H8S2472 Datasheet, PDF (929/1250 Pages) Renesas Technology Corp – 16-Bit Single-Chip Microcomputer H8S Family / H8S/2400 Series

Section 22 USB Function Module (USB)
22.7 Stall Operations
22.7.1 Overview
This section describes stall operations in this module. There are two cases in which the USB
function module stall function is used:
• When the application forcibly stalls an endpoint for some reason
• When a stall is performed automatically within the USB function module due to a USB
specification violation
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.
However, the internal status bit for EP0 is automatically cleared only when the setup command is
received.
22.7.2 Forcible Stall by Application
The application uses the EPSTL register to issue a stall request for the USB function module.
When the application wishes to stall a specific endpoint, it sets the corresponding bit in EPSTL (1-
1 in figure 22.18). 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 module
references the internal status bit, and if this is not set, references the corresponding bit in EPSTL
(1-2 in figure 22.18). If the corresponding bit in EPSTL is set, the USB function module sets the
internal status bit and returns a stall handshake to the host (1-3 in figure 22.18). 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 22.18), the USB function module 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 22.18). To clear a stall, therefore, it is necessary for
the corresponding bit in EPSTL to be cleared by the application, and also for the internal status bit
to be cleared with a Clear Feature command (2-1, 2-2, and 2-3 in figure 22.18).
Rev. 2.00 Aug. 20, 2008 Page 881 of 1198
REJ09B0403-0200