The main system clock is PCLK, which is used for all portions of MAX9880A. The PCLK is routed from either the crystal oscillator (X1/X2) or MCLK input. Thus, you will only need a clock input from either MCLK or a crystal oscillator (X1/X2). You can select between these clock inputs by configuring the crystal clock bits (XTEN and XTOSC) in the System Shutdown Register (0x27) for your desired configuration.