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USB SOF PIN?

Question asked by cuneo.john on Apr 25, 2014
A section of  code in the USB host lib 2.0 has been puzzling me for some time now, it is within the USB_OTG_BSP_Init function:

/* Configure SOF Pin.*/
//  GPIO_InitStructure.GPIO_Pin = HOST_OVRCURR_LINE;
//  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
//  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPU;
//  GPIO_Init(HOST_OVRCURR_PORT, &GPIO_InitStructure);

I've commented the code and everything's running OK (so far) but I don't want loose ends in my codes so I'm here to ask if anyone could tell me what does it do?

From my very primitive and limited understanding of USB protocol, it has something to do with Start of Frame packet, but what is a SOF "pin"?  Isn't SOF packet sent using NRZI coding using just one line DM/DP? (not counting VBUS and GND)

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