Datasheet

67
AT89C51IC2
4301D–8051–02/08
Power Management Two power reduction modes are implemented in the AT89C51IC2: the Idle mode and
the Power-down mode. These modes are detailed in the following sections. In addition
to these power reduction modes, the clocks of the core and peripherals can be dynami-
cally divided by 2 using the X2 mode detailed in Section “Enhanced Features”.
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 AT89C51IC2 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 V
DD
as shown in Figure 23. 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 Sec-
tion “DC Characteristics” of the AT89C51IC2 datasheet.
Figure 23. Reset Circuitry and Power-On Reset
Cold Reset 2 conditions are required before enabling a CPU start-up:
• V
DD
must reach the specified V
DD
range
• The level on X1 input pin must be outside the specification (V
IH
, V
IL
)
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:
• V
DD
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 KΩ and different oscillator startup and V
DD
rise times.
R
RST
RST
VSS
To CPU Core
and Peripherals
RST
VDD
+
Power-on ResetRST input circuitry
P
VDD
From Internal
Reset Source