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
VDD
From Internal
Reset Source
P
VDD
To CPU Core
and Peripherals
RST
+
RST
VSS
RST input circuitry
Power-on Reset
Cold Reset
2 conditions are required before enabling a CPU start-up:
•
•
V
DD must reach the specified VDD range
The level on X1 input pin must be outside the specification (VIH, 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 VIH1 is reached and when the pulse width covers the
period of time where VDD 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 RRST of
50 KΩ and different oscillator startup and VDD rise times.
80
AT89C51RB2/RC2
4180E–8051–10/06
ATMEL [ ATMEL ]
