A smart interface circuit includes: a first protocol processing circuit receiving several first protocol commands including a first command and a second command from a first device, storing the commands in an instruction register that is accessible to a second protocol processing circuit, and outputting first data and second data stored in a data buffer to the first device according to the first command and the second command respectively; and the second protocol processing circuit generating X second protocol command(s) according to the first command to request a second device to output the first data to the data buffer, and before the first protocol processing circuit finishes outputting the first data to the first device, the second protocol processing circuit generating Y second protocol command(s) according to the second command to request the second device to output the second data to the data buffer.