H8SXSLASH1663 datasheet(858/1158 Pages) RENESAS

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Section 16 USB Function Module (USB)

Rev.1.00 Jun. 07, 2006 Page 806 of 1102

REJ09B0294-0100

16.7

Stall Operations

16.7.1

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.

16.7.2

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 16.18). 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 16.18). 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 16.18). 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 16.18), 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 16.18). 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 16.18).

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