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

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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
VDD
Vrst
XTAL2
Reset is released
ALE

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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
2
ALE
The reset is applied
while the clock is not
stabilized
VDD
Vrst
XTAL2
ALE

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C51 Reset Guidelines
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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

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C51 Reset Guidelines
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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
Q1
Rrst
rstcon
Internal reset
CPU clock
X1
X2
ALE
RST
External reset
Reset
circuitry
Oscillator
Vrst VDD Rrstext
Rrstext Rstwt
+
-------------------------------------------
×=

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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.
0
VDD
0
Rrstwt
RrstRrstext
Crst
Internal reset
VDD
Vrst
Vih1

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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

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