Atmel C51 Installation and operating instructions

External Brown-out Protection for C51

Microcontrollers with Active High Reset Input

Features

•Low-voltage Detector

•Prevents Register and EEPROM/Flash Corruption

•One Discrete Solution

•Integrated IC Solution

•Low-power/Low-cost Solution

•Formulas for Component Value Calculations

•Complete with Sample Schematics

Introduction

This application note shows in detail how to prevent system malfunction during peri-

ods of insufficient power supply voltage. It describes techniques to prevent the MCU

from executing code during periods of insufficient voltage by using external low volt-

age detectors. These events are often referred to as “Brown-outs”, where power

supply voltage drops to an insufficient level, or “Black-outs” where power supply volt-

age totally disappear for a period of time.

One discrete solution is discussed in detail, allowing the user to calibrate the system

requirements. A complete guide to Integrated Circuit (IC) solutions is also included. By

implementing these techniques, the following can be prevented:

• MCU Register Corruption

• I/O Register Corruption

• I/O-pin Random Toggling

• On-chip Memory Corruption (SRAM, EEPROM, Flash)

• External Memory Corruption

C51

Microcontrollers

Application

Note

Rev. 4183A–80C51–11/02

2C51 MCUs 4183A–80C51–11/02

RST Pin Description A high on the RST pin for several machine cycles while the oscillator is running, resets

the device. An internal resistor to VSS permits a power-on reset using only an external

capacitor to VCC. While this pin is forced high, the MCU Core is halted from executing

code, the peripherals are stopped and the I/Os are tri-stated.

The RST pin is an output when the Hardware Watchdog Timer times out and forces a

system reset. When Watchdog Timer overflows, it drives an output RST HIGH pulse at

the RST pin. When the watchdog is used, a resistor Rrst mounted in serial with the exter-

nal capacitor Crst or reset circuit is necessary to allow the Hardware Watchdog to drive

the RST pin.

Theory of Operation For the MCU to successfully decode and execute instructions, the supplied voltage must

always stay above the minimum voltage level specified by the product datasheet. When

supplied voltage drops below this level, the MCU might to execute some instructions

incorrectly. The result is unexpected activity on the internal data and control lines. This

activity may cause MCU Registers, I/O Registers and Data Memories to get corrupted.

To avoid unexpected activity, the MCU should be prevented from executing code during

periods of insufficient supply voltage. This is best ensured by using a Power Supply Low

Voltage Detector. Below a fixed threshold voltage VT, the detector circuit forces the RST

pin high (active). Forcing RST high immediately stops the MCU from executing code.

While the supplied voltage is below the required threshold voltage VT, the MCU is

halted, making sure the system stays in a known state. When the supplied voltage rises

above this predefined voltage, the RST pin is again released, and the MCU starts to

execute code beginning at the Reset Vector.

Preventing SFR

Corruption When the Detector keeps the MCU in Reset, all MCU activity is halted. When released

from Reset, all the Special Function Registers (SFRs) are in their default state.

Without a Detector, random MCU activity such as described before might cause the

Special Function Registers to get corrupted. See “Volatile Memory”below.

Preventing I/O Register

Corruption When using a Detector to keep the MCU in Reset, all I/O Registers are kept in their

default state for the duration of the reset. Consequently, all on-chip peripherals will stay

in their reset state.

Without a Detector, random MCU activity such as described before might write an

unknown value to any I/O Register. This may cause unexpected behavior of the on-chip

peripherals.

Preventing I/O Pin

Random Toggling A Detector will keep the MCU in Reset, and all I/O pins are kept in their default state for

the duration of the Reset.

Without a Detector, random MCU activity such as described before might write a ran-

dom value to the I/O Registers. This may cause random toggling of the I/O pins.

Preventing Volatile On-

chip Memory Corruption Using a Detector to keep the MCU in Reset, there will be no access to the volatile on-

chip memory. The memory will keep its present content for the duration of the Reset.

Without a Detector, random MCU activity such as described in the introduction may

write an unknown value to any volatile on-chip memory location.

C51 MCUs

4183A–80C51–11/02

Preventing External

Components Corruption Using a Detector to keep the MCU in Reset, prevents access to the external compo-

nents such as memories, peripherals, latches, etc. These components will keep their

contents or status for the duration of the Reset.

Without a Detector, random MCU activity could write an unknown value to any External

Components address (SRAM locations, peripheral command registers, etc.).

Preventing Non volatile

On-chip Memory

Corruption

Non volatile memories such as EEPROM, and Flash are designed to keep their contents

even when power is completely removed from the system. By the use of a Detector to

keep the MCU in Reset, activity on the control lines ceases. The memory contents are

then prevented from unintentional writes from the MCU for the duration of the Reset.

Without a Detector, random MCU activity could initialize an unintended write to the non-

volatile memory. This may cause random corruption of the memory contents. Since the

C51 MCU is capable of writing to its own program memory, the internal Flash Program

memory contents could be affected by a power failure situation.

Notes: 1. For any write to non volatile memory, a minimum voltage is required to successfully

write the new values into the memory. If supplied voltage at any time during the write

cycle drops below the minimum voltage, the write might fail, corrupting the location

written to.

2. When the reset activates during a write to the internal EEPROM, the operation is

aborted. The result can be seen when the location being written get corrupted.

Design Criteria

Threshold Voltage It is recommended to set the threshold voltage below minimum VCC to allow for small

fluctuations in supplied voltage (VTMAX must be less than “Supply Voltage Min”). The

threshold voltage should always be selected to ensure that the Detector keeps the

device properly reset when supply voltage drops below the critical voltage required by

the MCU (VTMIN must be greater than “Component Specification VCC Min”). Care should

be taken to ensure sufficiently high detector threshold voltage even in worst case situa-

tions. See Table 4 on page 14 for examples.

Figure 1. Threshold Voltage Choice

Component

Specification

VccMAX

VccMIN

Supply

Voltage

VccMAX

VccMIN

Treshold

Voltage

VTMAX

VTMIN

Hysteresis

HIGH

LOW

4C51 MCUs 4183A–80C51–11/02

Hysteresis Hysteresis is the difference in the voltage between the positive-going switching thresh-

old and the negative-going switching threshold. Without hysteresis, it is possible that

system or environmental noise could cause a rising signal to toggle from low to high,

then high to low again in immediate succession. Hysteresis must ensure that any noise

on the VCC input is unable to cause a rising RST signal to toggle once it has gone from

low to high or a falling RST signal to toggle once it has gone from high to low. Typical

values for hysteresis are 0.2 - 0.3V, it must be selected according to VCC characteristics.

The hysteresis thresholds must be contained between VTMIN and VTMAX.

Operating Voltage Range Carefully monitor the MCU since it can operate with a supply voltage below its Vcc min.

However, its behavior may be erratic. The MCU oscillator can run with a supply voltage

near 1 volt. Under these conditions, the Brown-out circuit must generate a valid RST

signal.

Output Transition Delay Output transition (propagation delay and slew rate) must be fast enough to stop the

MCU before it causes memory corruption or unexpected behavior of peripherals.

C51 MCUs

4183A–80C51–11/02

Implementation A variety of Integrated Circuit (IC) solutions are available from different manufacturers.

These solutions offer high accuracy at a low price, typically they guarantee the threshold

voltage to be within ± 1%. Although the elementary three-pin fixed voltage detector is

available, there is also a whole range of devices offering additional features such as

Reset Pulse stretching, Power-on Reset Time-out, Watchdogs, Power regulation, dual

supply switching for uninterruptible Power Supply operation and more. Included in this

application note is a guide to integrated circuit solutions available. As an alternative, this

application note also presents one discrete Low-power Supply Voltage RESET

Detector.

•Alternative 1: Minimum Power Consumption. Well-suited for battery-powered

applications where power consumption is the most critical parameter.

•Alternative 2: High accuracy, low-power consumption using commercial

semiconductor ICs.

Design Hint: Supply

Voltage Filtering,

Oscillator Stability

Use low impedance capacitors (low ESR and ESL) on the VCC and multi-layer PCB with

power planes to improve transient rejection from the power supply.

To provide good MCU startup, the oscillator must be stable for several cycles before

releasing the RST signal (refer to the device documentation).

6C51 MCUs 4183A–80C51–11/02

Alternative 1: Low-

power Consumption

Characteristics •Very Low-power Consumption, (Typ 0.5 µA@3V, 1 µA@5V)

•Low-cost Solution

•Large Hysteresis, Typ. 0.3 Volts

•Fast Output Transitions

•Accuracy ± 5 - 10%

•High Component Count

•Long Response Time against VCC

Figure 2. Low-power Consumption Brown-out Detector (1) (2)

Notes: 1. Refer to the component specifications, some products such as T89C51CC02 have

an extended tolerance range from 20K to 200K.

2. HWDT: Hardware Watchdog Timer

Figure 3. Oscilloscope Plots Show how the Voltage on RST Varies with VCC

Vcc

Vss

RST

C51 MCU

OPTIONAL

RESET SWITCH

(1)

(2)

Rrst

50-200K

R1 T3

R4 R5

T1 HWDT

Crst

1µF

Vcc

Vss

RST

C51 MCU

OPTIONAL

RESET SWITCH

Rrst

1-5K

50-200K

R4 R5

T1 HWDT

Crst

1µF

VCC Optional

Reset Switch

C51 MCU

Vcc

RST

Vcc 3V

C51 MCUs

4183A–80C51–11/02

Introduction The circuit shown in Figure 2 on page 6 benefits from low-power consumption, which

makes it suitable for battery operated applications. Standard discrete components pro-

vide a low cost design.

The voltage transition on the RST pin is very steep. Combined with the large hysteresis,

the accuracy is high. However, the response time is slow, which makes it unsuitable for

rapidly varying supply voltages.

Theory of Operation The Brown-out Detector has two stages, the Detector and the Amplifier. In the Detector

stage, the threshold voltage is set by the resistors R1 and R2 in relation to the critical

voltage of transistor T1. Under normal operation, this transistor is conducting. When the

supply voltage drops below the threshold voltage, the transistor shuts off.

The output from this Detector leads to the input of the ultra low power Amplifier stage.

Under normal operation, the low voltage of the base of transistor T2 causes it to remain

shut, allowing resistor R5 to pull the RST input low. The Amplifier stage also contains a

hysteresis feedback loop through transistor T3, short-circuiting a resistor R3 in the

amplifier when the RST output is kept high.

Selecting Components

T1, T2, and T3 The production spread of current gain β(or hFE) in transistor T1 affects the threshold

voltage VT(typically ± 0.2 volts). Most small signal transistors can be used, but low pro-

duction spread transistors are recommended.

CAUTION: If transistor T1 is changed from one type to another. The emitter-base

threshold voltage of T1 affects the constant (VBE) in the equation for threshold voltage

(below). As a consequence, a change of transistor could cause a change in the thresh-

old voltage of the detector, which requires the voltage divider R1 + R2 to be re-

calculated.

R1 and R2 R1 and R2 form a voltage divider that defines the threshold voltage VT. As the threshold

voltage depends on these resistors, it is recommended to choose resistors with 1% tol-

erance or better. See “Noise Sensitivity”.

R1 is usually chosen equal to 10 MΩto ensure the lowest power consumption possible.

R2 is then found by the equation below. The constant (Vbe) in the equation may vary

slightly with variations in transistor T1:

Note: Vbe is the T1 base to supply voltage and typically it equals 0.4V to 0.6V.

R3 R3 is a non-critical pull-up resistor that has very little influence on the threshold voltage.

It should be selected as large as possible to minimize power consumption. A resistance

of R3 greater than 10 MΩis not recommended, see “Noise Sensitivity”below.

R4 Resistor R4 defines the hysteresis of the threshold voltage (VT). By choosing R4 to

3.3 MΩ, the resulting hysteresis will be approximately 0.3 volts. A smaller R4 will give a

larger hysteresis, a larger R4 gives smaller hysteresis. A larger R4 will also result in a

less sharp transition in the output slope of the RST signal. Large deviations from the rec-

ommended value will eventually alter the constant VBE in the threshold voltage equation

above. As the hysteresis is only slightly changed with variation in R4 resistance, the

accuracy is not critical.

VTR1 R2+()

Vbe

-----------





or R2 Vbe R1⋅

VTVbe–

------------------------=

,⋅=

8C51 MCUs 4183A–80C51–11/02

R5 Resistor R5 pulls the RST pin low in normal operating mode. A value in the range of

100 kΩis recommended to tie RST securely to GND. As no current goes through this

resistor in normal operating mode, its value and accuracy is of little importance. When

RST is pulled high, this resistor will start conducting a relatively large current.

C1 and C2 Capacitors C1 and C2 reduce RF noise picked up in the circuitry and amplified by the

transistors. Both capacitors can be omitted, but a value greater than 1 nF is recom-

mended. For maximum noise immunity, 100 nF (LF) or capacitors with lower ESR (HF)

should be selected when possible. Also see “Response Time”below. The accuracy is

not critical, but to ensure proper RF decoupling, the capacitors should have Z5U dielec-

tric or better.

C3 Capacitor C3 decouples the power lines. It can be omitted if there is RF decoupling of

the power lines somewhere nearby on the circuit board, otherwise 1 nF is recom-

mended. For maximum noise immunity, 100 nF (LF) or low ESR (HF) should be

selected.

Crst Capacitor Crst (typically 1 µF) from the RST pin to the VCC supply rail is the simplest way

to achieve a reset time of approximatively 100 msS. This reset time is dependant on the

sink current defined by the microcontroller (refer to the device specification). Crst must

be selected to ensure that the RST signal is high during at least two machine cycles

after the power on (refer to the device specification).

Rrst Rrst is necessary when the Hardware Watchdog of the MCU is used. It allows the MCU

to drive the RST pin high and it limits the output current of the RST pin. The recom-

mended value is 1 to 5K.

Reset Switch/In-System

Programming If a push button reset is required, it is simply connected in parallel as shown in Figure 2.

As the switch/programmer will pull RST high, power consumption in R5 will be relatively

high for the duration of the event. Also see “Power Consumption”.

Response Time Choosing large values for capacitors C1 and C2 will slow down the circuit’s response

time. This is not a problem with battery driven applications where the supply voltage

decreases slowly over time. Allthough the response time also applies to the time imme-

diately following power-on. This could affect operation when a flat battery is loaded.

When power can drop more rapidly, the long response time due to C1 and C2 should be

taken into consideration.

Noise Sensitivity Choosing values of R1 and R3 greater than 10 MΩis not recommended, as it makes the

circuitry sensitive to thermal noise generated in the resistor. When noise is not critical,

the values of R1 and R3 can be raised to 20 MΩ. Choosing larger values will result in

the resistors not conducting sufficient current, giving in a non-functional Detector. If

more noise immunity is required, these resistors can be reduced with smaller ones, at

the expense of increased power consumption.

Capacitors C1, C2 and C3 are decoupling capacitors to minimize noise sensitivity to

both RF and 50/60 Hz fields. They can all be omitted, but the noise immunity greatly

depends on the values selected.

C51 MCUs

4183A–80C51–11/02

Threshold Accuracy Because the threshold voltage is defined mainly by R1 and R2, inaccuracies in these

resistors directly influence the threshold voltage accuracy. It is recommended to choose

these with ± 1% tolerance.

Power Consumption The current consumption in normal operating mode (sufficiently high VCC) is found by:

When reset switch or programmer force RST to GND, the current increases to:

When voltage drops to the level where the detector activates, transistor T1 closes, T2

opens and the current is:

As resistor R5 is usually chosen much smaller than the other resistors R1-R4, the last

two expressions both simplify to:

Note: The operator indicates resistors are parallel.

In Table 1 the values are calculated to minimize the power consumption.

Table 1. Example Values when MCU RST Internal Pull-down is in the Range 50 - 200K

Component

Example Values

Recommended Tolerance3.0V 4.5V

T1, T2 BC558/BC888/2N3906 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100

T3 BC548/BC848/2N3904 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100

R1 10 MΩ≤1%

R2 1.54 MΩ976 kΩ≤1%

R3 6.8 MΩ≤20%

R4 3.3 MΩ≤20%

R5 100 kΩ≤20%

C1, C2, C3 100 nF ≤20%, Z5U dielectric or better

IVCC

R1 R2+()R3 R4+()

----------------------------------------------------------





VCC 1

R1 R2+

--------------------- 1

R3 R4+

---------------------+





=≈

IVCC

R1 R2+()R3 R4+()R5 RRESET

|| || ||

-------------------------------------------------------------------------------------------------

≈

IVCC

R1 R2+()R5 RRESET

|| ||

-----------------------------------------------------------------≈

IVCC

R5 RRESET

---------------------------------≈

10 C51 MCUs 4183A–80C51–11/02

In Table 2 the transistor T2 can source more current. The Vbe constant is approximately

0.5V.

Table 2. Example Values when MCU RST Internal Pull-down is in the Range 20 - 200K

Component

Example Values

Recommended Tolerance3.0V 4.5V

T1, T2 BC558/BC888/2N3906 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100

T3 BC548/BC848/2N3904 ICE ≥2.5 mA, VCE ≥8 V, β/hFE ≥100

R1 6.04 MΩ6.19 MΩ≤1%

R2 1.3 MΩ820 kΩ≤1%

R3 1.5 MΩ≤20%

R4 510 KΩ≤20%

R5 100 kΩ≤20%

C1, C2, C3 100 nF ≤20%, Z5U dielectric or better

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