5.2 User-Defined Supply Pins

Name	Type	Polarity/Bus Size	Description
VAREF0	Input	1	Analog reference voltage for first ADC The SmartFusion cSoC can be configured to generate a 2.56V 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 1.0V to 3.3V. 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 2.56V to initiate valid conversions by the ADC. If the lower capacitor value is chosen, the settling time required for VAREF0 to achieve 2.56V 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, you must connect the VAREFOUT pin to the appropriate ADC VAREFx input—either the VAREF0 or VAREF1 pin—on the PCB.
VAREF1	Input	1	Analog reference voltage for second ADC See VAREF0 above for more information.
VAREF2	Input	1	Analog reference voltage for third ADC See VAREF0 above for more.
VAREFOUT	Out	1	Internal 2.56V 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.

VAREF0

Input

1

Analog reference voltage for first ADC

The SmartFusion cSoC can be configured to generate a 2.56V 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 1.0V to 3.3V. 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 2.56V to initiate valid conversions by the ADC. If the lower capacitor value is chosen, the settling time required for VAREF0 to achieve 2.56V 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, you must connect the VAREFOUT pin to the appropriate ADC VAREFx input—either the VAREF0 or VAREF1 pin—on the PCB.

VAREF1

Input

1

Analog reference voltage for second ADC

See VAREF0 above for more information.

VAREF2

Input

1

Analog reference voltage for third ADC

See VAREF0 above for more.

VAREFOUT

Out

1

Internal 2.56V 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.