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22.7.3
Automatic Stall by USB Function Module
When a stall setting is made with the Set Feature command, or in the event of a USB specification
violation, the USB function module automatically sets the internal status bit for the relevant
endpoint without regard to the EPSTL register, and returns a stall handshake (1-1 in figure 22.19).
Once an internal status bit is set, it remains set until cleared by a Clear Feature command from the
host, without regard to the EPSTL register. After a bit is cleared by the Clear Feature command,
EPSTL is referenced (3-1 in figure 22.19). The USB function module continues to return a stall
handshake while the internal status bit is set, since the internal status bit is set even if a transaction
is executed for the corresponding endpoint (2-1 and 2-2 in figure 22.19). To clear a stall, therefore,
the internal status bit must be cleared with a Clear Feature command (3-1 in figure 22.19). If set
by the application, EPSTL should also be cleared (2-1 in figure 22.19).
(1) Transition from normal operation to stall
(1-1)
STALL handshake
(2) When transaction is performed when internal status bit is set, and Clear Feature is sent
(2-1)
Transaction request
(2-2)
STALL handshake
(3) When Clear Feature is sent before transaction is performed
(3-1)
Clear Feature command
Figure 22.19 Automatic Stall by USB Function Module
Internal status bit
0 → 1
To (2-1) or (3-1)
Internal status bit
1
Internal status bit
1
Stall status maintained
Internal status bit
1 → 0
Normal status restored
Section 22 USB Function Module (USB)
1. In case of USB
specification
EPSTL
violation, etc., USB
0
function module
stalls endpoint
automatically
1. EPSTL cleared to 0
by application
EPSTL
2. IN/OUT token
0
received from host
3. Internal status bit
already set to 1
4. EPSTL not
referenced
5. Internal status bit
not changed
1. Transmission of
EPSTL
STALL handshake
0
1. Internal status bit
cleared to 0
EPSTL
2. EPSTL not changed
0
Rev. 1.00 Mar. 12, 2008 Page 885 of 1178
REJ09B0403-0100
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