Measuring the current consumption of the ez430 is a slightly tricky task. To do so we must insert an ammeter in serial with the ground pin between the Spy-Bi-Wire programming interface and the target board. Since the program is loaded into flash on the tool, we can run our code, cease debugging, and the tool will run our program whenever it is powered. Thus, the following diagram shows how we can insert an ammeter in serial with the ez430 while the program runs even though we cannot debug it at this point (the two middle data pins aren't connected).

Figure 1: Connecting the Ammeter in serial as shown will allow for you to measure current consumption.

Connection Diagram

Figure 2: Here, I have created the setup using parts from around the lab.

Connection Picture

Most parts have a shutdown or sleep mode available that will reduce the current consumption of the component considerably. In general, digital parts consume significant current when their transistors switch because of the charging and discharging of the internal capacitances of the transistors. Analog integrated circuits also support shutdown modes to reduce power consumption. Datasheets will specify the current consumption in both on and shutdown modes of the component. It is important to note that when a device is in shutdown mode, power and ground voltages are still powered and connected to the device.

In order to shutdown most integrated circuits, all that is required is a shutdown or sleep pin to be asserted properly. Other devices require a shutdown command to be issued over the bus. The primary disadvantages of shutdown modes, apart from the fact that the device is inoperative is that recovering back into normal operating modes can impose a significant delay. A useful property of the MSP is that its recovery time from some low-power modes is fast enough to meet the response times of interrupts.

Parts without built-in shutdown modes must be shutdown by having its current supply controlled through a transistor or other switching device.

The MSP430 was designed with the low power modes in mind from its beginnings. In lower power mode, the processor can achieve current in the microamps while still monitoring its inputs. The principles of utilizing the MSP power modes are described in detail in the second chapter of the MSP User's Guide. The modes vary the degree to which the processor is aware of its surroundings and the clocks that the processor keeps running. The processor lowers power consumption partly by shutting off external and internal oscillators.

There are four low power modes in addition to regular operating mode on the MSP430:

To enter a low power mode the status register in the CPU must be set to indicate the desired mode. Specifically the bits SCG1, SCG0, OSCOFF, and CPUOFF. The User's Guide details the specific bits needed. Also provided in the chapter is some example code on changing power modes. To exit low power mode, an interrupt is needed. In the interrupt, the previous status register state can be altered so that exiting the interrupt will leave the processor awake. The User's Guide explains in detail the specifics of entering and leaving low power mode. Example code with the compiler also demonstrates the low power modes.

The User's Guide for the MSP also explains the principles needed to lower the power consumption of a design. Be sure to minimize wasteful code execution. This is the same idea as improving speed performance because every unnecessary instruction wastes a little bit of battery power. All of the techniques that improve code efficiency will improve power efficiency. Increasing clock speed will not yield similar power savings because faster execution increases power consumption. Similarly, unused peripheral modules on the processor should be de-activated to save power. Use interrupts to handle events to allow the processor to stay in Low Power Mode 3 as much as possible. By reducing the awake time of the processor, the average current consumption of the MSP can be reduced to levels approximately as low as LPM3 while maintaining the same functionality.