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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):
•
t
: time needed by the oscillator to reach the Vih1 or Vil1 level.
osc
•
t
: rise time of the power-supply taken between 10 to 90% of V
vddrise
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.
80C51
Microcontrollers
Application Note
.
DD
Rev. 4284A–8051–09/03
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Summary of Contents for Atmel 80C51
- Page 1 Application Note 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: •...
- Page 2 Figure 1. t and t are the Two Parameters to Evaluate, Depending on the vddrise Application treset Vih1 vddrise XTAL1 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.
- Page 3 C51 Reset Guidelines 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.
- Page 4 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. Table 1. Examples of Products Using Uni-directional and Bi-directional Reset...
- Page 5 C51 Reset Guidelines 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.
- Page 6 A spreadsheet can be down- vddrise loaded from the Atmel Web site to compute Crst. Four parameters have to be entered and Crst is directly computed by the spreadsheet while 1k Ω is chosen for Rrst. Here is how to do it: •...
- Page 7 C51 Reset Guidelines Table 2. Excel Spreadsheet Use to Calculate Crst Power Supply Characteristics Power Supply Voltage Power Supply Rise Time (10% to 90%) tvddrs = Oscillator Oscillator Startup Time time, measured at VIH1 tosct = Electrical Characteristics of the Reset Input k Ω...
- Page 8 No licenses to patents or other intellectual property of Atmel are granted by the Company in connection with the sale of Atmel products, expressly or by implication. Atmel’s products are not authorized for use as critical components in life support devices or systems.