22.12.2 STALL handshake and Retry mechanism
The Retry mechanism has priority over the STALL handshake. A STALL handshake is sent if the
STALLRQ request bit is set and if there is no retry required.
22.13 CONTROL endpoint management
A SETUP request is always ACK’ed. When a new setup packet is received, the RXSTPI inter-
rupt is triggered (if enabled). The RXOUTI interrupt is not triggered.
The FIFOCON and RWAL fields are irrelevant with CONTROL endpoints. The firmware shall
thus never use them on that endpoints. When read, their value is always 0.
CONTROL endpoints are managed by the following bits:
• RXSTPI is set when a new SETUP is received. It shall be cleared by firmware to
acknowledge the packet and to clear the endpoint bank.
• RXOUTI is set when a new OUT data is received. It shall be cleared by firmware to
acknowledge the packet and to clear the endpoint bank.
• TXINI is set when the bank is ready to accept a new IN packet. It shall be cleared by firmware
to send the packet and to clear the endpoint bank.
CONTROL endpoints should not be managed by interrupts, but only by polling the status bits.
22.13.1 Control Write
The next figure shows a control write transaction. During the status stage, the controller will not
necessary send a NAK at the first IN token:
• If the firmware knows the exact number of descriptor bytes that must be read, it can then
anticipate on the status stage and send a ZLP for the next IN token,
• or it can read the bytes and poll NAKINI, which tells that all the bytes have been sent by the
host, and the transaction is now in the status stage.
SETUP
DATA
STATUS
USB line
RXSTPI
RXOUTI
TXINI
SETUP
OUT
OUT
IN
IN
NAK
HW
SW
HW
SW
HW
SW
SW
274
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
