaloneAnemometer_019

The next step was to add the PIC32 with the crystal, the programming header and all the caps they need. This is a chip family that I’ve never used before so I wanted to first see if I can program it. All went well and I managed to get it to run on the crystal’s 8MHz boosted up to 40MHz by an internal PLL. So I was ready for the rest.

20160428_StandaloneAnemometer_031

I wrote some very basic software and confirmed that at least the basics were working ok. I was able to send and receive pulses, the pulses got amplified, the zero-crossing detector worked and so forth.

20160428_StandaloneAnemometer_024

As mentioned, I’m entirely new to the PIC32 microcontroller series. There are a lot of similarities to the PIC16 and PIC18 series that I’m quite familiar with but still it’s always a challenge to work with a new family of chips and the tools that come with it. I took me the better part of an afternoon to master the vectored interrupts with the different priority levels and so on.

20160428_StandaloneAnemometer_040
Driver circuit (front) and standalone anemometer (back) side by side.

By the way, with this project I’m using the free MPLAB X IDE with the also free XC32 C compiler from Microchip. So anyone is able to write, modify or compile code for this thing with free software. At least at the moment you need a programmer to actually burn the chip. But the PICkit3 only costs around 50 dollars and my idea is to write a USB bootloader so that any user can modify the software of a pre-programmed board.

20160514_StandaloneAnemometer_050

So now comes what I think might be the hardest part: Getting the USB to work. I’ve spent quite a few hours so far but haven’t managed to get it working properly yet. If anyone has experience with this kind of software development – Let me know, any help is highly appreciated.

It now works: Click here to view it.

Ultrasonic Anemometer Part 20: Standalone Anemometer Design

Last time I outlined my reasons to ‘go digital’ by adding a powerful on-board microcontroller and designing a standalone wind meter.

20160426_StandaloneAnemometer_001

In the weeks that followed that decision I tried to find a suitable microcontroller and to design a prototype. Today I’ll show you the result of that work.

20160426_StandaloneAnemometer_003

I looked at various series of microcontrollers from different manufacturers and finally decided to go for the Microchip PIC32 series. It offers everything I could possibly ask for: 32 bit architecture, inexpensive, vectored interrupts, integer division, any interface you want (depending on the model of course) , available in large, low pin count, hobbyist friendly packages and so on.

20160426_StandaloneAnemometer_011

As you can see above, the model chosen for my prototype is a PIC32MX220. This is low to mid-end representative of this series but even so the specs are quite impressive. CPU clock up to 50MHz, one instruction per clock cycle, full-speed USB 2.0, 32kB flash, 8kB RAM and all of that in a SOIC28 package at a price of CHF 3.58 in single quantities.

20160426_StandaloneAnemometer_005

After having chosen a chip the next task was to come up with an actual design. I took my anemometer driver design and tried to integrated the new PIC32. That driver circuit had performed extraordinary well so I changed very little. I left away the RELEASE signal since my tests had shown it to be unnecessary. I also replaced the LM5111-1M mosfet drivers by LM5111-2M. The difference is that the 2M is inverting while the 1M is not. The reason for changing this is because the 2M is available at a significantly lower price of CHF 1.35 vs CHF 2.25. Not a big deal if you just build a single prototype but I thought it might be smart to change this anyway. This also required some resistors to be changed from pull-downs to pull-ups. Except these details everything stayed the same with the drive circuit.

20160426_StandaloneAnemometer_012

I also had to re-design the power supply since the PIC needs a 3.3 volt rail as opposed to 5 volts in the driver test circuit. Since I had to re-design it anyway I also downsized the power supply somewhat. I’ve otherwise resisted all temptations to use smaller components but the power supply was just a bit too big with the two  SOIC8 chips and four size C tantalum caps.

The prototype now uses an MCP1755 linear 3.3V regulator in a SOT23-5 package with a 33uF size B tantalum cap at its input and a 10uF 0805 size ceramic cap at the output. A TCM829  (also in a SOT23-5 package) and two more 10uF caps produce the -3.3 volts output. So there is a total of three supply rails: +12V, +3.3V and -3.3V.

20160426_StandaloneAnemometer_006

A major challenge was to make do with the very limited number of I/O pins on the PIC. 28 pins seem like a lot for the task at hand but plenty of them are already used for things like supply voltages and the like.

In the end I managed to still get three independent interfaces to the outside world: USB 2.0, I2C and SPI. All these interfaces as well as just about any other signal of interest is easily accessible from one of the numerous 100mil headers along the edges of the board.

20160426_StandaloneAnemometer_007

The amplifier still uses a LMC6482 dual op amp , now running on a+/- 3.3 volt supply. The first stage amplifies the ground-referenced input signal by a fixed factor (currently 11 but this might change). The output signal is then biased to 1.65 volts and amplified once again. This time the gain can be adjusted digitally via a MCP4561 I2C digipot.

20160426_StandaloneAnemometer_008

Before I forget: The PIC gets its system clock from a 8MHz crystal. That might seem low but there are two (variable)  PLLs inside the PIC that (together with a number of pre- and post-scalers) let us produce the 48Mhz signal needed for USB as well as a reasonable clock speed to run the CPU and peripheral bus on (probably also 48MHz but we’ll see).

20160426_StandaloneAnemometer_013

The board layout proved to be a bit tricky because there isn’t that much space on the board and I had quite clear ideas where I wanted certain headers to be. So I was more than happy when all the traces were finally laid out.

As you can see, the PCB is already milled and the vias soldered. I can’t wait to build this thing up and start to do some programming. That will be the topic of my next post.

20160426_StandaloneAnemometer_014

As always, a zip file with the eagle files and PDFs of the schematic and board layout can be found on the overivew page.

And here‘s the finished board.