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Doc ID 7215 Rev 4
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 22. 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-
cy
– 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
powered by the USB or from a battery (Digital Au-
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-
cy
– The DTC operates at a frequency of 6MHz
– USBEN is kept floating by H/W.
– The microcontroller is supplied through the VDD
pin
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-
cy
– The DTC operates at a frequency of 24MHz
– 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
– The microcontroller can be USB bus powered
VDD1
VDD2
VDDA
USBVDD
ST7
4.0 - 5.5 V
Note: Ground lines not shown
1