The PCM6xx0 supports two functional modes when configured as an ASI controller:

• Auto Clock Generation with Internal PLL enabled. Enabling the PLL allows the auto clock generator engine to generate a system clock that can be greater than the provided MCLK.
• Auto Clock Generation with internal PLL disabled. Disabling the PLL limits the system clock to the MCLK frequency.

The system clock feeds the decimation filters and all the digital signal processing blocks (biquad filters, digital volume control, high pass filters, and so forth). Disabling the PLL limits the amount of digital signal processing available. However, with the low-jitter PLL disabled, the performance of the ADC can be degraded based on the jitter from the external clock source. For devices configured as controller mode in high-performance applications, the recommended operating mode is to enable the PLL.

Configuring the PCM6xx0 as an ASI controller requires that GPIO (for example, GPIO1) be configured as the MCLK input in GPIO_CFG0 (page 0, register 0x21, bits 7-4). The frequency of MCLK must be one of the supported frequencies or ratios supported by configuring the MCLK_FREQ_SEL frequency selection mode (page 0, MST_CFG0 register 0x13, Bits 2-0), as shown in Table 2-1. Note that when using auto clock generation with internal PLL disabled, MCLK_RATIO_SEL (page 0, CLK_SRC register 0x16, bits 5-3) must also be configured.