The PIO and state machine were hard wired, so this caused problems if they were not free. The approach used by the Servo library has been adopted so a free PIO and SM are searched.
The DMA_IRQ_0 was grabbed exclusively, but this conflicts with SPI DMA use. The interrupt is now shared, but has been allocated the highest possible priority.
Since the PDM PIO use is receive only, the PIO state machine RX FIFO's can be joined to reduce DMA interrupt load.
