IO13PDB0V0 datasheet(119/197 Pages) MICROSEMI

119 / 197 page

SmartFusion Customizable System-on-Chip (cSoC)

Re vi s i on 12

5 - 5

User-Defined Supply Pins

Name

Type

Polarity/

Bus Size

Description

VAREF0

Input

Analog reference voltage for first ADC.

The SmartFusion cSoC can be configured to generate a 2.56 V internal reference

that can be used by the ADC. While using the internal reference, the reference

voltage is output on the VAREFOUT pin for use as a system reference. If a

different reference voltage is required, it can be supplied by an external source

and applied to this pin. The valid range of values that can be supplied to the ADC

is 1.0 V to 3.3 V. When VAREF0 is internally generated, a bypass capacitor must

be connected from this pin to ground. The value of the bypass capacitor should be

between 3.3 µF and 22 µF, which is based on the needs of the individual designs.

The choice of the capacitor value has an impact on the settling time it takes the

VAREF0 signal to reach the required specification of 2.56 V to initiate valid

conversions by the ADC. If the lower capacitor value is chosen, the settling time

required for VAREF0 to achieve 2.56 V will be shorter than when selecting the

larger capacitor value. The above range of capacitor values supports the accuracy

specification of the ADC, which is detailed in the datasheet. Designers choosing

the smaller capacitor value will not obtain as much margin in the accuracy as that

achieved with a larger capacitor value. See the Analog-to-Digital Converter (ADC)

section in the SmartFusion Programmable Analog User’s Guide for more

information. The SoC Products Group recommends customers use 10 µF as the

value of the bypass capacitor. Designers choosing to use an external VAREF0

need to ensure that a stable and clean VAREF0 source is supplied to the VAREF0

pin before initiating conversions by the ADC. To use the internal voltage reference,

the VAREFOUT pin must be connected to the appropriate ADC VAREFx input on

the PCB. For example, VAREFOUT can be connected to VAREF0 only, if ADC0

alone is used. VAREFOUT can be connected to VAREF1 only, if ADC1 alone is

used. VAREFOUT can be connected to VAREF2 only, if ADC2 alone is used.

VAREFOUT can be connected to VAREF0, VAREF1 and VAREF2 together, if

ADC0, ADC1, and ADC2 all are used.

VAREF1

Input

Analog reference voltage for second ADC

See "VAREF0" above for more information.

VAREF2

Input

Analog reference voltage for third ADC

See "VAREF0" above for more.

VAREFOUT

Out

Internal 2.56 V voltage reference output. Can be used to provide the two ADCs

with a unique voltage reference externally by connecting VAREFOUT to both

VAREF0 and VAREF1. To use the internal voltage reference, you must connect

the VAREFOUT pin to the appropriate ADC VAREFx input—either the VAREF0 or

VAREF1 pin—on the PCB.

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