Answer: Constraints on a XlmSchema for Tullio Bettinazzi

Question

: Sep 16, 2002 03:01 PT I've a quite complex XmlSchema.
My problem is : I need to constraints some attributes depending of values of some other attributes.
By example : if the attribute att1 is defined then att2 is mandatory otherwise is non allowed.
I didn't find any simple way that.
Any suggestion ?
TIA
Tullio
Answer:
This is a coconstraint feature not suported in XMLSchema 1.0.

Answer 1

  the and gate output will have glitches on it 
Really? The karnaugh map of the three inputs to the 2 and gates only have one place where the output should be '1' so there is no place for transistion glithces.  I.e no changes in the input combination can transition such that the output transistion from '1' to '0' (momentarily) then back to '1'. So I assume this is not your reasoning here.
It is however true that a metastable condition can propagate throught the first two FF's which could then resolve on the second FF's ouput (then through the and gates) during the Setup and Hold time of the counter. So now you have a changing input on CE of the counter on a rising clock edge. In that case yes. I think it would be a better option to add a FF on the and gate's output.
and still be asyncronous to the faster clock,
I don't see how this can be true in the absence of a metastable condition which might resolving right on a clock edge.
It is true that the input is asynchronous for metastable conditions that resolve during the setup and hold time of a register but it is not true that in the absence of this condition that the signal is not synchronised. That is the purpose of the preceding syncronisers to get the probabilty that a metastable condition have reolved very high.
Under normal conditions (i.e. non-metastable feed through) taking into account logic propagation delay through the and gates, clock skew and setup time for the counter should be shorter than the clock period. If this condition is satisfied then all should be okay.