AT83SND1CXXX-ROTIL datasheet(48/212 Pages) ATMEL

48 / 212 page

4109J–8051–10/06

AT8xC51SND1C

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

be chosen. Table 48 gives some capacitor values examples for a minimum R

RST of 50 KΩ and

different oscillator startup and V

DD rise times.

Table 48. Minimum Reset Capacitor Value for a 50 k

Ω Pull-down Resistor(1)

Note:

1. These values assume V

DD starts from 0V to the nominal value. If the time between 2 on/off

sequences is too fast, the power-supply de-coupling capacitors may not be fully discharged,

leading to a bad reset sequence.

9.1.2

Warm Reset

To achieve a valid reset, the reset signal must be maintained for at least 2 machine cycles (24

oscillator clock periods) while the oscillator is running. The number of clock periods is mode

independent (X2 or X1).

9.1.3

Watchdog Reset

As detailed in section “Watchdog Timer”, page 60, the WDT generates a 96-clock period pulse

on the RST pin. In order to properly propagate this pulse to the rest of the application in case of

external capacitor or power-supply supervisor circuit, a 1 k

Ω resistor must be added as shown in

Figure 9-2.

Reset Circuitry for WDT Reset-out Usage

9.2

Reset Recommendation to Prevent Flash Corruption

An example of bad initialization situation may occur in an instance where the bit ENBOOT in

AUXR1 register is initialized from the hardware bit BLJB upon reset. Since this bit allows map-

ping of the bootloader in the code area, a reset failure can be critical.

If one wants the ENBOOT cleared in order to unmap the boot from the code area (yet due to a

bad reset) the bit ENBOOT in SFRs may be set. If the value of Program Counter is accidently in

the range of the boot memory addresses then a Flash access (write or erase) may corrupt the

Flash on-chip memory.

It is recommended to use an external reset circuitry featuring power supply monitoring to prevent

system malfunction during periods of insufficient power supply voltage (power supply failure,

power supply switched off).

Oscillator

Start-Up Time

VDD Rise Time

1 ms

10 ms

100 ms

5 ms

820 nF

1.2 µF

12 µF

20 ms

2.7 µF

3.9 µF

12 µF

RST

VSS

To CPU Core

and Peripherals

VDD

From WDT

Reset Source

VSS

VDD

RST

To Other

On-board

Circuitry

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