Memory device, memory system, and operation method of memory device

An operating method of a memory controller for ZQ calibration, the method comprising:

• transmitting a mode register write (MRW) command to a memory device for setting a background ZQ calibration mode;
• after a first time period, transmitting a mode register read (MRR) command to the memory device for detecting a ZQ update flag (ZQUF);
• and
• upon detecting the ZQUF, transmitting a ZQ latch command to the memory device for updating a ZQ code register, and upon failing to detect the ZQUF, repeating the step of transmitting the MRR command,
• wherein, in response to the ZQ latch command, the memory device updates a ZQ code register with new ZQ code generated from a background ZQ calibrating operation.