In sync mode there is no nice transition from internal to SOF clock

[xross]

When running in sync mode the internal clock takes over when the host is unplugged.

E.g. when unplugged the device is driving a 500Hz signal at the CS2100, when SOF's are detected (due to plug event) the reference clock is immediately toggled with no account for when the last edge from the internal timer was driven.

Un-plug events are fine - the internal timer is updated every SOF.

To solve this issue, rather than toggling directly on SOF reception we could use a timer to measure the time between two SOF's, then use this offset from the last timer driven edge.

Clearly this effects self-powered devices only. Additionally this is not an issue for most devices, it could be an issue if an sync mode device has a 'stand-alone' mode (say a mixer) - there could be audible artefacts on USB plug.

[xross]

Test authored in test_sync_clk_plugin.py

[xross]

This is now lower priority due to the use of lib_sw_pll and the Application PLL in XS3 devices.