The Arduino USB Host Shield allows you to connect a USB device to your Arduino board. The Arduino USB Host Shield is based on the MAX3421E (), which is a USB peripheral/host controller containing the digital logic and analog circuitry necessary to implement a full-speed USB peripheral or a full-/low-speed host compliant to USB specification rev 2.0.
Arduino communicates with the MAX3421E using the SPI bus (through the ICSP header). This is on digital pins 10, 11, 12, and 13 on the Uno and pins 10, 50, 51, and 52 on the Mega. On both boards, pin 10 is used to select the MAX3421E. Pins 7, 8 and 9 are used for GPX, INT and RES pins.