Memory device and system having multiple physical interfaces

A memory device comprising:

• a buffer die comprising a first interface circuit and a second interface circuit configured to communicate with an external device, wherein the first interface circuit is configured to activate responsive to a first selection signal, and the second interface circuit is configured to activate responsive to a second selection signal;
• and
• a memory die stack mounted on the buffer die and including a plurality of memory dies, wherein the plurality of memory dies are electrically connected to the first interface circuit and the second interface circuit,
• wherein, which of the first interface circuit and the second interface circuit is activated to communicate with an external memory controller is based on whether the first selection signal is fixed to a first voltage level or a second voltage level different from the first voltage level and whether the second selection signal is fixed to the first voltage level or the second voltage level.

A memory device comprising:

• a buffer die comprising a first interface circuit and a second interface circuit configured to communicate with an external device, wherein the first interface circuit is configured to activate responsive to a first selection signal, and the second interface circuit is configured to activate responsive to a second selection signal;
• and
• a memory die stack mounted on the buffer die and including a plurality of memory dies, wherein the plurality of memory dies are electrically connected to the first interface circuit and the second interface circuit,
• wherein the first selection signal and the second selection signal are generated inside the memory device responsive to an interface selection signal received from a memory controller external to the memory device.