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32142D–06/2013
ATUC64/128/256L3/4U
6.
Supply and Startup Considerations
6.1
Supply Considerations
6.1.1
Power Supplies
The ATUC64/128/256L3/4U has several types of power supply pins:
VDDIO: Powers I/O lines. Voltage is 1.8 to 3.3V nominal.
VDDIN: Powers I/O lines, the USB pins, and the internal regulator. Voltage is 1.8 to 3.3V
nominal if USB is not used, and 3.3V nominal when USB is used.
VDDANA: Powers the ADC. Voltage is 1.8V nominal.
VDDCORE: Powers the core, memories, and peripherals. Voltage is 1.8V nominal.
The ground pins GND are common to VDDCORE, VDDIO, and VDDIN. The ground pin for
VDDANA is GNDANA.
When VDDCORE is not connected to VDDIN, the VDDIN voltage must be higher than 1.98V.
For decoupling recommendations for the different power supplies, please refer to the schematic
checklist.
6.1.2
Voltage Regulator
The ATUC64/128/256L3/4U embeds a voltage regulator that converts from 3.3 V nominal to
1.8 V with a load of up to 60 mA. The regulator supplies the output voltage on VDDCORE. The
regulator may only be used to drive internal circuitry in the device. VDDCORE should be exter-
nally connected to the 1.8V domains. See
Section 6.1.3 for regulator connection figures.
Adequate output supply decoupling is mandatory for VDDCORE to reduce ripple and avoid
oscillations. The best way to achieve this is to use two capacitors in parallel between VDDCORE
decoupling capacitors values and regulator characteristics.
Figure 6-1.
Supply Decoupling.
The voltage regulator can be turned off in the shutdown mode to power down the core logic and
keep a small part of the system powered in order to reduce power consumption. To enter this
mode the 3.3V supply mode, with 1.8V regulated I/O lines power supply configuration must be
used.
3.3V
1.8V
VDDIN
VDDCORE
1.8V
Regulator
C
IN1
C
OUT1
C
OUT2
C
IN2
IN3
C