ST7FOXA0 datasheet(106/123 Pages) STMICROELECTRONICS

106 / 123 page

Electrical characteristics

ST7FOXA0

106/123

Figure 39.

RESET pin protection when LVD is enabled

The reset network protects the device against parasitic resets. The output of the external reset circuit must

have an open-drain output to drive the ST7 reset pad. Otherwise the device can be damaged when the

ST7 generates an internal reset (LVD or watchdog). Whatever the reset source is (internal or external), the

user must ensure that the level on the RESET pin can go below the VIL max. level specified in

Section 12.9.1 on page 105. Otherwise the reset will not be taken into account internally. Because the

reset circuit is designed to allow the internal Reset to be output in the RESET pin, the user must ensure

that the current sunk on the RESET pin is less than the absolute maximum value specified for IINJ(RESET) in

Section Table 40. on page 94.

When the LVD is enabled, it is recommended not to connect a pull-up resistor or capacitor. A 10nF pull-

down capacitor is required to filter noise on the reset line.

In case a capacitive power supply is used, it is recommended to connect a 1M

Ω pull-down resistor to the

RESET pin to discharge any residual voltage induced by the capacitive effect of the power supply (this will

add 5µA to the power consumption of the MCU).

Tips when using the LVD

●

Check that all recommendations related to ICCCLK and reset circuit have been applied

(see caution in Table 2 on page 11 and notes above).

●

Check that the power supply is properly decoupled (100nF + 10µF close to the MCU).

Refer to AN1709 and AN2017. If this cannot be done, it is recommended to put a

100nF + 1M

Ω pull-down on the RESET pin.

●

The capacitors connected on the RESET pin and also the power supply are key to

avoid any start-up marginality. In most cases, steps 1 and 2 above are sufficient for a

robust solution. Otherwise: replace 10nF pull-down on the RESET pin with a 5µF to

20µF capacitor.”

0.01

µF

ST7xxx

PULSE

GENERATOR

Filter

RON

VDD

INTERNAL

RESET

EXTERNAL

Required

Ω

Optional

(note 3)

WATCHDOG

LVD RESET

ILLEGAL OPCODE

Obsolete

Product(s)

- Obsolete

Product(s)

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