When working with Arduino projects, you might encounter unexpected issues when combining different functionalities. A common problem arises when using both the Servo and Tone libraries in the same project. This is because both libraries rely on timers and interrupts to operate, and conflicts can occur when they try to use the same resources.
Let’s delve into why this problem happens. Both the Servo library, which is crucial for controlling servo motors, and the Tone library, used for generating sounds, utilize Timer interrupts within the Arduino microcontroller. Specifically, they both depend on TIMERn_COMPA_vect interrupts.
Looking at the code from these libraries, you can see this clearly. The Tone.cpp library uses the following Interrupt Service Routine (ISR):
ISR(TIMERn_COMPA_vect) {...}This is used for timers n = 0 to 5 in the Arduino architecture. Similarly, the Servo.cpp library employs the same ISR:
ISR(TIMERn_COMPA_vect) {...}but for a subset of timers: n = 1, 3, 4, and 5.
The conflict emerges during the compilation and linking process. When both libraries define the same interrupt vector (like __vector_32 which corresponds to TIMERn_COMPA_vect), it creates a symbol clash. The microcontroller’s architecture is designed to have a single, unique interrupt handler for each specific interrupt vector. These vectors are essentially fixed slots in a lookup table in the flash memory, each pointing to the address of its corresponding interrupt handler. You can refer to the microcontroller’s datasheet, specifically Table 9-1 in documents like the ATmega16U4-32U4 datasheet, to understand more about interrupt vector tables.
To resolve this conflict, you essentially have two main paths. One approach is to attempt to merge the interrupt handlers from both libraries. This is a complex task requiring a deep understanding of interrupt handling and the inner workings of both libraries. The other, potentially more feasible, solution is to modify one or both libraries to use different timer interrupts. The Arduino Pro Micro, for example, offers multiple TIMER interrupts. If the hardware supports it, you could reassign the Servo library to use a different timer interrupt than the Tone library. This would require examining the hardware specifications of your specific Arduino board to see which timers are available and how they can be configured.
Ultimately, resolving interrupt conflicts between libraries like Servo and Tone often necessitates modifying the libraries themselves or developing custom code that integrates the desired functionalities without relying on conflicting interrupt assignments. Understanding the concept of interrupt service routines and timer usage is key to tackling these kinds of Arduino programming challenges.
