Microchip MCP112 Manual

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Side 1/8

 2002 Microchip Technology Inc. DS00820A-page 1

MAN820

INTRODUCTION

Semiconductor manufacturers have designed several

types of circuit supervisors with varying types of func-

tionality over the past few years. Some supervisors

incorporate watchdog features as well as complex func-

tions, such as programmable threshold levels. As it

turns out, 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 manufac-

turers should also address system supervisors

designed into systems where microcontrollers (MCUs)

and programmable logic devices (PLDs) are pro-

grammed in-circuit. Programming PICmicro® micro-

controllers in this fashion is known as In-Circuit Serial

ProgrammingTM (ICSPTM), which can be implemented

for a variety of reasons, including field upgrades.

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-

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

techniques included below are intended to guide

designers with supervisors of all kinds for ICSP

circuitry.

CIRCUITRY BACKGROUND

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

not driving low and when a voltage is applied to the out-

put that is higher than the power supply level, the out-

put can handle sinking less than 1 mA. Other pertinent

electrical specifications for the device are shown in the

data sheet, which includes test conditions for the chip.

The MCP120 has an open drain output, though it is not

a true open drain. Specifically, the PMOS transistor on

the high side of the output stage is diode-connected, as

shown in Figure 1. When the voltage applied to the out-

put of the supervisor exceeds the power supply for the

chip, the PMOS transistor acts like a forward biased

diode. Lastly, since the output stage is open drain, a

pull-up resistor is required between the supervisor out-

put and VDD.

FIGURE 1: MCP120 output stage simplified.

Author: Ken Dietz

Microchip Technology Inc.

VDD

OutIn

GND

System Supervisors in ICSPTM Architectures

AN820

DS00820A-page 2  2002 Microchip Technology Inc.

ICSP Circuit Configuration

A schematic showing ICSP circuitry with the MCP120 is

shown in Figure 2. A current limiting resistor, RCL, limits

the current driven into the output stage of the supervisor

when the programming voltage is applied to the MCU.

A pull-up resistor, RPU, is placed between the supervi-

sor output and VDD, since the MCP120 output is open

drain and active low. Calculations for the resistors are

explained in the Design Methodology section.

The pull-down resistor, RPD, shown in Figure 2 is useful

during power-up and power-down sequences. Supervi-

sor functionality is not specified at power supply volt-

ages typically lower than 1 Volt, so the output stage of

the supervisor could be in a high impedance state. If

the supervisor output is high impedance and voltage is

applied to the MCU from an external source, the pro-

cessor could potentially run its program until the super-

visor takes over and resets the MCU again. This is

especially important in systems where multiple printed

circuit boards are interfaced together and a secondary

board might end up driving the MCU I/O pins before

power is applied to the primary board.

Since the voltage on the MCLR pin on a PICmicro®

MCU is very close to the power supply level, RPD can

be fairly large to minimize current consumption when

the circuit is normally operating. Furthermore, during

program and verify sequences, up to 13.25 Volts are

applied to the pull-down resistor. Because of this, R

should also be large enough to minimize current con-

sumption for the programming voltage supply. A value

of 100 kΩ (+/-5%) results in a maximum of 140 µA

when VPP is applied to the microcontroller, or 134 µA if

1% resistors are used, which is a small load for most

programmers. For battery powered applications, sub-

stantially larger valued resistors may be desirable for

this purpose.

FIGURE 2: Active low open drain ICSP circuit.

Key Programmer Specifications

The PRO MATE® II and the ICSP Socket Module, part

number AC004004, were used to test the circuit shown

in Figure 2. Current drive for the programming voltage

signal and current drive for the power supply signals are

critical specifications for the ICSP socket module. For

the VPP signal, the output of the ICSP module can pro-

vide as much as 100 mA, and for the power supply sig-

nal, the ICSP socket module can drive as much as

400 mA. Furthermore, the PRO MATE II programs

MCUs only at 5 Volts. However, it does have the capa-

bility to verify the memory contents of microcontrollers

at power supply levels ranging from 2.5 Volts to

5.5 Volts.

Another important aspect to consider for In-Circuit

Serial Programming includes the cable length for the

interface. Not only are sufficient current drive capabili-

ties required, but if a lengthy cable is used, reflections

and oscillations may cause programming errors.

Because of this phenomenon, manufacturers imple-

menting ICSP architectures should keep their cables

as short as possible. The circuits tested for the purpose

of this article implemented the ICSP cable that is nor-

mally shipped with the ICSP module, which is about 6

feet long. Lastly, the end of the cable connected to the

printed circuit board was modified to interface to a

modular connector.

VDD

(Programmer Supplied)

Bypass

Capacitor MCP120

VSS

VDD

RST

RPU RCL

3.3 Volts

Isolation

Microcontroller

VDD

MCLR

VSS

RPD

VPP

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	MCP112

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