Hi,
The readdatavalid signal will indicate the data is ready after the read signal is requested.
You need to read through the Avalon Interface Specification document to get some ideas on the read and write transfer of Avalon Memory Mapped interface.
Here the link for instance: https://www.intel.com/content/www/us/en/docs/programmable/683091/20-1/pipelined-read-transfer-with-variable.html
Are you using TSoM Evaluation Kit?
If yes, does the kit working normally with the DDR3 example design from the TSoM Evaluation Kit CD (rev.B Hardware) resources?
Regards,
Adzim