Intel PXA27 Series Design Manual page 59

3.5.2.1.1 Power Supply Design Requirements
VCC_BATT powers the real time clock (RTC) and power management circuitry during initial
power-on, sleep, deep sleep and sleep wake-up, so the RTC must remain powered from the backup
battery when the main power source is discharged or removed. When the system main battery is
installed, VCC_BATT must be driven by a regulator whose output matches the output of VCC_IO
regulator. This ensures that VCC_IO and VCC_BATT remain within 200 mV of each other when
the VCC_IO regulator is enabled. Power the external output drivers for the logic signals nRESET,
nVDD_FAULT, nBATT_FAULT, PWR_SDA, GPIO0 and GPIO1 from the VCC_BATT supply.
This also applies to all other digital outputs such as the JTAG signals driving PXA27x processor
inputs on the VCC_REG domain. Refer to
a PXA27x processor digital output and is powered from the VCC_REG domain, must tolerate
output high drive levels between 2.25 V and 3.75 V.
VCC_PLL must driven by a regulator which cannot be shared with any other devices.
VCC_IO is the fixed supply for standard CMOS I/O interfacing to external components. VCC_IO
must be the highest potential in the system (excluding VCC_BATT and VCC_USB) and must be
turned on at the same time or before the other supplies enabled by SYS_EN. VCC_IO are
connected to any of the VCC_USB, 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.
Caution: When VCC_IO is connected to a VCC_USIM supply with voltage higher than 3.0V, it does not
damage the silicon, but exceeds the USIM card specification, which in turn can cause damage to
USIM card.
VCC_IO must be enabled when SYS_EN is asserted and disabled when SYS_EN is de-asserted.
The I/Os for external components connected to the corresponding I/O pins on the PXA27x
processor must be supplied from the same regulator generating VCC_IO. VCC_USB powers the
differential I/O signals for the USB client and host 1 interface. Be aware that the USB differential
signals D+ and D- are out of compliance with the USB specification if VCC_USB is below 2.8 V.
The +5 V V
must be supplied from an external source.
®
Intel PXA27x Processor Family Design Guide
source from USB host controller, which is available for bus-powered peripherals,
BUS
Clocks and Power Interface
Figure 3-1. Any device that has a digital input driven by
§§
II:3-9