I would like to use multiple (three) serial ports on an Arduino Due. Instead of polling the ports continuously, I would like to use a Serial.Event function for each of the ports. These handler functions will read out the incoming string from the port and parse the string (i.e.
Check for a pre-defined command and call a corresponding sub-routine). I was wondering what is going to happen when serial data arrives at the same time at two different ports?
For example, if the program is in the process of handling/parsing the first serial message, will it interrupt/abandon this process as soon as another SerialEvent on another port is triggered? Or will it complete the first SerialEvent routine before going to the second one?
Arduino Interrupt Service Routine
That should be fairly simple to implement. Each serial port has TX and RX interrupts.
You just need to tie into the various interrupts. For a mbed, but it should be similar (both use ARM NVIC interrupt controllers).
To be clear, this wouldn't involve touching the serialEvent function stuff at all, but rather implementing what it does using your own code. If you set up your own interrupt handlers, you also get the benefit of being able to control interrupt priority yourself. The ATSAM3X has 16 interrupt priority levels, and a interrupt of a higher priority can pre-empt a lower-level interrupt. I Strongly suggest you read the. It describes the interrupt controller, as well as the rest of the processor peripherals.
Skill Level: Intermediate by January 24, 2012 Interrupts! Interrupts, what are they? They are people that intermittently prevent you from doing your current work. Haha, well maybe. But we really want to know is what are they in the context of embedded electronics and microprocessors. So, what really is an interrupt in that context?
Well, there is a method by which a processor can execute its normal program while continuously monitoring for some kind of event, or interrupt. This event can be triggered by some sort of sensor, or input like a button, or even internally triggered by a timer counting to a particular number. We see the event, then what? So what happens when the event, or interrupt happens? The processor takes immediate notice, saves its execution state, runs a small chunk of code often called the interrupt handler or interrupt service routine, and then returns back to whatever it was doing before. How does it know what code to execute?
This is set up in the program. The programmer defines where the processor should start executing code if a particular interrupt occurs.
In Arduino, we use a function called to do this. This function takes three parameters. The first is the number of the interrupt, which tells the microprocessor which pin to monitor. The second parameter of this function is the location of code we want to execute if this interrupt is triggered. And the third, tells it what type of trigger to look for, a logic high, a logic low or a transition between the two. Let's look at a simple coding example to make more sense of this.
The AVR INT0.5 can trigger interrupts on edge transitions: on low, rising edge, or falling edge, or changing (both rise or fall). And available as noted above through attachInterrupt. Though not natively available from Arduino APIs,the ATmega 48, 88, 168, 328 family (and others I’m sure) provide pin change interrupt capability on other pins, labeled PCINT0.23. I found some code here if you want to make use of this capability within the wonderful world of Arduino.:) (I haven’t tested/tried the code myself) HTH.
Another thing to note is that there aren’t interrupt priority levels on AVR’s (and thus arduino). In effect, they are all blocking, as the default ISR behavior in AVR.h is to disable interrupts when entering an ISR and re-enable them afterwards. Meaning it is even more important that the ISR be as fast as possible, especially if you have other things running in the background that use interrupts (like PWM, for example).
Also, something to note, while your explanation is technically correct as to why interrupts can be superior to polling, you then state that the ISR should simply set status variables and return. While again also technically correct, it implies that you then need to poll these status variables to notice that there is a change, somewhat muddling your point. You might want to clarify that doing it this way will ensure that you never miss the change, even if you don’t process it immediately. Which is appropriate for the majority of cases, assuming that rate of polling the status variables and processing them is faster than the probable frequency of interrupts. But if something needs to happen RIGHT NOW as a result of the interrupt, it needs to happen inside the ISR. Just pointing out a possible point of confusion.
All MCUs have a priority mapping in memory related to interrupts, a timer interrupt will have a higher priority compared to a flag interrupt, but a lower priority than an IRQ. Consult user guides to determine the interrupt priority mapping. Lastly, it perfectly acceptable to have an interrupt do calculations. I have done a motor speed control design where I did the conversions in the interrupt function. Granted this was the only interrupt in the design, but it was effective. Another design was measuring gravity based upon dropping a steel ball down a tube and it breaks 3 IR sensors points, each of these sensors went to an OR gate that went to an IRQ pin. In this case, I had to only send data to global variable for reading.
If I remember correctly, for Freescale MCUs, there is an interrupt stack which will run. So if 3 interrupts occur at the same time, it executes them in order of priority. I could be wrong, so someone correct me. No, I was replying to you:) The biggest problem with nested, non-priority interrupts is the possibility of ‘torn writes’, be that to IO or to sections of memory. I’ll use a toy example: Say you have an interrupt which, when a pin changes states, reads in a value from the ADC and sets 4 output pins to some value. Say it is triggered, and writes to the first two pins.
Arduino Due Serial Communication
Before it writes to the last two pins, it is triggered again. The value on the ADC has changed enough to cause it to write a different set of values to the pin.
It finishes setting the outputs, then pops back to finish the first ISR. Except now once it writes to the last two pins, you have a set of outputs that doesn’t correspond to either the correct output for the first interrupt or for the second interrupt. Nested interrupts are useful with priorities (because, presumably, well written higher priority interrupts wouldn’t have interaction with lower ones), but are generally just bad news without them. Another thing to note is that there aren’t interrupt priority levels on AVRs (and thus arduino).
Actually, this isn’t true. There aren’t interrupt priority levels on the Tiny or the Mega, but there are on the Xmega (high, medium, and low priority). This essentially allows interrupts of a higher priority to take control from interrupts of a lower priority.
The awesome part? The Xmega implements round-robin interrupt scheduling in hardware. Dji phantom 3 drone professional setup. So let’s say we have four interrupts: A (low-level), B (medium-level), C (medium-level), and D (high-level). Interrupt A is triggered, but while it’s running, Interrupt B is triggered as well. Now, Interrupt B is running. Since it’s medium-level, it took priority over Interrupt A, which took priority over the standard application code. What if Interrupt D is triggered while Interrupt B is running?
The AVR will immediately switch to Interrupt D because it’s high-priority and Interrupt B is medium-priority. Now, let’s say Interrupt C is triggered while Interrupt D is running. Interrupt C is a lower priority than Interrupt D, and Interrupt B is already running. Here’s what will happen.
The AVR will wait for Interrupt D to finish, then will also wait for Interrupt B to finish. Once both are done, it will call Interrupt C. What about Interrupt A? Well in this scenario, it will finish when Interrupt C is done. I believe (to my best knowledge) this is how interrupt priority works on the Xmega. If anyone has any corrections, you’re welcome to make them.
Here’s what the call stack looks like (I think) Application Code Interrupt A (low-priority) Interrupt B (medium-priority) Interrupt D (high-priority) Interrupt C (medium-priority). Hello, I’m developing an Automatic Tool Changer for my Milling machine using an Arduino-UNO to control my Tool-Turret. The Mill passes pulses via a relay sending the same number of pulses as the Next-Tool position to pin-2 (example–it sends 7 LOWs/FALLINGs for tool-position-7). Arduino code then calculates the number of tool positions to rotate from the current tool position. I’ve tried Jordan’s Interrupt code but received various Counter Increases (x) without sending the same number of pulses.
I solved the problem by moving the “buttontime = millis;” line Out of the Interrupt routine, “void increment”, and placed it into the void Loop routine. Now x increments only when my Mill passes a Low. I thought of this after reading the Arduino Reference page on “attachinterrupt” where it says millis will not increment if it’s within an ISR. Thank You Much for this Interrupt example–saved me tons of time. Ok, here is my two-pence worth. In the example, you have millis refreshing a 32 bit word. Now, it doesn’t matter if it is non-volatile or not.
As the chips are 8 bits, there will be a number of operations involved in copying the value from millis to the variable buttontime. What happens if the interrupt is called smack in the middle of that, say in a rollover from 0x00ffffff to 0x01111111 - I can’t remember which way around the bytes are handled - but let’s say the value has reached 0x0100ff - that is of course entirely wrong. Someone with a deeper understanding of the architecture will no doubt tell me the updates are the other way around. So is it not then important to disable interrupts around that statement where buttontime is updated? LastButtonTime needs to be global because it has to have its state saved between ISR calls, and ISRs cannot take or return parameters so there’s no other way to get it the information. But I agree that there’s no reason that ButtonTime needs to be global, since it’s basically just a temp variable used to check against/replace the value in LastButtonTime.
In this situation Volatile isn’t needed because the values in the variables will never be red/written outside the scope of the ISR. Volatile simply forces the compiler to read the value from its memory location every time, rather than performing the optimization of loading its value once into a register and reading from that instead. But volatile would absolutely be needed if the ISR was going to change status variables that the main program needed to read. Otherwise the ISR would change the value in memory, but the main program might never see it because it’s just reading from the copy inside a register. I agree with Signal7.
The lastbuttontime variable is only touched when initialized, and when it is checked or set within the ISR function. A static variable with function scope is ideal. See for a good description. The buttontime variable could be a stack variable or static with function scope as well, just to minimize stack use during the ISR. Not a big deal with non-reentrant ISRs; either way is fine.
Also, where are the pinMode calls in setup? Am I missing a reference somewhere that explains the default state of each pin and why it’s not necessary to set pin2 to INPUT or 13 to OUTPUT? I just recently got in trouble using interrupts with I2C. I am using a port expander (MCP23008) which will generate an interrupt on any of the bit changes. Great function, easy to use, except - that during the ISR (Interrupt Service Routine) I was trying to read the device to determine which bit changed. Bad idea, this chip uses the I2C communications which require interrupts, but interrupts are turned off during an ISR and everything goes kinda south. Bottom line - don’t do things that will require an interrupt during an ISR; rather, set a flag that will be “POLLED” to let some non-interrupt routine read the device.
The “standard” way of handling this is to make the counter variables unsigned, but make the comparison signed using the same size variable. If the counters wrap 0xffffffff to 0x00000000 then all is OK.
The code as written works correctly on gcc 4.2.1 (i686-apple-darwin10; haven’t checked arduino) because the subtraction in the if statement is being done into long. One could be more explicit about it by casting the subtraction as a long before making the comparison: if ((long)(buttontime - lastbuttontime) 250) If the subtraction gets up-cast into a bigger variable, then there will be a wrap problem. For example if the counters were unsigned char, and the subtraction was not cast as char, then there will be a wrapping problem: unsigned char t1=0xff; unsigned char t2=0x01; if (t2 - t1 0) printf (“greater”); // Does not work! If ((char)(t2 - t1) 0) printf (“greater”); // Works! The Arduino framework appears to only support the AVR’s external interrupts. The ATmega48/8/88/168/328 used in most Arduinos only has two external interrupts.
The 40 pin xmegas have three, and the larger ones even more. However most recent mega AVR’s also have a pin change interrupt.
This is activated when ANY pin in a port (that isn’t masked out) changes state. For the 28 pin megas that would give us three more interrupt vectors (one for each port). However, I don’t think the Arduino IDE supports port change interrupts directly (though you could still program for this by writing your own library functions). The port change interrupts could be used to support a rotary encoder. Simply mask off all but two pins on a given port and attach the two signals from the encoder to those pins. The function called by the interrupt resulting from the pin state change will read the new state of the two pins, and compare them in a state machine to process the new position of the encoder. While this is technically true, AVR’s don’t work very well as encoder counters unless that’s all they’re dedicated to doing.
The frequency of interrupt calls for even a single quadrature encoder of reasonable resolution on a motor spinning at even a few hundred RPM will eat up the vast majority of your CPU time. You’re much better off using a dedicated encoder IC such as the HCTL-20XX series (and talking to it over SPI.
AttachInterrupt function is used for creating external interrupt. Syntax attachInterrupt(interrupt, ISR, mode) attachInterrupt(pin, ISR, mode) (Arduino Due only) Parameters interrupt:the number of the interrupt (int) pin:the pin number(Arduino Due only) ISR:the ISR to call when the interrupt occurs; this function must take no parameters and return nothing. This function is sometimes referred to as aninterrupt service routine.mode:defines when the interrupt should be triggered. Four contstants are predefined as valid values: (1) LOW: to trigger the interrupt whenever the pin is low, (2) CHANGE: to trigger the interrupt whenever the pin changes value (3) RISING: to trigger when the pin goes from low to high, (4) FALLING: for when the pin goes from high to low.
The Due board allows also: HIGH to trigger the interrupt whenever the pin is high. (Arduino Due only) Returns: none. You can also see this from arduino page.Link.
Well, I have got to the bottom of this problem. Not sure it's the best answer but it's a solution. The long and short of it is to avoid TXBE interrupts. If I use TXEMPTY interrupts instead it works fine. A line on page 168 of the Atmel data sheet says (sic) 'A interrupt can enter pending state even it is disabled' so I wondered if the problem with TXBE was because I was not clearing the pending interrupt before or even inside the ISR so I added NVICClearPendingIRQ(UARTIRQn); at the start of the ISR and also just before I enabled the TXBE interrupt but the (mis)behaviour didn't change. The operation of TXEMPTY is still a little odd (to me) because it appears that the interrupt is generated by the transmit shift register just being empty, not when it goes empty. If you enable interrupts without having loaded the transmit buffer you will immediately get an interrupt.
Some may like this 'self=priming' behaviour, but it doesn't do it for me. I am writing my sending routine such that the TXEMPTY interrupt is not enabled until the transmitter has been loaded with the first byte to be sent. Based on this post on the Arduino Forum: I presume that the USARTs have a similar issue. Hopefully this will help others. I just realised what could be the real error at the source of my problem. The UART interrupt register descriptions talk about the TXBUFE bit in the context of transmit buffer empty and so my assumption was that this is the bit that tells me when I can put another byte into the transmit holding register.
However the UART Status Register description say that the TXBUFE bit is 'the buffer full signal from the transmitter PDC channel'. The latter puts a whole different slant on what this bit does. According to the UART Status Register description the bit I need to be looking at is the TXRDY bit!
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