Atmel 80C51 Installation and operating instructions

How to Calculate the Capacitor of the Reset

Input of a C51 Microcontroller

This application note explains how the reset of the 80C51 microcontroller works when

the RST pin is a pure input pin and when the RST input is bi-directional. It gives rules

to determine the extra components required to operate the reset function properly.

The reset process can be active on low or high level depending on the product. In this

application note only the high level case is discussed.

Introduction

The reset is used to start-up or to restart the 80C51 microcontroller activities. It forces

the 80C51 in a known state by reinitializing all the internal registers needed to properly

start the program execution. The reset must be kept active until all three of the follow-

ing conditions are respected:

• The power supply must be in the specified range.

• The oscillator must reach a minimum oscillation level to ensure a good noise to

signal ratio and a correct internal duty cycle generation.

• The reset pulse width duration must be at least two machine cycles.

If one of the conditions is not respected the microcontroller will not startup properly.

Theory of Reset Operation

To ensure a good startup, the reset pulse width has to be wide enough to cover the

period of time where the electrical conditions are not met. Two parameters should be

considered for a proper reset sequence to determine the reset pulse width (see

Figure 1):

•tosc: time needed by the oscillator to reach the Vih1 or Vil1 level.

•tvddrise: rise time of the power-supply taken between 10 to 90% of VDD.

When these two parameter conditions are met, the reset has to be maintained at least

two machine cycles in order to synchronize the internal activity of the core. In normal

mode, a machine cycle is 12-clock periods and in X2 mode is 6-clock periods.

Rev. 4284A–8051–09/03

80C51

Microcontrollers

ApplicationNote

C51 Reset Guidelines

4284A–8051–09/03

Figure 1. tosc and tvddrise are the Two Parameters to Evaluate, Depending on the

Application

Vih1 is the reference parameter taken to calculate and determine the time constant of

the reset. Indeed, normally the input is considered to be low when its level reaches Vil1.

But in reality the trigger is somewhere between Vih1 and Vil1. So, the worst case condi-

tion is considered at the Vih1 level.

When the reset is released, the program execution starts and the ALE signal toggles as

it is illustrated in Figure 2. and showing a proper startup condition:

•VDD is within the voltage operating range,

• The level of Xtal1 is greater than the Vih1 level specification,

• The reset reached its active level (Vih1) and is maintained at least two machine

cycles.

Figure 2. Reset Conditions to Properly Startup a Microcontroller

tvddrise

tosc

Vih1

treset

VDD

XTAL1

VDD

Vrst

XTAL2

Reset is released

ALE

C51 Reset Guidelines

4284A–8051–09/03

Why Does the Reset Does Not

Properly Start the

Microcontroller?

The Oscillator is not Stabilized Figure 3 shows the case where the RST signal is applied while the oscillator is not stabi-

lized and there is no clock to reset the internal registers of the CPU. Due to this bad

reset, the first address fetched can be anywhere in the program space except address

0000h.

Figure 3. If electrical Conditions are not Met, the Reset Signal is Applied but without the

Clock

Reset is Released before VDD is

Stable Figure 4 shows the case where the reset is released before the VDD is stable. The micro-

controller will never see the reset and can start anywhere in the program space and with

a bad register initialization.

Figure 4. The Reset is Released before the Clock and the VDD are Stable

VDD

Vrst

XTAL

ALE

The reset is applied

while the clock is not

stabilized

VDD

Vrst

XTAL2

ALE

C51 Reset Guidelines

4284A–8051–09/03

Reset Input Circuitry

Description At least two kinds of reset input structure exist in C51 products. The first one is a pure

input which allows an external device to reset the microcontroller. The second one is bi-

directional. The microcontroller can be reset by an external device. The microcontroller

can reset an external device when, for example the internal watchdog expires. Table 1

lists some C51 Atmel products which have uni-directional or bi-directional reset.

uni-directional Reset

Input

Description The uni-directional reset input circuitry is shown in Figure 5. A pull-down resistor, Rrst,

is connected between the RST input and the ground. An external capacitor, Crst, is con-

nected between the RST input and the VDD. The value of Crst determines the reset time

duration. The calculation of Crst is explained in the next chapter.

Figure 5. Reset Structure for an uni-directional Circuitry

Theory of Operation When a reset is applied on the RST input, Crst is discharged and then charged through

Rrst. The reset is active until the level applied on the RST pin is below Vih1. Crst deter-

mines the reset pulse width duration.

Table 1. Examples of Products Using Uni-directional and Bi-directional Reset

Product Main Features Uni/Bidir

T83C51RB2 16 KB of ROM, Watchdog B

T89C51RD2 64 KB of Flash, Watchdog B

AT89C51CC01 10-bit ADC, CAN controller B

AT89C51SND1 MP3 decoder, TWI, MMC, USB U

C51 Reset Guidelines

4284A–8051–09/03

Bi-directional Reset

Input

Description The bi-directional reset circuitry is shown in Figure 6. In addition to the uni-directional

structure, the RST pin is able to drive an external reset for example when a watchdog

expires. To do this, a pull-up resistor (Rrstwt) controlled by the rstcon signal drives a

high level on the RST pin. An extra resistor (Rrstext) must be added between the RST

input and Crst (Figure 6.).

Figure 6. Reset Structure in a Bi-directional Mode

Theory of Operation

External Reset During a power-up or when an external reset is applied to the RST input, the Crst

capacitor is charged through the two resistors Rrstext and Rrst. The reset is active until

the level applied on the RST pin is below Vih1. The Rsrtext resistor is required when an

internal reset is applied by the microcontroller and will be explained in the next session.

Internal Reset In some cases, such as a watchdog reset, the microcontroller generates an internal

reset by driving the rstcon signal and consequently by applying a high level on the RST

pin. The pulse duration depends on the product and is typically equal to 96 clock periods

(see the product datasheet).

The Rrstext allows a reset pulse to occur on the RST pin. The reset time constant (sev-

eral ms) is large in comparison to the reset pulse duration (96 clock periods). In that

condition, Crst maintains its charge (VDD) for all the duration of the reset pulse even if

Rrstwt is active (see Figure 7). VDD is applied across the Rrst and Rrstwt resistors and

expression of Vrst is given below:

For a given Rrstwt resistance, Rrstext determines the active level of the reset pin. To

take into account on the external and internal reset constraints, Rrstext must be chosen

in the 1 kΩand 10 kΩrange.

vrst(t)

VDD

Rrstwt

1 2

Rrstext

Crst

1 2

Rrst

rstcon

Internal reset

CPU clock

ALE

RST

External reset

Reset

circuitry

Oscillator

Vrst VDD Rrstext

Rrstext Rstwt

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

×=

C51 Reset Guidelines

4284A–8051–09/03

Figure 7. Reset Circuitry When an Internal Reset is Applied

Without Rrstext no external reset signal will be generated.

Use Excel File to

Determine Crst To determine Crst, the reset pulse width needs to be calculated using the following

equation:

treset = tvddrise + tosc

•tvddrise (typically 1 ms to 100 ms), is the rise time of the VDD (10% and 90% of the

VDD). It depends on the power supply and the decoupling capacitors used.

•tosc (typically 1 ms to 50 ms), time taken by the oscillator at startup. It depends on

the crystal characteristics and the capacitors connected to the crystal.

Because the power supply has a finite transition time (several hundreds of microsec-

onds to several milliseconds), Crst is not so easy to compute by hand. Excel tool is

used to calculate Crst versus tvddrise and tosc parameters. A spreadsheet can be down-

loaded from the Atmel Web site to compute Crst.

Four parameters have to be entered and Crst is directly computed by the spreadsheet

while 1kΩis chosen for Rrst. Here is how to do it:

•VDD, the power supply voltage, is entered in the cell F3

•tvddrise, the rise time of the power-supply, is entered in the cell F4

•tosc, the oscillator startup time, is entered in cell F8

•Rrstmin, the minimum pull-down resistor, is entered in the cell F13

After these steps, the cell E31 has to be clicked to compute the Crst and the minimum

reset pulse width.

VDD

Rrstwt

RrstRrstext

Crst

Internal reset

VDD

Vrst

Vih1

C51 Reset Guidelines

4284A–8051–09/03

Table 2. Excel Spreadsheet Use to Calculate Crst

Table 3 gives the Crst value computed from the Excel file for different values of tvddrise

and tosc parameters.

Table 3. Minimum Reset Capacitor Value for a 50kΩPull-down Resistor (Rrstmin)

Table 4. Minimum Reset Capacitor Value for a 100KΩPull-down Resistor

Power Supply Characteristics

Power Supply Voltage VDD = 5 V

Power Supply Rise Time (10% to 90%) tvddrs = 1ms

Oscillator

Oscillator Startup Time time, measured at VIH1 tosct = 10 ms

Electrical Characteristics of the Reset Input

Minimum pull-down resistance Rrstmin = 100 kΩ

Minimum Pulse Width

Calculation of the reset pulse width trstmin = 22, 726 ms

Evaluation of Reset Capacitor: C

Minimum reset pulse width Trst = 65, 545 ms

Value of reset capacitor C C = 0.649 µF

tosc

tvddrise

1 ms 10 ms 100 ms

5 ms 820 nF 1.2 µF 12 µF

20 ms 2.7 µF 3.9 µF 12 µF

tosc

tvddrise

1 ms 10 ms 100 ms

5 ms 390 nF 0.56 µF 5.6 µF

20 ms 1.2 µF 2 µF 5.6 µF

Printed on recycled paper.

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warranty which is detailed in Atmel’s Terms and Conditions located on the Company’s web site. The Company assumes no responsibility for any

errors which may appear in this document, reserves the right to change devices or specifications detailed herein at any time without notice, and

does not make any commitment to update the information contained herein. No licenses to patents or other intellectual property of Atmel are

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