Intel PXA27X manual Power Supply Configuration in a Minimal System, Application Note, Voltage

Intel® PXA27x Processor Family Power Requirements

Table 2. Intel® PXA27x Processor Voltage Domains (Sheet 2 of 2)

Voltage	Description
VCC_IO	Fixed 3.0 V or 3.3 V (±10%) for standard CMOS I/Os interfacing to external
	components, which are also supplied from fixed 3.0 V or 3.3 V. The I/Os for external
	components connected to the corresponding signals on the PXA27x processor must be
	supplied from the same regulator. Driving VCC_BATT in this manner prevents forward-
	biasing of protection diodes and inadvertent charging of the backup battery through
	inputs on the PXA27x processor VCC_REG domain. The VCC_IO supply must be the
	highest potential in the system (excluding VCC_BATT and VCC_USB) and must be
	sequenced on at the same time or before the other supplies enabled by SYS_EN.
	VCC_IO are connected to any of the VCC_LCD, VCC_MEM, VCC_BB or VCC_USIM
	supplies as long as none of these supplies are driven at a voltage higher than VCC_IO.
	VCC_IO must be enabled when SYS_EN is asserted and disabled when SYS_EN is
	de-asserted.
	NOTE: When the main battery is installed, VCC_BATT must be driven by a regulator
	whose output is matched to the VCC_IO regulator so that VCC_IO and
	VCC_BATT remain within 200 mV of each other when the VCC_IO regulator is
	enabled.
VCC_LCD	Power for output drivers to LCD panel, 1.8 V (+20%, -5%), 2.5 V, 3.0V or 3.3 V (± 10%).
	Optionally, these are strapped to one of the existing I/O supplies at 3.3 V, 2.5 V, or
	1.8 V if appropriate for the panel used. This supply must be enabled when SYS_EN is
	asserted and disabled when SYS_EN is de-asserted.
VCC_MEM	Power for memory/system bus I/O at 1.8 V (+20%, -5%), 2.5 V, 3.0V or 3.3 V (±10%);
	fixed, strappable by input signals on the power controller to one of these voltages. The
	power controller automatically powers up VCC_MEM to the voltage specified by its
	input control signals when this regulator is enabled. Corresponding I/Os of the memory
	components or companion chips must be powered from the same regulator. This
	supply must be enabled when SYS_EN is asserted and disabled when SYS_EN is
	de-asserted.
VCC_BB	Power for I/Os to an external baseband module or device, at 1.8 V (+20%, -5%), 2.5 V,
	3.0V,or 3.3 V (±10%). Corresponding I/Os of the baseband device must be powered
	from the same regulator. In systems that use PCMCIA or Compact flash and the
	baseband interface, VCC_BB must be tied to VCC_MEM because some of the card
	interface signals are multiplexed with baseband interface signals. This supply must be
	enabled when SYS_EN is asserted and disabled when SYS_EN is de-asserted if any of
	these GPIOs are used (as either a GPIO or as an alternate function): GPIO<48>,
	GPIO<57:50>, GPIO<85:81>.
VCC_USIM	Power for I/Os to an external Universal Subscriber Identity Module (USIM) card. The
	VCC_USIM voltage generated by the PMIC is software configurable at settings of 1.8 V
	(+20%, -5%) or 3.0 V (± 10%) or disabled (0 V). The software voltage control is
	implemented using I2C commands or the PMIC decodes the PXA27x processor UVS0,
	nUVS1, and nUVS2 outputs. Refer to Section 4.7, “Universal Subscriber Identity
	Module (USIM)” on page 21 for more information.
VCC_USB	Power for USB at 3.0 V or 3.3V (±10%) for standard differential USB I/Os interfacing to
	external components, which are also supplied from fixed 3.0 V or 3.3 V.
	NOTE: VCC_USB powers the I/O for the USB interfaces, the USB differential signals
	D+. D- is out of compliance with the USB specification if VCC_USB is below
	2.8 V.
	NOTE: The +5 V VBUS source from USB host controller, which must be available for
	bus-powered peripherals, must be supplied from an external source, but it is
	not part of the PXA27x processor silicon.

2.1.2Power Supply Configuration in a Minimal System

For minimal systems, only five (four if VCC_USIM is disabled) regulators are required to power the PXA27x processor and its input/output interfaces, as shown in Table 3.

10 Application Note