Auto Page CPX-3600 Specifikace Strana 208

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Electrical characteristics ST72321xx-Auto
208/243 Doc ID 13829 Rev 1
Figure 90. RESET pin
protection when LVD is enabled
Note: 1 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).
Whether the reset source is internal or external, the user must ensure that the level on the
RESET
pin can go below the V
IL
maximum level specified in Section 19.9.1 on page 207.
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 I
INJ(RESET)
in Section 19.2.2 on page 187.
2 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.
3 In case a capacitive power supply is used, it is recommended to connect a 1M
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).
4 Tips when using the LVD:
A. Check that all recommendations related to reset circuit have been applied (see notes
above).
B. 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
pull-down on the RESET pin.
C. The capacitors connected on the RESET
pin and also the power supply are key to avoid
any start-up marginality. In most cases, steps A and B above are sufficient for a robust
solution. Otherwise, replace 10nF pull-down on the RESET
pin with a 5µF to 20µF
capacitor.
0.01µF
ST72XXX
PULSE
GENERATOR
Filter
R
ON
V
DD
WATCHDOG
LVD RESET
INTERNAL
RESET
RESET
EXTERNAL
Required
1M
Optional
(note 3)
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