In an embedded system, and specifically the MSP430 microcontrollers, the usage of interrupts is key to obtain real-time and low-power performance. It is highly recommended to develop an interrupt-driven system to service external and peripheral events during main application runtime. Additionally, servicing the interrupts should be executed in a short and timely manner to improve real-time response of the system. Hence interrupt service routines should be kept to execute a minimal amount of instructions, allowing the CPU to quickly return to sleep or its previous task. Most of the processing should take place in the main application in active mode. Consequently, function calls within an interrupt service routine should be avoided to prevent considerable amount of additional code execution from being introduced (i.e. on entry: storing the current PC, stack, and CPU state, loading the function and preparing for its own stack; on exit: restoring the stack and the CPU state before going back to continue executing code in the original ISR).
Invoking function calls within an Interrupt Service Routine (ISR) significantly adds instruction execution cycles to the ISR. This time might increase depending on the number of parameters being passed, and whether they are passed by value or reference.
An interrupt service routine (ISR) in the code project
issues/invokes a function call.
Instead of calling functions in the ISR, try to inline the instructions, i.e. move the code instructions directly into the ISR, or move the entire function call out of the ISR if possible (especially for processing intensive functions). For the latter option, a global flag can be setup to notify the main program to invoke the function call when the device is out of the ISR and back in active mode.
Alternatively, if the procedure must be executed within the ISR context, use macros or inline the function to minimize context switching sequences.
unsigned char interrupt_triggered = 0; void xxx_ServiceFunction(xxx_param); #pragma vector=xxx_VECTOR __interrupt void xxx_ISR(void) { interrupt_triggered = 1; _BIC_SR_IRQ(LPM3_bits); // Clear LPM3 bits from 0(SR), go back to Active Mode } void main(void) { while(1) { /* go to low power mode, wait for xxx interrupt */ if (interrupt_triggered) xxx_ServiceFunction(xxx_param); } }
See the
rest of the code examples for all MSP430 devices here!
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