A keyboard and a keyboard circuit, wherein the keyboard acts as an input device for a portable device or a computer. The keyboard circuit comprises an interface for connecting with the computer or the portable device, a detection circuit for detecting the logic level signal of the power supply from the computer or the portable device connected to the keyboard, and a microprocessor determining whether a computer or a portable device is connected to the keyboard in accordance with the logic level signal of the power input. The keyboard circuit uses a first communication circuit to communicate with the computer and a second communication circuit to communicate with the portable device, thus the keyboard is compatible with the computer or the portable device. The keyboard is capable of entering a sleep mode after finishing the transmission of the key codes to the computer or the portable device.