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DIFFERENT WAYS TO RESET ST7 MICROCONTROLLER

tion cannot be used for very slow supply voltage rise time. It would suitable when the rise time

is fairly fast.

In all ST7 microcontrollers you have a weak pull-up equivalent resistor whose values differ for

different MCU. This equivalent resistor is sensitive to temperature so while calculating the

component values based on the maximum supply rise time, please remember to take this re-

sistor into account as it would come in parallel with the external resistor. In case your applica-

tion would be running at very low temperature an internal pull-up resistor at the RESET pin

would have a minimum value.

This circuit cannot be used when the power supply oscillates during power-down. As the

supply may drop momentarily (oscillate) below the minimum safe level, with the RESET pin

held high, as a consequence internal logic is not guaranteed any more, and the RESET pin

cannot be seen at a logic low level to ensure a safe restart.

3.2 Transistor based External Reset circuits

This solution is necessary if the supply voltage rise time is unspecified or it is very slow.

The transistor is closed as long as the supply voltage is below the threshold value, this allows

The transistor is turned on once the voltage from emitter to base drops below a certain value,

usually around 0.6-0.7V.

Figure 5. Transistor based External Reset circuit with potential divider to bias the

transistor.

Resistors, R1 and R2 form a voltage divider which controls the emitter-base voltage. The

threshold is defined by {(R1+R2)/R1}*Vbe

Depending on the threshold required and the emitter-base voltage of the transistor, you would

get the ratio of two resistance, R2/R1. Selecting the resistor values in order to satisfy this ratio

would give the desired threshold.

While making the selection of the pull-down resistance connected at the RESET pin there are

certain points to be considered. The RESET pin in ST7 has a pull-up resistance of minimum

20K, when the transistor is off this pull-up resistance and the pull-down resistance at the

10K

RESET

1K

R1

R2