You can "fix" certain facets by setting the 'fixed' attribute to 'true' on the facet declaration:
<length value="8" fixed='true'/>
The following facets can be fixed:
length, minLength, maxLength
minInclusive, maxInclusive, minExclusive, maxExclusive
totalDigits, fractionDigits
whitespace
Fixing a facet means a derived type cannot specify any other value than the fixed value.