ST STM32L4x6 Reference Manual page 1608

USB on-the-go full-speed (OTG_FS)
Unmask (NAK/TXERR/XFRC/BBERR/STALL/FRMOR/DTERR)
if (XFRC)
{
Reset Error Count
Mask ACK
if (OTG_HCTSIZx.PKTCNT == 0)
{
De-allocate Channel
}
else
{
Transfer Done = 1
Unmask CHH
Disable Channel
}
}
else
if (STALL or FRMOR or NAK or DTERR or BBERR)
{
Mask ACK
Unmask CHH
Disable Channel
if (STALL or BBERR)
else
}
else
if (TXERR)
{
Increment Error Count
Unmask ACK
Unmask CHH
Disable Channel
}
else
if (CHH)
{
Mask CHH
if (Transfer Done or (Error_count == 3))
else
}
1608/1693
{
Reset Error Count
Transfer Done = 1
}
if (!FRMOR)
{
Reset Error Count
}
{
De-allocate Channel
}
Re-initialize Channel (in next b_interval - 1 /Frame)
DocID024597 Rev 3
RM0351