STW4811MBRATSLASHHF datasheet(50/85 Pages) STMICROELECTRONICS

50 / 85 page

Functional description

STw4811M/STw4811N

50/85

CD00131784

UART mode

UART mode is entered by setting the ‘uart_en’ bit to 1 (

USB control register 1 - Table 11).

The transceiver contains two digital logic level translators between the following balls:

●

TXD signal: from USBVM ball to DN ball

●

RXD signal: from DP ball to USBVP ball

When not in UART mode the level translators are disabled.

VBUS monitoring and control

The monitoring is made of three comparators that determine if the VBUS voltage is at a valid

level for operation:

●

VBUS VALID: It corresponds to the minimum level on VBUS. Any voltage on VBUS

below the threshold is considered to be a fault. During power-up, it is expected that this

comparator output is ignored.

●

VBUS SESSION VALID: This threshold is necessary for session request protocol to

detect the VBUS pulsing.

●

VBUS SESSION END: Session is ended. In this USB block, a B-device Session End

threshold is defined within the range [0.2; 0.8] V. The reason for a low 0.2 V limit is that

the leakage current could charge the VBUS up to 0.2 V (maximum).

When the A-device (default master) is power supplied and does not supply VBUS, it

presents an input impedance RA_BUS_IN on VBUS of no more than 100 k

Ω. If the A-device

responds to the VBUS pulsing method of SRP, then the input impedance RA_BUS_IN may

not be lower than 40 k

Ω.

When the A-device supplies power, the rise time TA_VBUS_RISE on VBUS to go from 0 to

4.4 V is less than 100 ms when driving 100 mA and with an external load capacitance of

10 µF (in addition to VBUS decoupling capacitance). If VBUS does not reach this voltage

within TA_VBUS_RISE maximum time, it indicates that the B-device is drawing more

current that the A-device is capable of providing and an over-current condition exists. In this

case, the A-device turns VBUS off and terminates the session.

VBUS capacitance

A dual-role device must have a VBUS capacitance CDRD_VBUS value comprised between

1 µF and 6.5 µF (see charge pump specification). The limit on the decoupling capacitance

allows a B-device to differentiate between a powered-down dual-role device and a powered-

down standard host. The capacitance on a host is higher than 96 µF.

Data line pull-down resistance

The two bits of

USB control register, dp_pulldown and dn_pulldown (Table 12) are used to

connect/disconnect the pull-down resistors.

When an A-device is idle or acting as host, it activates the pull-down resistors RPD on both

DP and DN lines.

When an A-device is acting as peripheral, it disables RPD on DP, not DN.

The A-device can disable both pull-down resistors during the interval of a packet

transmission when acting as either host or peripheral.

When the line is not used, the pull-down is activated and the maximum level on this ball

should not exceed 0.342 V.

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