AN820 M TM System Supervisors in ICSP Architectures CIRCUITRY BACKGROUND Author: Ken Dietz Microchip Technology Inc. MCP120 Output Stage A simplified schematic for the MCP120 output stage is shown in Figure 1. Nominally, the output stage of the MCP120 can handle sinking less than 1 mA of current in a high impedance state. That is, when the output is INTRODUCTION not driving low and when a voltage is applied to the out- Semiconductor manufacturers have designed several put that is higher than the power supply level, the out- types of circuit supervisors with varying types of func- put can handle sinking less than 1 mA. Other pertinent tionality over the past few years. Some supervisors electrical specifications for the device are shown in the incorporate watchdog features as well as complex func- data sheet, which includes test conditions for the chip. tions, such as programmable threshold levels. As it The MCP120 has an open drain output, though it is not turns out, most system supervisor data sheets address a true open drain. Specifically, the PMOS transistor on typical supervisor functions related to Power-on Reset, the high side of the output stage is diode-connected, as power-down, and brown-out conditions. In order to shown in Figure 1. When the voltage applied to the out- serve a wide customer base, semiconductor manufac- put of the supervisor exceeds the power supply for the turers should also address system supervisors chip, the PMOS transistor acts like a forward biased designed into systems where microcontrollers (MCUs) diode. Lastly, since the output stage is open drain, a and programmable logic devices (PLDs) are pro- pull-up resistor is required between the supervisor out- ® micro-grammed in-circuit. Programming PICmicro .put and V DD controllers in this fashion is known as In-Circuit Serial TM TM (ICSP ), which can be implementedProgramming for a variety of reasons, including field upgrades. V DD System supervisors are available with several types of output stages. Some have active low output stages, some active high, and there are others similar to the MCP100, with output stages that drive RESET lines both high and low. Supervisor output stages are extremely important to understand for ICSP circuitry, since programming equipment actually drives the out- In Out put stages when the MCU or PLD is being pro- grammed. While there is a wide variety of supervisor types available on the market, this Application Note pri- marily 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 GND techniques included below are intended to guide designers with supervisors of all kinds for ICSP circuitry. FIGURE 1: MCP120 output stage simplified.