Siemens HC15 User Manual download pdf (Page 13)

AN39: USB Interface Description

3.4 Transition from Composite to Mass Storage Device

HC15_USB_Interface_v02 Page 13 of 28 2007-04-05

Confidential / Preliminary

3.4 Transition from Composite to Mass Storage Device

As a rule the module’s USB interface is invoked as an "MdmNet" or "Mdm" enumerated USB

composite device (see Table 1; parameter “startup“). However, if no USB composite host driver

is installed or active, the host will not send any USB communication device class request after

enumeration (see Section 3.4.1). Instead, it only sends SOF packets (=Start-Of-Frame pack-

ets; sent every 1ms for a USB full speed interface).

In this case, the module will re-enumerate as a USB mass storage device after a specified time

period (see Table 1; parameter “mnto“). The mass storage device holds the Windows XP host

driver software for the default USB composite device. The following section describes the tim-

ing mechanism that controlls the transition from composite to mass storage device. This mech-

anism is enabled only if the module is configured to initially enumerate as a composite

communication device ("MdmNet" or "Mdm"). See note in Section 3.1 for the steps required to

terminate the mass storage device.

3.4.1 Transition Timer

The transition from composite to mass storage device is controlled by a timer configured for the

module (see Table 1; parameter “mnto“).

The USB host starts the timer with its initial GET_DESCRIPTOR (DEVICE) request (according

to [5], Section 9.4.3) to which the module replies with the appropriate device descriptor (see

Chapter 4). Any further GET_DESCRIPTOR (DEVICE) request received within the configured

time period, i.e., while the timer is running, will restart the timer. Any GET_DESCRIPTOR

(DEVICE) request received after the timer has expired does not restart the timer. The timer is

only started once after power up.

Note: The timing mechanism operates with low priority in the firmware task context. The con-

figured time period does therefore only represent the minimum period of time before the timer

expires. The exact period cannot be guaranteed. If an application is busy with higher priority

tasks, the timeout may be delayed for some 10ms. Considerably longer periods are also pos-

sible.

If the configured time period (i.e., the USB communication device class request timeout)

expires without any of the USB requests listed below being received by the module, a USB SE0

condition (see [5], Section 7.1.7.1) is driven on the bus by the module for t > 10ms (typical

200ms) to trigger re-enumeration by the USB host, i.e., a restart as mass storage device:

• CLEAR_FEATURE (ENDPOINT)

This is actually no communication class request but a standard request according to [5],

Section 9.4.1. Because this request is mandatory to get the endpoints associated with com-

munication device interfaces into operation, it is also an indicator for a successfully installed

USB modem host driver.

• SET_LINE_CODING

Communication class request according to [6], Section 6.2.12

• GET_LINE_CODING

Communication class request according to [6], Section 6.2.13

• SET_CONTROL_LINE_STATE

Communication class request according to [6], Section 6.2.14

Only if any of the mentioned USB requests are received within the configured time period will

the module remain enumerated as "MdmNet" or "Mdm" USB composite device.

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