Lesson 27 Notes



Lesson Outline

Review Interrupts

What did we talk about last time? What's an interrupt? What are the benefits of an interrupt? What's stored in the Interrupt Vector Table? What happens when an interrupt is triggered?

Remember what the programmer's job was in configuring interrupts - cover this again, emphasizing local / global switch.

Any questions?

Let's take a look back at the interrupt code I presented last lesson for toggling the Launchpad LED's on button push. What am I not doing in my button handling code that could cause problems? I'm not debouncing! What are some ways I could handle that?

Is there another, better way?

What if I changed the nature of my interrupt inside my ISR? So once I detect a high-to-low edge transition, I change the interrupt to sense low-to-high edge transitions. Then I can detect a release and debounce appropriately! Sweet!

Just a reminder to be creative about how you use the tools at your disposal to suit your needs.

Practice on your own:

Low Power Modes

The MSP430 is designed for ultralow-power applications. It has five low power operating modes, known by the designators LPM0, LPM1, LPM2, LPM3, and LPM4. These can disable various system components depending on application needs.

MSP430 Low Power Modes

These modes are configured using bits in the Status Register (SR) - namely CPUOFF, OSCOFF, SCG0, and SCG1. The benefit of including these in the SR is that they are saved when an ISR is triggered and restored afterwards!

What happens to the bits of the SR once the ISR is triggered? They're cleared - so the MSP430 can 'wake up' from a low-power mode.

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Here are the subsystems that are disabled based on each Low Power Mode:

Device Status Based on Operating Mode

If a peripheral depends on a clock that is disabled, it is disabled until the clock becomes active.

Manufacturers recommend maximizing time in LPM3 and using the ACLK to for peripherals.

Your header file provides some macros for entering / exiting these modes:

// from msp430g2553.h

#define LPM0      _BIS_SR(LPM0_bits)     /* Enter Low Power Mode 0 */
#define LPM0_EXIT _BIC_SR_IRQ(LPM0_bits) /* Exit Low Power Mode 0 */
#define LPM1      _BIS_SR(LPM1_bits)     /* Enter Low Power Mode 1 */
#define LPM1_EXIT _BIC_SR_IRQ(LPM1_bits) /* Exit Low Power Mode 1 */
#define LPM2      _BIS_SR(LPM2_bits)     /* Enter Low Power Mode 2 */
#define LPM2_EXIT _BIC_SR_IRQ(LPM2_bits) /* Exit Low Power Mode 2 */
#define LPM3      _BIS_SR(LPM3_bits)     /* Enter Low Power Mode 3 */
#define LPM3_EXIT _BIC_SR_IRQ(LPM3_bits) /* Exit Low Power Mode 3 */
#define LPM4      _BIS_SR(LPM4_bits)     /* Enter Low Power Mode 4 */
#define LPM4_EXIT _BIC_SR_IRQ(LPM4_bits) /* Exit Low Power Mode 4 */

More detailed info is in the reading from today.