Memory modules, memory systems, and methods of operating memory modules

A memory system comprising:

• a memory module including a plurality of semiconductor memory devices and a control device configured to control the plurality of semiconductor memory devices, wherein the plurality of semiconductor memory devices and the control device are mounted on a circuit board;
• and
• a memory controller configured to control the memory module,
• wherein the control device is configured to receive a command, an address, and a clock signal from the memory controller and to provide the command, the address, and the clock signal to the plurality of semiconductor memory devices,
• wherein the control device, in a hidden training mode during a normal operation, is configured to performing a command/address training on at least one semiconductor memory device of the plurality of semiconductor memory devices by transmitting a first command/address signal and a first clock signal to the at least one semiconductor memory device, and receiving a second command/address signal and a second clock signal in response to the first command/address signal and the first clock signal, from the at least one semiconductor memory device,
• wherein the at least one semiconductor memory device includes a first loopback pin and a second loopback pin, the first loopback pin configured to transmit a data signal to the control device in an operation mode other than the hidden training mode, and to transmit the second command/address signal to the control device in the hidden training mode, and the second loopback pin configured to transmit a data strobe signal to the control device in the operation mode other than the hidden training mode, and to transmit the second clock signal to the control device in the hidden training mode, and the at least one semiconductor memory device configured to perform the command/address training using the first loopback pin and the second loopback pin in the hidden training mode.

A method of operating a memory module, wherein the memory module includes a plurality of semiconductor memory devices and a control device configured to control the plurality of semiconductor memory devices, and the plurality of semiconductor memory devices and the control device are mounted on a circuit board, the method comprising:

• performing, by the control device, an initial command/address training on each of the plurality of semiconductor memory devices to store, in a register in the control device, initial skew information based on a skew between a command/address signal and a clock signal associated with each of the plurality of semiconductor memory devices in an initial training mode of the memory module;
• performing a normal operation in each of the plurality of semiconductor memory devices based on a respective data signal and a respective data strobe signal;
• determining, by the control device, whether at least one semiconductor memory device of the plurality of semiconductor memory devices enters into a hidden training mode during the normal operation;
• receiving, by the at least one semiconductor memory device in the hidden training mode, a first command/address signal rather than the respective data signal, and a first clock signal rather than the respective data strobe signal;
• transmitting, by the at least one semiconductor memory device in the hidden training mode, a second command/address signal based on the first command/address signal, and a second clock signal based on the first clock signal;
• adjusting a skew, by the control device, by receiving the second command/address signal and the second clock signal from the at least one semiconductor memory device entering into in the hidden training mode;
• and
• performing, by the at least one semiconductor memory device, a normal operation based on the adjusted skew.