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EEPROM protection

in AVR controllers

031

H. Bonekamp

AVR controllers have the unfortunate property of their data EEP-

ROM being affected when the supply voltage drops below a cer-

tain level, which can, of course, be prevented by making the reset

low in good time to disable the processor. Unfortunately, this

requires a circuit for monitoring the supply voltage and for taking

the requisite action automatically when needed.

This requirement is met by the circuit in the diagram, which

draws a low enough current to enable it being powered by a bat-

tery. The circuit may be split into a detector, T 1 , and an amplifier,

T 2 –T 3 .

The trip voltage of the detector is determined by the values of

R 1 and R 2 . Normally, the transistor conducts, but as soon as the

supply voltage drops below the trip level, it is cut off. The output of

Elektor Electronics

7-8/99

59

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T 1 is applied to a low-power amplifier.

During normal operation, transistor T 3 is off, so that R 5 func-

tions as pull-up resistor to retain the RST input of the AVR

processor high. When the detector goes off, T 3 is switched on and

the RST input goes low, a process that is enhanced by transistor

T 2 being switched on, whereupon R 3 is shorted out. The resulting

hysteresis requires the supply voltage to exceed the trip voltage

before this situation can change. A manual reset is possible at all

times with witch S 1 .

As stated earlier, the trip voltage is determined by the values

of R 1 and R 2 (and, to some degree, the base-emitter potential of T 1

– about 540 mV), and the tolerance of these resistors should there-

fore be 1%. If the trip voltage needs to be altered, it is best to

retain the value of R 1 at 10 M

5V

R1

R4

R5

ISP

VCC

BC

557B

R3

T2

AVR

AT90S.....

RST

T3

and change the value of R 2

T1

BC

GND

according to

547B

R2

S1

C3

C1

C2

R 2 =0.54R 1 /( U b –0.54),

100n

547B

100n

100n

RESET

where R 2 is in ohms and U b is the supply voltage.

The hysteresis is determined by the value of R 4 : the smaller this

is, the larger the hysteresis. The specified value of 3.3 MΩ

994083 - 11

is fine

for most cases, but some experimentation does no harm.

[994083]

BC

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