EHCI

The accessory port found only on the original PS Vita models is a custom port USB interface. The port must be activated in software and so far, there does not seem to be any official public software that uses it. It was thought that all API's that reference the accessory port were permanently disabled as of firmware 1.69, however this is not actually true: even on FW 3.73 the accessory port can be enabled using SceUsbServ#sceUsbServAccessoryActivate and disabled with SceUsbServ#sceUsbServAccessoryDeactivate.

The "CBPS" were also able to get the accessory port working and able to mount a USB drive via VitaShell by hacking Mini-USB cables.

Pin	Name	Description
1	GND	Ground - On the side with the mounting hole that has threads
2	ID (1.8v)	USB OTG
3	D-	USB Data
4	D+	USB Data
5	VBUS (3.3v)	Power

From FCC filing for AK8PDE1001A

The only documented official usage of this port is from an FCC filing for the PDEL-1001 development unit. It seems that some dongle attached to the device is used as part of their hardware testing framework.