#17 interrupts Part-2: How most CPUs (e.g. MSP430) handle interrupts?

Summary

In this lesson, Miro Samek explores interrupt handling in embedded systems, comparing the ARM Cortex-M and MSP430 processors. He demonstrates how interrupt service routines (ISRs) work, focusing on the SysTick interrupt for ARM Cortex-M and the Timer0 interrupt for MSP430. The lesson highlights key differences, including how interrupts are handled, the special instructions used, and the need for saving CPU registers. Samek also provides hands-on examples, such as setting breakpoints and simulating interrupt preemption, to emphasize the challenges and techniques in debugging embedded systems. The video serves as an in-depth introduction to interrupt mechanisms in embedded programming.

Takeaways

• 😀 The lesson focuses on interrupt handling in embedded systems, specifically for the MSP430 processor and how it differs from ARM Cortex-M.
• 😀 The speaker uses a project prepared in advance (lesson 17) to demonstrate interrupt behavior, which is available for download from state-machine.com/quickstart.
• 😀 The SysTick interrupt, introduced in the previous lesson, is modified to be called directly from the main() function in the code.
• 😀 A visual effect of the green LED blinking is created using the modified code to help understand how interrupts preempt linear code execution.
• 😀 The ARM Cortex-M processor allows interrupt handlers to be regular C functions, unlike most other processors, such as the MSP430.
• 😀 MSP430 interrupt handlers require special handling with the `__interrupt` keyword and the `pragma-vector` directive, which are specific to the IAR toolset and MSP430.
• 😀 The MSP430 LaunchPad is used to demonstrate an interrupt-driven blinky program, and the debugger is configured to use hardware breakpoints for accurate step-through.
• 😀 A technique to trigger interrupts manually is demonstrated by manipulating the TA0R register to force an interrupt to preempt normal program flow.
• 😀 The MSP430 processor uses a stack to save and restore the Program Counter (PC) and Status Register (SR) during interrupt entry and exit.
• 😀 Interrupt Service Routines (ISRs) require saving and restoring additional CPU registers compared to regular functions, as they must preserve the state of the CPU to prevent clobbering during preemption.

Q & A

What is the primary focus of this lesson?

The primary focus of the lesson is on interrupts and how they function, particularly in the MSP430 processor, and how ARM Cortex-M differs from other processors in terms of interrupt handling.

How does the interrupt service routine (ISR) work in the MSP430 processor?

In the MSP430 processor, the ISR is a special function marked with the '__interrupt' keyword and the 'pragma-vector' directive. This ensures the ISR is treated differently from regular functions and cannot be directly called from the program.

What is the difference between how ARM Cortex-M and MSP430 handle interrupts?

ARM Cortex-M allows interrupt handlers to be regular C functions that can be called directly, whereas MSP430 uses special syntax and directives like '__interrupt' and 'pragma-vector' to define interrupt handlers, which cannot be called directly from the main program.

What is the significance of the 'RETI' instruction in the MSP430 ISR?

'RETI' (Return from Interrupt) is crucial in the MSP430 ISR as it restores the saved registers, including the Program Counter (PC) and Status Register (SR), allowing the processor to return to the exact location where the interrupt occurred.

Why is the ability to trigger interrupts at will useful in embedded systems programming?

Triggering interrupts at will is helpful in debugging and troubleshooting, as it allows developers to simulate rare or intermittent events and test the system's response without waiting for these events to naturally occur.

What is the role of the stack pointer (SP) during interrupt handling in the MSP430?

During interrupt handling, the stack pointer (SP) decreases as the processor pushes the Program Counter (PC) and Status Register (SR) onto the stack. This allows the processor to save its state before jumping to the interrupt service routine.

What happens when the interrupt handler in the MSP430 is invoked?

When the interrupt handler is invoked, the processor pushes the necessary registers (like PC and SR) onto the stack, disables further interrupts by clearing the Global Interrupt Enable bit, and executes the interrupt service routine (ISR).

What does the 'BIS' instruction do in the MSP430 interrupt process?

The 'BIS' (Bit Set) instruction in the MSP430 is used to trigger an interrupt by setting specific bits in the timer register. This action causes the interrupt line to go high, thus initiating the interrupt process.

How does the MSP430 timer (Timer0) trigger an interrupt?

The MSP430 Timer0 triggers an interrupt when the timer register (TA0R) reaches the value in the TACCR0 register. This is set up by incrementing the TA0R register at a specific rate, generating an interrupt when the counter exceeds the set limit.

What are the challenges when working with interrupt-driven code on different processors?

One of the challenges is that interrupt handling differs between processors, such as ARM Cortex-M and MSP430. For example, MSP430 requires special handling for interrupt vectors and uses specific keywords and pragmas, while ARM Cortex-M allows regular C functions to be used as ISRs.