Electronic – Arduino – interrupt based decoding of serial data, race between ISR and loop()

arduinointerruptsRF

Trying to decode serial data (encoded using proprietary method) from a RF module.

Having switched back and forth between the fast-sampling (4x the fastest freq) and of late an interrupt based method, I've reached a dead-end and not sure what all I am doing wrong, because apparently people have been successful in decoding ISM-band RF data (Manchester, NRZ, VirtualWire and HT12E/PT2260-62 encoded), in software.

In my case, the RF modules's AGC (which I've no idea how to turn off, since there's no data-sheet, and the co. doesn't respond to emails… a cheap east-asian make) picks up noise and has the fastest level-transitions in durations ~40us, although valid (encoded data) transitions not lesser than 400us. My ISR has an elementary task of measuring the duration of last state to reject the spurious noise, and save only those transitions that could possibly be part of the encoded data. So far, I have not used a circular-queue (which is what I plan as the next step), but before doing that, I did a small experiment. In the ISR I check the state of a flag to see if it is true, and if true I do Serial.print("x"), else I set it true. In the loop(), I check if flag is true, then do Serial.print("-") and set it false. The flag is declared volatile. I find that I get very long continous streams of "x"s and very rare "-". My ISR is triggering on every transition of pin-2 (i.e. external interrupt#0).

Now my question is, would adding a queue really solve my problem, because apparently the ISR is running far too frequently and thereby starving my loop() of cycles. Counting the distance between the "x"s and "-", I was hoping to find the ideal queue length, but I can't figure out a pattern. Is there something inherently wrong in my approach ?

An outline of the source code is here.

Best Answer

You probably want to take the transmission time (and software UART management methods) of the Serial.print() routine into account.

If you have access to a scope, toggling I/O pins would be a lot faster. You could toggle one in the ISR and the other in the loop.

Or you could increment a counter in the ISR and sample/clear it in the loop (inside an interrupt disable/re-enable) then print it (after re-enabling interrupts). More sophisticated would be to only print the number if it exceeds the previous maximum.

However, such tests are fundamentally flawed unless the testing code constitutes a small fraction of the loop operations - just figuring out how many interrupts can occur during a max-check/print routine is meaningless in terms of the ultimate operation which would presumably be doing something else in the loop.

Generally, you will want to use circular buffering if you are acquiring data via an interrupt which must be aggregated into a larger assembly (bits into bytes, or bytes into buffers) and then acted upon in a way that is at all time consuming. It shouldn't be too hard to change your buffer size later to optimize memory consumption, especially if you keep them power-of-two sized. You may want to build in overflow alarms, and try running the finished system with an artificially reduced buffer size.

Do pay very careful attention to ensuring that the ISR can't jump in the middle of where the loop trying to update the buffer's tracking variables - if you do a read-modify-write, you'll need to protect it.

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Electronic – arduino – Circuit to remove noise from digital communication line

When I have a design problem like this, I first think "how would I decode this by hand" and then I try to implement my thought process through use of electronics.

So, when I look at your two diagrams I see only one thing that my brain can use to determine what is data and what is noise, that is the width of the pulse. The problem with passive components in a situation like this is that for one you have a square wave that will have harmonics much higher then the cut off point for your filter. This will result in a not so square wave.

The easiest way to do this will probably be in software. You can still use interrupts, but it will take a bit more work. Essentially what you will have to interrupt on an edge, and then count time until the next edge. If the length of time is long enough then you can call it a logic 1 otherwise you can consider it noise on top of a logic 0.

If you must go with passive components you might want to consider a low pass filter followed by a comparator. You would have to play with the low pass filter to get it just right, but essentially you would aim to have the capacitor charge just high enough to cause the comparator to switch its output when the bit has been on long enough. What makes this difficult is that your "glitches" are very similar to your data and it is difficult to tune your circuit just right to get your data with no glitches. Even if you are able to perfect it on paper your components wont be very precise either. It is because of this that you should seriously consider doing this digitally.

Electronic – Arduino Uno SoftwareSerial and Serial conflict

I tried a voltmeter over GND and DI10 and while measuring it showed 0V.

That doesn't sound right. When a UART is not transmitting any data, it stays in the idle "1" state. I expected the wires connected to the Arduino to be so-called RS232TTL levels of +5V in the "1" state and close to GND in the "0" state. (d) When the UART is transmitting lots of data, a multimeter typically shows some sort of average voltage between the "1" state and the "0" state, bouncing around 2 V to 4 V. Perhaps a power or data line got disconnected or wired up wrong?

As a side note, if I remove the pinMode(rxPin, INPUT) line then nothing is ever received (and rf.available() is always 0).

That's very unexpected. Most Arduino documentation says things like "Arduino (Atmega) pins default to inputs, so they don't need to be explicitly declared as inputs with pinMode()." (a)

Some tutorials for SoftwareSerial suggest explicitly declaring the TX pins to be output. (b) Perhaps whatever is listening to "txPin" is picking up noise, making it do something unexpected?

Most Arduino tutorials seem to use 9600 bps for the hardware Serial uart. (c)

rf.available() is always > 0 (and also becomes 63)

How could you possibly know that? I'm beginning to suspect the code in your Arduino is some program other than what you posted.

What happens when you test exactly the same program, but with a known serial source? For example, rather than connect Arduino D10 (your SoftwareSerial rxPin) to the radio, instead connect D10 to Arduino D0 (the data you type in your PC's serial monitor), and type a few words. What happens then?

Maybe the SoftwareSerial works fine with normal UART data, but it can't handle the high-frequency glitches common in low-cost radio receivers. In that case, maybe it would work better to

(a) connect the Arduino hardware UART (D0 Rx and D1 Tx) to the radio, and the SoftwareSerial to your debugging serial monitor. Or
(b) use more sophisticated hardware that does clock recovery, etc. like the HopeRF RFM12B as used in the JeeNode and the Moteino, or
(c) use more sophisticated software, such as the protocol Roman Black invented and describes in "RF modules made easy" or the frequency-shift keying system developed by Tom Boyd in "Pulse train detection with an Arduino".

Some test code:

#include <SoftwareSerial.h>
#define rxPin 10
#define txPin 11
SoftwareSerial rf(rxPin, txPin);

void setup() {
  pinMode(rxPin, INPUT);  
  pinMode(txPin, OUTPUT);  
  Serial.begin(9600);
  rf.begin(2400);
  Serial.println("Hello, I was compiled " __DATE__ );
}

void loop() {
  if( rf.available() ){
    int incomingByte = rf.read();
    Serial.print(incomingByte, DEC);
    Serial.print(" ");
    Serial.println( rf.available(), DEC);
  }
}

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Electronic – arduino – Circuit to remove noise from digital communication line

Electronic – Arduino Uno SoftwareSerial and Serial conflict

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