External device power management is another aspect to low power system design. In this case, two examples of power management of external devices are shown. Often with low power system design, if an external device needs to be chosen, in this example an op-amp is shown, the designer will often go to a manufacturer’s Web site and pick a device with a low current consumption. On the right, an MCU is connected to a 1 microamp op-amp. In this case, the op-amp consumes 1 microamp, 100% of the time, adding 1 microamp to the standby current consumption of the application. The example on the left shows an op-amp with a shutdown function. In this case, the op-amp active current is much higher but the shutdown current is much lower. The averages are shown for using the op-amp for one minute four times per day. The average current consumption contribution on the left is 0.06 microamps for the op-amp with shutdown capability, where as the 1 microamp op-amp contributes 1 microamp 24 hours a day. Therefore, the higher current op-amp with shutdown capability is a wiser choice for low power system design. Every application is different and the duty cycle of external devices varies with its application. However, the power consumption budgeting analysis should be done each time with each external device to see if it can be power managed by the MSP430 or to see if shutdown capability can be incorporated. In addition, if devices cannot be found with shutdown capability, sometimes they can be powered from port pins directly.