1. Lecture 8: TI MSP430 Interrupts, ISRs
ECE 447 Fall 2009
Lecture 8: TI MSP430
Interrupts, ISRs

2. ECE447: Interrupt Service Routines in Assembly
PORT1_ISR: ; respond to inputs on port1 reti ; return from interrupt ORG 0FFE8 DW PORT1_ISR -- or -- COMMON INTVEC ORG PORT1_VECTOR
The service routine itself is quite simple. (Note the “reti” instruction.)
To store the address of this ISR in the proper vector table location an ORG directive is used.
PORT1_VECTOR is defined in the header file relative to the INTVEC segment. The COMMON directive must be used to place the defined address (PORT1_ISR) in the correct location. This is useful for defining ISRs in multiple files.

3. ECE447: Interrupt Service Routines in C
// tell the compiler the vector for this ISR #pragma vector = PORT1_VECTOR __interrupt void port1_isr(void) { // respond to inputs on port1 }
“#pragma vector” tells the compiler to store the address of this routine in the PORT1_VECTOR location.
“__interrupt” is used to tell the compiler that this function should end with a “reti” instruction.

4. ECE447: Interrupt Vector Jump Table
The jump table tells the program where to go when an interrupt occurs.
A vector points to the address where the interrupt service routine starts.
The MSP430 jump table is stored in RAM from 0xFFC0 to 0xFFFF and can be modified by the programmer.

5. ECE447: MSP430 Interrupts

6. ECE447: MSP430 Status Register
GIE (bit 3): Global Interrupt Enable
1: Enables all interrupts (interrupts must also be enabled individually)
0: Disable all interrupts
NOTE: SR is saved on the stack and cleared when an ISR is executed.

7. ECE447: Setting and Clearing the General Interrupt Mask
Assembly
EINT – Enable interrupts (set GIE)
DINT – Disable interrupts (clear GIE) C
asm(“ EINT”);
asm(“ DINT”);
C (#include intrinsics.h)
__enable_interrupt();
__disable_interrupt();

8. ECE447: MSP430 Interrupt execution flow
RAM JUMP TABLE
0xFFFF PC SR EE
10EE
00F8 10
0000
0xFFC0
STACK F8 00 SP
RAM SERVICE ROUTINE
0x10EE

9. Interrupts Example: Toggling LEDs
// Listing 6.5: Toggles LEDs in ISR using interrupts from timer A CCR0
// in Up mode with a period of about 0.5s
#define LED1 BIT3
#define LED2 BIT4
void main (void) {
WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer
P2OUT = ~LED1; // Preload LED1 on, LED2 off
P2DIR = LED1 | LED2; // Set pins with LED1,2 to output
TACCR0 = 49999; // Upper limit of count for TAR
TACCTL0 = CCIE; // Enable interrupts on Compare 0
TACTL = MC_1 | ID_3 | TASSEL_2 | TACLR; // Set up and start Timer A
// “Up to CCR0” mode, divide clock 8, clock SMCLK, clear timer
__enable_interrupt ( ); // Enable interrupts (intrinsic)
for (;;) { // Loop forever doing nothing
} // Interrupts do the work }
// Interrupt Service Routine for Timer A channel 0
#pragma vector = TIMERA0_VECTOR // Assoc. the funct. w/ an interrupt vector
__interrupt void TA0_ISR (void) // name of the interrupt function (can be anything)
P2OUT ^= LED1 | LED2; // Toggle LEDs

10. ECE447: MSP430 Interrupt Class Exercise
Write a program that collects samples from input Port 2. A signal line that indicates a new data value is available is connected to pin P Use an interrupt to detect a rising edge on the P1.0. When an interrupt is triggered the value on Port2 should be stored into a byte array of 32 entries. When 32 samples have been collected, P1.7 should be driven as an output high to indicate the memory is “full”