8.3.6.1 Clocking Overview

The PCM512x devices have flexible systems for clocking. Internally, the device requires a number of clocks, mostly at related clock rates to function correctly. All of these clocks can be derived from the serial audio interface in one form or another.

Figure 62. Audio Flow with Respective Clocks

As shown in Figure 62 the data flows at the sample rate (f_S). Once the data is brought into the serial audio interface, it gets processed, interpolated and modulated all the way to 128 × f_S before arriving at the current segments for the final digital to analog conversion.

The clock tree is shown in Figure 63.

Figure 63. PCM512x Clock Distribution Tree

The serial audio interface typically has 4 connections: SCK (system master clock), BCK (bit clock), LRCK (left right word clock), and DIN (data). The device has an internal PLL that is used to take either SCK or BCK and create the higher rate clocks required by the interpolating processor and the DAC clock. This allows the device to operate with or without an external SCK.

In situations where the highest audio performance is required, it is suggested that the SCK is brought to the device, along with BCK and LRCK. The device should be configured so that the PLL is only providing a clock source to the audio processing block. By ensuring that the DACCK (DAC Clock) is being driven by the external SCK source, jitter evident in the PLL (in all PLLs) is kept out of the DAC, charge pump, and oversampling system.

Everything else should be a division of the incoming SCK. This is done by setting DAC CLK Source Mux (SDAC in Figure 63) to use SCK as a source, rather than the output of the SCK/PLL Mux. Code examples for this are available in SLASE12.

When the Auto Clock Configuration bit is set (Page 0/ Register 0x25), no additional clocks configuration is required. However, when setting custom PLL values and so forth, the target output rates should match those shown in the recommended PLL values of Table 131.