AT89C51RC2-3CSIM datasheet(80/127 Pages) ATMEL

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AT89C51RB2/RC2

4180C–8051–12/03

Power Management

Two power reduction modes are implemented in the AT89C51RB2/RC2: the Idle mode

and the Power-down mode. These modes are detailed in the following sections. In addi-

tion to these power reduction modes, the clocks of the core and peripherals can be

dynamically divided by 2 using the X2 mode detailed in Section “X2 Feature”.

Reset

In order to start-up (cold reset) or to restart (warm reset) properly the microcontroller, an

high level has to be applied on the RST pin. A bad level leads to a wrong initialization of

the internal registers like SFRs, Program Counter… and to unpredictable behavior of

the microcontroller. A proper device reset initializes the AT89C51RB2/RC2 and vectors

the CPU to address 0000h. RST input has a pull-down resistor allowing power-on reset

by simply connecting an external capacitor to VDD as shown in Figure 32. A warm reset

can be applied either directly on the RST pin or indirectly by an internal reset source

such as the watchdog timer. Resistor value and input characteristics are discussed in

the Section “DC Characteristics” of the AT89C51RB2/RC2 datasheet.

Figure 32. Reset Circuitry and Power-On Reset

Cold Reset

2 conditions are required before enabling a CPU start-up:

•VDD must reach the specified VDD range

•

The level on X1 input pin must be outside the specification (V

IH, VIL)

If one of these 2 conditions are not met, the microcontroller does not start correctly and

can execute an instruction fetch from anywhere in the program space. An active level

applied on the RST pin must be maintained till both of the above conditions are met. A

reset is active when the level V

IH1 is reached and when the pulse width covers the

period of time where V

DD and the oscillator are not stabilized. 2 parameters have to be

taken into account to determine the reset pulse width:

•VDD rise time,

•

Oscillator startup time.

To determine the capacitor value to implement, the highest value of these 2 parameters

has to be chosen. Table 1 gives some capacitor values examples for a minimum R

RST of

50 K

Ω and different oscillator startup and V

DD rise times.

RST

VSS

To CPU Core

and Peripherals

RST

VDD

Power-on Reset

RST input circuitry

VDD

From Internal

Reset Source

AT89C51RC2-3CSIM Datasheet(PDF) 80 Page - ATMEL Corporation

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