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The USB function module has internal status bits that hold the status (stall or non-stall) of each
endpoint. When a transaction is sent from the host, the module references these internal status
bits and determines whether to return a stall to the host. These bits cannot be cleared by the
application; they must be cleared with a Clear Feature command from the host.
(2) Forcible Stall by Firmware
The firmware uses UESTL to issue a stall request for the USB function module. When the
firmware wishes to stall a specific endpoint, it sets the corresponding EPnSTL bit (1-1 in figure
15.22). The internal status bits are not changed.
When a transaction is sent from the host for the endpoint for which the EPnSTL bit was set, the
USB function module references the internal status bit, and if this is not set, references the
corresponding EPnSTL bit (1-2 in figure 15.22). If the corresponding EPnSTL bit is not set,
the internal status bit is not changed and the transaction is accepted. If the corresponding
EPnSTL bit is set, the USB function module sets the internal status bit and returns a stall
handshake to the host (1-3 in figure 15.22). If the SCME bit in UESTL1 is set, the EPnSTL bit
is automatically cleared (1-4 in figure 15.22).
Once an internal status bit is set, it remains set until cleared by a Clear Feature command from
the host, without regard to EPnSTL. Even after a bit is cleared by the Clear Feature command
(3-1 in figure 15.22), the USB function module continues to return a stall handshake while the
EPnSTL bit is set, since the internal status bit is set each time a transaction is executed for the
corresponding endpoint (1-2 in figure 15.22). To clear a stall, therefore, it is necessary for the
corresponding EPnSTL bit to be cleared by the firmware (or set the SCME bit so that the
EPnSTL bit is automatically cleared when the USB function module returns a stall handshake),
and also for the internal status bit to be cleared with a Clear Feature command (2-1, 2-2, and 2-
3 in figure 15.22).
Rev. 3.0, 10/02, page 509 of 686
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