This is a tiny breakout board for the VS1011e MP3 decoder IC. It has all the required components onboard. It measures 25mmx52mm and uses 3V.
This module doesn't do much on it's own but it can be used to add simple MP3 functionality to an other project. As playing an MP3 is a trivial task I simply wired it up to my reference design 2 SD-Card board for testing and even played internetradio with help of the userver.
The bus activity.
When the SM_SDISHARE bit in the SCI_MODE register is set, there is only one pin needed for the command and data SPI mode. In this seaquence the AVR is streaming an MP3 from the SD card. The CS is high for data input (and low for command input). The white blocks on the Sck and Mosi line show the times when the data is being shovelld into the VS. The Dreq line is pulled low when the VS1011e is full up with data. One can see that the AVR is waiting for Dreq to go high, thus the AVR has a few cycles free for other tasks. The low intervals on the CS SD line show the time when data is being loaded from the SD card in 512 byte loads (this takes about 8ms with a 12MHz Xtal).