a) Control-IN transfer - Transaction (packet) sequence: SETUP - IN - IN -...- IN - OUT (data stage) (status stage) - setup stage SETUP packet is received by Control-OUT EP This packet is handled by "Control Out Handler" - left main path MCU set "Control State" to USB_TRANSMIT - data stage MCU interpret SETUP packet in mainloop, fill IN buffer with appropriate data IN transaction is received by Control-IN EP D12 returns a packet filled to IN buffer in the mainloop After sending the packet, "Control IN Handler" is called by interrupt - status stage OUT packet (ZLP) is received by Control-OUT EP This packet is handled by "Control Out Handler" - rightmost small path MCU set "Control State" to USB_IDLE a) Control-OUT transfer (with data stage) - Transaction (packet) sequence: SETUP - OUT - OUT -...- OUT - IN (data stage) (status stage) - setup stage SETUP packet is received by Control-OUT EP This packet is handled by "Control Out Handler" - left main path MCU set "Control State" to USB_RECEIVE - data stage MCU interpret SETUP packet in mainloop, prepare to receive data OUT packet is received by Control-OUT EP This packet is handled by "Control Out Handler" - center path When all data is received, MCU set "Control State" to USB_IDLE MCU handle the command told by host - status stage MCU set ZLP to Control-IN EP in mainloop IN transaction is received by Control-IN EP D12 returns ZLP After sending the packet, "Control IN Handler" is called by interrupt "Control IN Handler" return immediately, because "Control State" is USB_IDLE a) Control-OUT transfer (without data stage) - Transaction (packet) sequence: SETUP - IN (status stage) - setup stage SETUP packet is received by Control-OUT EP This packet is handled by "Control Out Handler" - left main path MCU set "Control State" to USB_IDLE, because of no data - status stage MCU interpret SETUP packet in mainloop, handle the command told by host MCU set ZLP to Control-IN EP in mainloop IN transaction is received by Control-IN EP D12 returns ZLP After sending the packet, "Control IN Handler" is called by interrupt "Control IN Handler" return immediately, because "Control State" is USB_IDLE