SH7720 Datasheet, PDF (918/1524 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7700 Series

Section 25 USB Function Controller (USBF)
25.8 Stall Operations
25.8.1 Overview
This section describes stall operations in this module. There are two cases in which the USB
function controller stall function is used:
• When the application forcibly stalls an endpoint for some reason
• When a stall is performed automatically within the USB function controller due to a USB
specification violation
The USB function controller 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.
However, the internal status bit to EP0 is automatically cleared only when the setup command is
received.
25.8.2 Forcible Stall by Application
The application uses the EPSTL register to issue a stall request for the USB function controller.
When the application wishes to stall a specific endpoint, it sets the corresponding bit in EPSTL (1-
1 in figure 25.16). The internal status bits are not changed at this time. When a transaction is sent
from the host for the endpoint for which the EPSTL bit was set, the USB function controller
references the internal status bit, and if this is not set, references the corresponding bit in EPSTL
(1-2 in figure 25.16). If the corresponding bit in USBEPSTL is set, the USB function controller
sets the internal status bit and returns a stall handshake to the host (1-3 in figure 25.16). In this
time, if the CTLR/ASCE bit is set to 1, the corresponding bit in EPSTL is automatically cleared to
0 and a stall handshake is returned to the host (1-4 in figure 25.16). If the corresponding bit in
EPSTL is not set, the internal status bit is not changed and the transaction is accepted.
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. Even after a bit is cleared by the Clear Feature
command (3-1 in figure 25.16), the USB function controller continues to return a stall handshake
while the bit in EPSTL is set, since the internal status bit is set each time a transaction is executed
for the corresponding endpoint (1-2 in figure 25.16). To clear a stall, therefore, it is necessary for
the corresponding bit in EPSTL to be cleared automatically when a stall is returned from the USB
controller while the CTLR/ASCE bit is set to 1, or to be cleared by the application, and also for
Rev. 3.00 Jan. 18, 2008 Page 856 of 1458
REJ09B0033-0300