Texas Instruments OMAP5912 Reference Manual page 1369

Table 36. Status Register (STAT_FLG) (Continued)
Bit Name
5 STALL
4 NAK
3 ACK
SPRU761A
Description
The transaction stall (non-ISO) bit only concerns non-ISO
endpoints.This status bit is set at the end of a transaction if a STALL
handshake packet was returned to the USB host, and if no
non-handled interrupt is pending on the current buffer (see Section
3.11, Important Note on USB Device Interrupts). The core
automatically returns a STALL packet if a valid IN token is received
by a halted TX endpoint, if a valid OUT transaction is received by a
halted RX endpoint, or if there is a request error (endpoint 0). The bit
is cleared when the USB device controller has finished handling the
corresponding interrupt (at EP_NUM.EP_SEL bit deselection).
0: No STALL handshake was returned.
1: A STALL handshake packet was returned.
Value after MPU or USB reset is low.
The transaction non-acknowledge (non-ISO) bit only concerns
non-ISO endpoints with SYSCON1.NAK_EN bit asserted.This status
bit is set at the end of a transaction if a NAK handshake is returned to
the USB host, and if no non-handled interrupt is pending on the
current buffer. The USB core automatically returns a NAK handshake
to the USB host if a valid IN token is received by a TX endpoint or if a
valid OUT transaction is received by an RX endpoint, and the
STAT_FLG.FIFO_EN bit is not set for the endpoint. The bit is cleared
when the USB device controller has finished handling the
corresponding interrupt (at EP_NUM.EP_SEL bit deselection).
0: No NAK handshake was returned (SYSCON1.NAK_EN bit is set).
1: A NAK handshake packet was returned and SYSCON1.NAK_EN
bit is set.
Value after MPU or USB reset is low.
The transaction acknowledge (non-ISO) bit only concerns non-ISO
endpoints.This bit is set at the end of a non-transparent valid IN
transaction if the data packet was sent successfully to the USB host
and the ACK handshake was received, or at the end of a
non-transparent valid OUT transaction if the data packet was
received successfully by the USB device and the ACK handshake
was returned. The bit is cleared when the USB device controller has
finished handling the corresponding interrupt (at EP_NUM.EP_SEL
bit deselection).
0: No ACK handshake packet was returned.
1: An ACK handshake packet was returned.
Value after MPU or USB reset is low.
USB Device Controller
Universal Serial Bus (USB) 77