ST72F324J2T3 datasheet(137/163 Pages) STMICROELECTRONICS

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ST72324

137/163

CONTROL PIN CHARACTERISTICS (Cont’d)

Figure 77. RESET pin protection when LVD is enabled.1)2)3)4)5)6)7)

Figure 78. RESET pin protection when LVD is disabled.1)2)3)4)

1. The reset network protects the device against parasitic resets.

2. 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).

3. 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 12.10.1. Otherwise the reset will not be taken into account internally.

4. 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 (by an external pull-up for example) is less than the absolute maximum value

specified for IINJ(RESET) in Section 12.2.2 on page 117.

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

to filter noise on the reset line.

6. In case a capacitive power supply is used, it is recommended to connect a1M

Ω pull-down resistor to the RESET pin

to discharge any residual voltage induced by this capacitive power supply (this will add 5µA to the power consumption of

the MCU).

7. Tips when using the LVD:

– 1. Check that all recommendations related to ICCCLK and reset circuit have been applied (see notes above)

– 2. Check that the power supply is properly decoupled (100nF + 10µF close to the MCU). Refer to AN1709. If this

cannot be done, it is recommended to put a 100nF + 1M

Ω pull-down on the RESET pin.

– 3. The capacitors connected on the RESET pin and also the power supply are key to avoiding any start-up margin-

ality. 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

ST72XXX

PULSE

GENERATOR

Filter

RON

VDD

WATCHDOG

LVD RESET

INTERNAL

RESET

EXTERNAL

Recommended

Ω

Optional

(note 6)

0.01

µF

VDD

0.01

µF

EXTERNAL

RESET

CIRCUIT

USER

VDD

4.7k

Ω

Required

Recommended

ST72XXX

PULSE

GENERATOR

Filter

RON

VDD

WATCHDOG

INTERNAL

RESET

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