9202 datasheet(51/175 Pages) STMICROELECTRONICS

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UPSD3254A, UPSD3254BV, UPSD3253B, UPSD3253BV

Each RESET source will cause an internal reset

signal active. The CPU responds by executing an

internal reset and puts the internal registers in a

defined state. This internal reset is also routed as

an active low reset input to the PSD Module.

External Reset

The RESET pin is connected to a Schmitt trigger

for noise reduction. A RESET is accomplished by

holding the RESET pin LOW for at least 1ms at

power up while the oscillator is running. Refer to

AC spec on other RESET timing requirements.

Low VDD Voltage Reset

An internal reset is generated by the LVR circuit

when the VDD drops below the reset threshold. Af-

ter VDD reaching back up to the reset threshold,

the RESET signal will remain asserted for 10ms

before it is released. On initial power-up the LVR

is enabled (default). After power-up the LVR can

be disabled via the LVREN Bit in the PCON Reg-

ister.

Note: The LVR logic is still functional in both the

Idle and Power-down Modes.

The reset threshold:

5V operation:

4V +/- 0.25V

3.3V operation: 2.5V +/-0.2V

This logic supports approximately 0.1V of hystere-

sis and 1µs noise-cancelling delay.

Watchdog Timer Overflow

The Watchdog timer generates an internal reset

when its 22-bit counter overflows. See Watchdog

Timer section for details.

USB Reset

The USB reset is generated by a detection on the

USB bus RESET signal. A single-end zero on its

upstream port for 4 to 8 times will set RSTF Bit in

UISTA register. If Bit 6 (RSTE) of the UIEN Regis-

ter is set, the detection will also generate the

RESET signal to reset the CPU and other periph-

erals in the MCU.

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