28.5.2 Endpoint Status
The Endpoint Status register is modified by the USB
hardware. The application modifies this register using the RMW register
interface to avoid data hazards. See the Endpoint Registers – Concurrency
and Hazards section for more details. Flag or status registers in
AVR MCUs are usually cleared by writing a ‘1’ and can be
set only by hardware. These status bits in RAM are an exception where you
can write ‘0’ to clear them or ‘1’ to set
them.
| Name: | STATUS |
| Offset: | 0x00 + n*0x08 [n=0..1] |
| Reset: | 0x00 |
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| UNFOVF | TRNCOMPL | EPSETUP | STALLED | BUSNACK | TOGGLE | ||||
| Access | R/W | R/W | R/W | R/W | R/W | R/W | |||
| Reset | x | x | x | x | x | x |
Bit 6 – UNFOVF Underflow / Overflow Endpoint Flag
The UNF flag is set when an input endpoint is not ready to send data to the host
in response to an IN token for input endpoints.
The OVF flag is set when an output endpoint is not ready to accept data
from the host following an OUT token for output
endpoints.
Bit 5 – TRNCOMPL Transaction Complete Flag
This flag is set when an IN or OUT
transaction on the endpoint has successfully completed. When setting
this flag, the USB hardware will also set the Transaction Complete
(TRNCOMPL) flag in the Interrupt Flags B (INTFLAGSB) register - unless
the TRNCOMPL Interrupt Disable (TCDSBL) bit in the Endpoint Control
(EPn.CTRL) register is ‘1’.
For multi-packet transfers, TRNCOMPL is not set for each transaction but only when a multi-packet transfer has been completed.
Bit 4 – EPSETUP Endpoint SETUP Complete Flag
This flag is set when a SETUP transaction has been completed
successfully. When this flag is set, all IN and
OUT transactions to control endpoints are
responded to with a NAK.
This flag must be cleared by the application before decoding the request sent in the setup stage of a control transfer.
Bit 3 – STALLED Endpoint STALL Flag
This flag is set when a STALL handshake has been sent to the host
in response to an IN or OUT
transaction.
Bit 1 – BUSNACK Data Buffer Not Acknowledge
This flag is set when a packet has been received. When this bit is set,
the USB peripheral will discard incoming data to the data buffer in an
OUT transaction and will not return any data
from the data buffer in an IN transaction. Control,
bulk and interrupt endpoints will return a NAK handshake.
After processing the packet, this flag must be cleared by the application. This bit can be written by the application to force returning a NAK to the host, e.g., during initialization. The setting of BUSNACK does not affect the ACKing of the SETUP packets.
Bit 0 – TOGGLE Data Toggle
This indicates if a DATA0 or DATA1 PID is expected in the next data packet for an output endpoint, and if a DATA0 or DATA1 PID will be sent in the next transaction for an input endpoint. In most cases, TOGGLE is updated by hardware. The Programming Manual describes the exceptional cases where TOGGLE needs to be updated by the application.
| Value | Name | Description |
|---|---|---|
| 0x0 | DATA0 | Data packet DATA0 |
| 0x1 | DATA1 | Data packet DATA1 |