Power estimation for CMOS circuits appears to be deceptively straightforward. Most vendors provide a table or an equation that basically splits out the various components of power dissipation for a CMOS part (see Figure 1). Typically, these include a component for the input receivers, which must be de-rated if driven from TTL rather than CMOS external drivers. Next, there is a component for the internal core of the chip, which usually includes a negligible d.c. component and an a.c. component that requires detailed knowledge of the various switching frequencies encountered. It also requires knowledge of exactly how much circuitry is used or unused at any time. Finally, there is a component attributed to the output stages which are functions of both the switching frequencies as well as the external load capacitance. The power is found as the sum of all three components: Power = PIN + PCORE + POUT