Semiconductor manufacturers have designed several types of circuit supervisors with varying types of functionality over the past few years. Most system supervisor data sheets address typical supervisor functions related to power-on reset, power-down, and brown-out conditions. In order to serve a wide customer base, semiconductor manufacturers should also address system supervisors designed into systems where microcontrollers (MCUs) and programmable logic devices (PLDs) are programmed in-circuit. Programming PICmicro micro-controllers in this fashion is known as In-Circuit Serial Programming (ICSPTM), which can be implemented for a variety of reasons, including field upgrades.

While there is a wide variety of supervisor types available on the market, this application note primarily focuses on the MCP120, which has an open drain, active low, output stage. Even though the MCP120 was chosen for this ICSP example, the design techniques covered are intended to guide designers with supervisors of all kinds for ICSP circuitry.