STW4810 datasheet(46/79 Pages) STMICROELECTRONICS

46 / 79 page

STw4810

Functional description

45/78

The two bits of USB control register dp_pullup and dn_pullup (

Table 12) 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

Table 12).

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.

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