STW4810 datasheet(40/72 Pages) STMICROELECTRONICS

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4 Functional description

STw4810

40/72

Rev3

The two bits of USB control register dp_pullup and dn_pullup (

Table 11) are used to connect/

disconnect pull-up resistors.

Session Request Protocol (SRP)

To save power, the OTG supplement allows an A-device to leave the VBUS turned off when the

bus is not being used. If the B-device wants to use the bus when VBUS is turned off, then it

requires the A-device to supply power on VBUS using the Session Request Protocol (SRP).

●

Initial conditions

The B-device does not attempt to start a new session until it has determined if the A-device has

detected the end of the previous session. The B-device must ensure that VBUS is below

VBUS_SESSION_END before requesting a new session.

Additionally, the B-device switches a pull-down resistor (R_VBUS_PD) from VBUS to ground in

order to quicken the discharge process as long as the B-device does not draw more than 8 mA

from VBUS. R_VBUS_PD is activated by bit ‘vbus_dischrg’ of USB control register 2,

(

Table 11).

When the B-device detects that VBUS is below the VBUS_SESSION_END and that both DP

and DN have been low (SEO) for at least 2 ms, then any previous session on the A-device is

over and a new session can start.

●

Data-line pulsing

To indicate a request for a new session using the data line pulsing, the B-device turns on the

DP pull-up resistor for 5 ms to 10 ms (only at full speed, no DN pulsing). The DP pull-up resistor

is connected to VUSB (regulator output voltage). Timing is controlled by the USB digital control.

●

VBUS pulsing

To indicate a request for a new session using the VBUS pulsing method, the B-device waits for

the initial conditions and then drives VBUS. VBUS is driven for a long enough period for a

capacitance on VBUS that is smaller than 2x6.5 µF to be charged to 2.1 V while a capacitance

on VBUS higher than 97 µF is not charged above 2.0 V. In this USB block, the

VBUS_SESSION_VALID threshold is used to determine if an A-device is DRD (dual role

device) or a standard host.

The B-device VBUS pulsing block is designed so that the maximum drawn current does not

exceed 8 mA. In this USB block, the pull-up is 600

Ω +/- 30%.

If a B-device is attached to a standard device, the pull-up must be disconnected after the

defined timing to prevent damage of standard hosts not designed to withstand a voltage

externally applied to VBUS.

●

Session Request Protocol (SRP)

If the B-device is in correct condition to start a new session, it first performs data line pulsing,

followed by VBUS pulsing. When VBUS next crosses the SESSION VALID threshold, the B-

device considers a session to be in progress and asserts the DP or DN data line within 100 ms.

After SRP initialization, the B- device is set up to wait for at least 5 seconds for the A-device to

respond before informing the user that the consumption attempt has failed.

●

Host Negotiation Protocol (HNP)

At the start of a session, the A-device has the role of host as default. During a session, the host

role can be transferred back and forth between the A-device and the B-device any number of

times using the Host Negotiation Protocol (HNP). The process for this exchange of host role is

described in the “On the Go Supplement to the USB 2.0 Specification” (rev 1.0).

STW4810 数据表(PDF) 40 Page - STMicroelectronics

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