altddio_in, altddio_out msb, lsb confusion

I remember being equally confused when I first use these megafunctions. Assuming that nothing has changed since I last used these megafunctions in 6.1:

datain_h is the data that will be driven to pin on rising clock edge

datain_l is the data that will be driven to pin on falling clock edge

dataout_h is the data on the pin while clock was low (captured rising clock edge)

dataout_l is the data on the pin while clock was high (captured falling clock edge)

Connect an altddio_in and an alt_ddio_out together and run a quick sim on it to see the results. The short answer is whatever you clock out on datain_h and datain_l will show up on dataout_l and dataout_h respectively on the following clock cycle.

Jake