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12-36 Intel® PXA255 Processor Developer’s Manual
USB Device Controller
12.6.8.4 Transmit Underrun (TUR)
The transmit underrun bit is be set if the transmit FIFO experiences an underrun. When the UDC
experiences an underrun, NAK handshakes are sent to the host. UDCCSx[TUR] does not generate
an interrupt and is for status only. UDCCSx[TUR] is cleared by writing a 1 to it.
12.6.8.5 Sent STALL (SST)
The sent stall bit is set by the UDC in response to FST successfully forcing a user induced STALL
on the USB bus. This bit is not set if the UDC detects a protocol violation from the host PC when a
STALL handshake is returned automatically. In either event, the core does not intervene and the
UDC clears the STALL status when the host sends a CLEAR_FEATURE command. The endpoint
operation continues normally and does not send another STALL condition, even if the
UDCCSx[SST] bit is set. To allow the software to continue to send the STALL condition on the
USB bus, the UDCCSx[FST] bit must be set again. The core writes a 1 to the sent stall bit to clear
it.
12.6.8.6 Force STALL (FST)
The core can set the force stall bit to force the UDC to issue a STALL handshake to all IN tokens.
STALL handshakes continue to be sent until the core clears this bit by sending a Clear Feature
command. The UDCCSx[SST] bit is set when the STALL state is actually entered, but this may be
delayed if the UDC is active when the UDCCSx[FST] bit is set. The UDCCSx[FST] bit is
automatically cleared when the UDCCSx[SST] bit is set. To ensure that no data is transmitted after
the Clear Feature command is sent and the host resumes IN requests, software must clear the
transmit FIFO by setting the UDCCSx[FTF] bit.
12.6.8.7 Bit 6 Reserved
Bit 6 is reserved for future use.
12.6.8.8 Transmit Short Packet (TSP)
Software uses the transmit short to indicate that the last byte of a data transfer has been sent to the
FIFO. This indicates to the UDC that a short packet or zero-sized packet is ready to transmit.
Software must not set this bit if a packet of 8 bytes is to be transmitted. When the data packet is
successfully transmitted, the UDC clears this bit.
These are read/write registers. Ignore reads from reserved bits. Write zeros to reserved bits.
12.6.9 UDC Interrupt Control Register 0 (UICR0)
UICR0, shown in Table 12-20, contains 8 control bits to enable/disable interrupt service requests
from data endpoints 0 - 7. All of the UICR0 bits are reset to a 1 so interrupts are not generated on
initial system reset.