Microcontrollers, like those at the heart of Arduino boards, rely heavily on timers for a multitude of tasks, from precise timekeeping to generating waveforms for Pulse Width Modulation (PWM). These timers often feature comparator units, allowing for actions to be triggered when the timer counter reaches a specific value – the compare value. Interrupt Service Routines (ISRs) are fundamental in responding to these timer events, enabling efficient and timely execution of code. However, a subtle yet critical aspect often overlooked is the correct naming of these ISR functions. As we’ll explore, misnaming an ISR can lead to silent failures, making debugging a frustrating endeavor.
The original post highlights an intriguing experiment concerning Timer1 overflow interrupts on an Arduino. The author meticulously sets up a scenario to test the recognition of Interrupt Service Routines (ISRs) by the Arduino IDE and compiler. They correctly defined an ISR for the TIMER1_OVF_vect interrupt, observing the expected behavior – the interrupt triggered as programmed.
ISR(TIMER1_OVF_vect) {
asm volatile (
"neg %[ctr]" "nt"
"subi %[ctr], 1" "nt"
"neg %[ctr]" "nt"
: [ctr] "=&r" (ctr)
: "[ctr]" (ctr)
:
);
flag = (ctr == 0);
}This code snippet, designed for demonstration, successfully executed when the timer overflowed. However, the experiment took a turn when the author deliberately misspelled the ISR name. Instead of TIMER1_OVF_vect, they used names like “Melbourne,” “Sydney,” and “Canberra”—clearly not recognized interrupt vector names.
The results were revealing. Changing the ISR definition to an incorrect name, such as ISR(Melbourne) { ... }, resulted in the Arduino resetting repeatedly. This indicated that the intended interrupt vector was not correctly associated with the provided ISR code. Upon examining the compiled code, the author found that the Timer1 interrupt vector pointed to a “bad interrupt” handler (jmp 0), the same default handler used for unassigned interrupts. Crucially, the code within the misnamed ISR was completely absent from the compiled program, and the sketch size was smaller, confirming that the ISR was simply ignored during compilation.
Disassembled code snippet showing ISR absence
This experiment underscores a vital lesson for Arduino and embedded systems programmers: the Arduino IDE and compiler do not flag incorrect ISR names as errors. They silently ignore them. This “silent treatment” can be particularly problematic when working with Comparator Timers.
Imagine configuring a comparator timer to trigger an interrupt when the timer count matches a predefined value. This interrupt is meant to initiate a specific action, perhaps controlling a motor, reading a sensor, or updating a display. If you were to inadvertently misname the ISR associated with this comparator match interrupt – for instance, using TIMER1_COMPA_vect instead of the correct TIMER1_COMPB_vect if you intended to use Compare Match B – the code inside your ISR would never execute.
The Arduino program would compile and upload without any warnings or errors, leading you to believe everything is functioning correctly. However, the comparator timer interrupt would effectively be disabled because the system has no correctly linked ISR to call. This silent failure can be incredibly difficult to diagnose, as there are no explicit error messages to guide you.
To avoid these silent ISR errors, especially when working with comparator timers, it’s crucial to:
- Double-check ISR names: Carefully consult the microcontroller datasheet or Arduino documentation for the precise and correct names of interrupt vectors, such as those related to timer compare matches (
TIMERx_COMPA_vect,TIMERx_COMPB_vect, etc.) and timer overflows (TIMERx_OVF_vect). - Use descriptive variable names: Employ meaningful variable names in your code to enhance readability and reduce the chances of typos when referencing interrupt vectors.
- Test ISR execution: Incorporate debugging techniques, such as toggling an LED or sending serial messages within your ISR, to confirm that the interrupt is indeed firing and the ISR code is being executed as expected.
In conclusion, while the Arduino environment simplifies microcontroller programming, it’s essential to be aware of potential pitfalls like silent ISR errors. The experiment highlighted in the original article serves as a valuable reminder of the importance of meticulous attention to detail, particularly when naming interrupt service routines for timers and comparator units. By understanding this behavior and adopting careful coding practices, developers can prevent frustrating debugging sessions and ensure the reliable operation of their Arduino-based projects that utilize timer interrupts.
