Microchip MCP112 Handleiding

Microchip Niet gecategoriseerd MCP112

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 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

 2002 Microchip Technology Inc. DS00820A-page 3

AN820

DESIGN METHODOLOGY

Absolute Maximum Clarifications

A critical question about designing this type of circuit

revolves around the worst case voltages and currents

applied to the circuit. Additionally, a circuit designer

needs to determine whether or not the absolute maxi-

mum ratings for any of the components on the board

are being exceeded. Knowing the largest voltage drops

across the circuit enables a designer to calculate the

resistor sizes so that absolute maximum ratings for all

components are not exceeded. From the PIC16LF872

data sheet (DS30221), the maximum voltage that can

be applied to the MCLR pin of the microcontroller can-

not exceed 13.25 Volts. Additionally, the maximum volt-

age that can be applied to any pin on the MCP120, with

respect to VSS, is from -0.6 Volts to (VDD + 1.0) Volts.

Calculations

Since the lowest voltage applied to V

DD is 0 Volts dur-

ing programming, the voltage applied to the RST pin

should not exceed 1.0 Volt. Knowing this, the circuit

can be designed to limit the voltage applied to RST to

a nominal 700 mV. This means that RCL has to drop at

most 12.55 Volts. If the assumption is made that all of

the current flowing through RCL is also flowing through

RPU when the supervisor output is high impedance,

then RPU needs to be 700 and RΩCL should be at least

12.55kΩ. For standard 1% tolerance resistors, 715Ω

and 12.7kΩ would be the correct selections for R

and RCL, respectively. If standard 5% resistors are

being used in the circuit, then 750Ω and 13 kΩ are the

correct solutions.

In order to determine if this combination of resistors will

work in the design, all of the permutations for power

supply levels and output drive levels for the supervisor

must include calculations for minimum and maximum

currents and voltages in the circuit. While the microcon-

troller is being programmed, power supply levels can

be as high as 5.5 Volts for the PIC16LF872 during ver-

ify operations. This can be further clarified by reviewing

Figure 3. With this in mind, minimum and maximum cir-

cuit calculations show that this selection of resistors will

work within tolerance.

Results

Figure 3 shows a successful programming waveform

for two key nodes in an ICSP circuit. Channel 1 mea-

sured the voltage levels on the supervisor output pin,

RST, and Channel 2 measured the voltage applied to

the node between the power supply pins of the micro-

controller and the supervisor. The circuit schematic for

this system was shown in Figure 2. Notice that

Channel 2 in Figure 3 shows the verification voltage

levels at 5.5 Volts and 2.5 Volts. Notice also that Chan-

nel 1 shows the voltage on the RST pin at about

700 mV when the supervisor output is driving low.

Lastly, when the supervisor output is high impedance,

Channel 1 shows a voltage level of 5.7 Volts applied to

RST, which is about 700 mV above the power supply.

FIGURE 3: Successful programming waveform.

RST Node

VDD Node

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	MCP112

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