2157 - [LatticeECP3] Will the LatticeECP3's read datavalid generation circuit work properly when I remove the VTT termination from the DQS pad?
The datavalid generation circuit works properly as long as you provide the READ pulse input to the DQSBUF block in the right timing. LatticeECP3 devices have preamble detection function which is a level detection circuit instead of edge detection. This means that the circuit does not use the DQS transition timing to preamble. Instead, the datavalid generation circuit uses the output of the preamble detection function, incoming DQS and the READ pulse timing to determine the actual read data valid timing.
If the parallel termination to VTT is not used, compared to the interface with the VTT termination, the preamble detect signal (PRMBDET) may behave differently until the preamble stage starts but will be the same once preamble starts. Since the data valid generation for the incoming read data is started when the READ pulse falling edge occurs, the PRMBDET status before the preamble becomes "don't care". This is because the READ pulse falling edge is required to occur during the preamble period.