ST7265X datasheet(34/166 Pages) STMICROELECTRONICS

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ST7265x

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POWER SUPPLY MANAGEMENT (Cont’d)

6.4.2.1 Switching from Stand-Alone Mode to

USB Mode

In Stand-Alone Mode, when the user plugs in the

USB cable, 4V min. is input to USBVDD. The on-

chip power Supply Manager generates an internal

interrupt when USBVDD reaches USBVIT+ (if the

PLGIE bit in the PCR register is set). The user pro-

gram then can finish the current processing, and

MUST generate a software RESET afterwards.

This puts the microcontroller into reset state and

all I/O ports go into input high impedance mode.

During and after this (software induced) reset

phase, the USBEN pin is set to output low level by

hardware. This causes the USB SWITCH to be

turned ON. Consequently, VDD pin is powered by

USBVDD supply. See Figure 24.

Once in USB mode, no power is drawn from the

step-up converter output.

For more details, refer to Figure 25.

Figure 24. External Power Supply Switch

VDD1

USB SWITCH

VDD2

VDDA

USBVDD

USBEN

ST7

(True OD, H/W crtl)

Step-up converter

4V min. from USB

Note 1: Ground lines not shown

PCR REGISTER

PLG

General Purpose I/O (I/O port DR, DDR)

Option bit

USBEN H/W

CONTROL

USBVIT-

USBVIT+

USBVIT-

PLG bit

USBVDD

Alternate Function (USBEN)

PMOS

VITPF

USBVIT+

VITMF Bit

VITPF Bit

PLGIE

VITMF

Interrupt Request

RESET

LOGIC

S/W RESET

EDGE DETECTOR

USB VOLTAGE

DETECTOR

WITH LATCH

DETEN

(Note 2)

Note 2: Suggested device: IRLML6302 (International

rectifier) or Si230DS (Siliconix)

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