Here is a more detailed example of an audio application. The major features typically required in audio applications are: two audio class I²S interfaces supporting 16 and 32-bit data, a 147.46MHz audio PLL able to provide the master clock with less than 0.5% error, audio sampling frequency ranges from 8 to 96kHz, USB Full Speed OTG with embedded PHY for connecting USB devices such as an MP3 player or mass storage devices, and a SPI peripheral for connecting to SD cards. In this example, the audio files can be stored in either the SD card or the USB devices. The processor is also required to run advanced software such as audio codecs, a file system and the USB host stack for mass storage and audio classes. The STM32’s I/Os are used to connect the buttons, while a second SPI peripheral is used to connect an LCD display. Together, a sophisticated human machine interface can be built for the file browsing and playback functions. To achieve the best audio quality, the STM32’s embedded oscillator accepts audio crystals such as a 14.7456MHz quartz.