ST7265X datasheet(33/166 Pages) STMICROELECTRONICS

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ST7265x

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6.4 POWER SUPPLY MANAGEMENT

6.4.1 Single Power Supply Management

In applications operating only when connected to

the USB (Flash writers, Backup systems), the mi-

crocontroller must operate from a single power

supply (i.e. USB bus power supply or the local

power source in the case of self-powered devic-

es). Devices with LVD (no E suffix) or without LVD

(E suffix) can support this configuration.

In order to enable the Single Power Supply Man-

agement, the PLGIE bit in the PCR register should

kept cleared by software (reset default value).

In this case, pin VDD and USBVDD of the micro-

controller must be connected together and sup-

plied by a 4.0 to 5.5V voltage supply, either from

the USB cable or from the local power source. See

Figure 23.

Figure 23. Single Power Supply Mode

In this mode:

– The PLL is running at 48 MHz

– The on-chip USB interface is enabled

– The core can run at up to 8MHz internal frequen-

– The microcontroller can be either USB bus pow-

ered or supplied by the local power source (self

powered)

– The USBEN function is not used. The PF4 pin

can be configured to work as a normal I/O by pro-

gramming the Option Byte.

6.4.2 Dual Power Supply Management

In case of a device that can be used both when

dio Player, Digital Camera, PDA), the microcon-

troller can operate in two power supply modes:

Stand-alone Mode and USB Mode. This configura-

tion is only available on devices without LVD (E

suffix). Devices with LVD are kept under reset

when the power supply drops below the LVD

threshold voltage and thus Stand-Alone mode can

not be entered.

In order to enable Dual Power Supply Manage-

ment:

– the USBEN pin function must be selected by pro-

gramming the option byte.

– the user software must set the PLGIE bit in the

PCR register in the initialization routine.

Stand-Alone Mode

This mode is to be used when no USB communi-

cation is needed. The microcontroller in this mode

can run at very low voltage, making the design of

low power / battery supplied systems easy. In this

mode:

– The USB cable is unplugged (no voltage input on

USBVDD pin)

– The PLL is off

– The on-chip USB interface is disabled

– The core can run at up to 6 MHz internal frequen-

– USBEN is kept floating by H/W.

– The microcontroller is supplied through the VDD

USB Mode

When connected to the USB, the microcontroller

can run at full speed, still saving battery power by

using USB power or self power source. To go into

USB mode, a voltage from 4.0V to 5.5V must be

provided to the USBVDD pin. In this mode:

– The USB cable is plugged in

– USBVDD pin is supplied by a 4.0 to 5.5V supply

voltage, either from the USB cable or from the

self powering source

– The PLL is running at 48 MHz

– The on-chip USB interface is enabled

– The core can run at up to 8 MHz internal frequen-

– USBEN is set to output low level by hardware.

This signal can be used to control an external

transistor (USB SWITCH) to change the power

supply configuration (see Figure 24).

– The microcontroller can be USB bus powered

VDD1

VDD2

VDDA

USBVDD

ST7

4.0 - 5.5 V

Note: Ground lines not shown

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