SmartFusion Customizable System-on-Chip (cSoC)
Revision 10 5-5
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 2.56 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,
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 2.56 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.
