App note: Termination recommendations for single-ended oscillator driving single or multiple Loads

Proper clock signal terminations app note from SiTime. Link here (PDF)

Clock signals with fast edges see traces on Printed Circuit Boards (PCB) as transmission lines rather than simple wire connections. If the length of PCB trace exceeds certain limit it requires matching of the trace impedance to one or both of the source and load impedances. Impedance mismatch causes signal reflections travelling back and forth the transmission line causing signal distortions such as ringing, overshoots, and undershoots.

from Dangerous Prototypes

App note: Interfacing an STM32L1xx microcontroller with an external I2S audio codec to play audio files

I2S protocol interfacing with an STM32 microcontroller app note from ST Microelectronics. Link here (PDF)

The I2S protocol is widely used to transfer audio data from a microcontroller/DSP to an audio codec in order to play melodies (stored in a memory) or, to capture analog sound (from a microphone).
The STM32L1xx allows I2S audio communication using the SPI peripheral, and implements specific functionalities for this communications mode.

from Dangerous Prototypes