<xs:attribute name="source" use="optional">
    <xs:simpleType>
        <xs:restriction base="xs:string">
            <xs:enumeration value="Low" />
            <xs:enumeration value="High" />
            <xs:enumeration value="CT0" />
            <xs:enumeration value="CT1" />
            <xs:enumeration value="CT2" />
            <xs:enumeration value="CT3" />
            <xs:enumeration value="TrigSel" />
            <xs:enumeration value="EncASel" />
            <xs:enumeration value="EncBSel" />
            <xs:enumeration value="VFG0CT0" />
            <xs:enumeration value="VFG0CT1" />
            <xs:enumeration value="VFG0CT2" />
            <xs:enumeration value="VFG0CT3" />
            <xs:enumeration value="EncoderDivider" />
            <xs:enumeration value="EncoderQuadrature" />
            <xs:enumeration value="AuxCT0" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>