NEC switch User Manual page 146

Figure 8-25. Flowchart of Transmit/Receive Pointer Operation (7/7)
(4) Handshake packet transmission
Idle state
Transmit/receive
signal?
Set USBPOW to 7FH
Set USBPOB to 00H
Bit read
Y
Does bit stuffing
signal = 1?
USBPOB increment
N
USBPOB
overflow?
Transmit area?
(reservation flag
judgment)
Set USBPOW to n0H
146
CHAPTER 8
Y
Data/handshake transmission
N
Y
Transmit buffer
Go to (3)
Handshake
n = 4: ACK
n = 5: NAK
n = 6: STALL
1
User's Manual U12978EJ3V0UD
USB FUNCTION
Y
USBPOB increment
N
1
Bit read
Does bit stuffing
signal = 1?
N
USBPOB
overflow?
Y
Set EOP
transmit signal
Idle state