AT89C5115-RATUM datasheet(18/113 Pages) ATMEL

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AT89C5115

4128F–8051–05/06

Power Management

Two power reduction modes are implemented in the T89C5115: 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 dynamically

divided by 2 using the X2 Mode detailed in Section “Clock”.

Reset Pin

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

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

the internal registers like SFRs, PC, etc. and to unpredictable behavior of the microcon-

troller. A warm reset can be applied either directly on the RST pin or indirectly by an

internal reset source such as a watchdog, PCA, timer, etc.

At Power-up (cold reset)

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

•

VDD must reach the specified VDD range,

•

The level on xtal1 input must be outside the specification (VIH, VIL).

If one of these two 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 until 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. Two parameters have

to be taken into account to determine the reset pulse width:

•

VDD rise time (vddrst),

•

Oscillator startup time (oscrst).

To determine the capacitor the highest value of these two parameters has to be chosen.

The reset circuitry is shown in Figure 5.

Figure 5. Reset Circuitry

Table 12 and Table 13 give some typical examples for three values of VDD rise times,

two values of oscillator start-up time and two pull-down resistor values.

Table 12. Minimum Reset Capacitor for a 50K Pull-down Resistor

oscrst/vddrst

1ms

10ms

100ms

5ms

820nF

1.2µF

12µF

20ms

2.7µF

3.9µF

12µF

VDD

Rrst

Crst

RST pin

Internal reset

Reset input circuitry

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