An interrupt is an event in hardware that triggers the processor to jump from its current program counter to a specific point in the code. Interrupts are designed to be special events whose occurrence cannot be predicted precisely (or at all). The MSP has many different kinds of events that can trigger interrupts, and for each one the processor will send the execution to a unique, specific point in memory. Each interrupt is assigned a word long segment at the upper end of memory. This is enough memory for a jump to the location in memory where the interrupt will actually be handled. Interrupts in general can be divided into two kinds- maskable and non-maskable. A maskable interrupt is an interrupt whose trigger event is not always important, so the programmer can decide that the event should not cause the program to jump. A non-maskable interrupt (like the reset button) is so important that it should never be ignored. The processor will always jump to this interrupt when it happens. Often, maskable interrupts are turned off by default to simplify the default behavior of the device. Special control registers allow non-maskable and specific non-maskable interrupts to be turned on. Interrupts generally have a "priority;" when two interrupts happen at the same time, the higher priority interrupt will take precedence over the lower priority one. Thus if a peripheral timer goes off at the same time as the reset button is pushed, the processor will ignore the peripheral timer because the reset is more important (higher priority).

The function that is called or the particular assembly code that is executed when the interrupt happens is called the Interrupt Service Routine (ISR). Other terms of note are: An interrupt flag (IFG) this is the bit that is set that triggers the interrupt, leaving the interrupt resets this flag to the normal state. An interrupt enable (IE) is the control bit that tells the processor that a particular maskable interrupt should or should not be ignored. There is usually one such bit per interrupt, and they are often found together in a register with other interrupt enable bits. The most important interrupt on MSP430 is the reset interrupt. When the processor detects a reset or powers up for the first time, it jumps to the beginning of memory and executes the instructions there. The highest priority interrupt vector begins at the address 0xFFFE. The lowest priority interrupt begins at 0xFFE0. The complete set of interrupts is ranked by priority:

When the interrupt first occurs on the MSP there is a precise order of events that will occur. This process takes 6 instruction cycles AFTER the current instruction completes.

The interrupt service routine is the code that the programmer writes to take care of the work that needs to be done when a particular interrupt happens. This can be anything you need it to be. Because entering the interrupt turned off the GIE bit, you will not receive any interrupts that happen while you are still in the interrupt service routine. You can turn the interrupts back on if you need to receive interrupts during your interrupt, but usually it is a better idea to make interrupt service routines shorter instead. In C interrupts are simply functions with special declarations. You never call these functions; the compiler simply sets up the interrupt vector table to call your function when the particular interrupt occurs.

This example interrupt is pulled from the msp430x20x3_wdt_01.c example file by Mark Buccini. The complete file can be found on the Ti-ez430 homepage.

// Watchdog Timer interrupt service routine
#pragma vector=WDT_VECTOR
__interrupt void watchdog_timer(void)
{
          P1OUT ^= 0x01;   // Toggle P1.0 using exclusive-OR
} 
       

Interrupt functions should always be void and accept no arguments. This particular interrupt service routine (ISR) is called watchdog_timer, but the name does not matter. The way the compiler knows that this function should handle the watchdog timer interrupt is what follows the function name. the #pragma vector = indicates that this is an interrupt and WDT_VECTOR is a macro from the MSP header file that indicates the interrupt vector being used. Every interrupt vector in the processor has a macro defined for it (which can be found in the header file). To attach this interrupt service routine to a different interrupt, all you need to do is change the WDT_VECTOR to one of the other macros defined in the ez430 header file.

When the end of the ISR is reached the MSP executes a precise set of steps to pick up the execution of the program where it left off before the interrupt occurred. This process takes 5 cycles.

Footer

This work is licensed by Naren Anand under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.

Last edited by Naren Anand on Jun 6, 2006 5:02 pm GMT-5.