USB3370 datasheet(27/75 Pages) MICROCHIP

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DS00001915B-page 27

USB3370

5.7.3

SESSVLD COMPARATOR

The SessVld comparator is used when the PHY is configured as both an A and B device. When configured as an A

device, the SessVld is used to detect Session Request protocol (SRP). When configured as a B device, SessVld is used

to detect the presence of VBUS. The SessVld comparator output can also be read from the USB Interrupt Status regis-

ter. The SessVld comparator will also generate an RX CMD, as detailed in Section 6.3.1, anytime the comparator

changes state. The SessVld interrupts can be disabled by clearing this bit in both the USB Interrupt Enable Rising and

USB Interrupt Enable Falling registers. When the interrupts are disabled, the SessVld comparator is still operational and

will generate RX CMD’s. The SessVld comparator trip point is detailed in Table 4-8.

5.7.3.1

VbusVld Comparator

The VbusVld comparator is only used when the USB3370 is configured as a host that can supply less than 100mA

VBUS current. In the USB protocol, the A-device supplies the VBUS voltage and is responsible to ensure it remains

within a specified voltage range. The VbusVld comparator can be disabled by clearing this bit in both the USB Interrupt

Enable Rising and USB Interrupt Enable Falling registers. When disabled, bit 1 of the USB Interrupt Status register will

return a 0. The VbusVld comparator threshold values are detailed in Table 4-8.

If the USB3370 is used as a Device only the VbusValid Comparator should be disabled, using the method described

above.

The USB3370 includes the external VbusVld indicator logic as detailed in the ULPI Specification. The external VbusVld

indicator is tied to a logic one. The decoding of this logic is shown in Table 5-4 below. By default this logic is disabled.

Note 5-4

A peripheral should not use VbusVld to begin operation. The peripheral should use SessVld to detect

the presence of VBUS on the USB connector. VbusVld should only be used for USB Host and OTG

A-device applications.

5.7.3.2

VBUS Pulsing with Pull-up and Pull-down Resistors

In addition to the internal VBUS comparators, the USB3370 also includes the integrated VBUS pull-up and pull-down

resistors used for VBUS Pulsing during OTG Session Request Protocol. To discharge the VBUS voltage so that a Ses-

sion Request can begin, the USB3370 provides a pull-down resistor from VBUS to GND. This resistor is controlled by

the DischargeVbus bit 3 of the OTG Control register. The pull-up resistor is connected between VBUS and VDD33. This

resistor is used to pull VBUS above 2.1 volts so that the A-Device knows that a USB session has been requested. The

state of the pull-up resistor is controlled by the bit 4 ChargeVbus of the OTG Control register. The Pull-Up and Pull-Down

resistor values are detailed in Table 4-8.

The internal VBUS Pull-up and Pull-down resistors are designed to include the RVBUS external resistor in series. This

external resistor is used by the VBUS Over voltage protection described below.

Note:

The OTG Supplement specifies a voltage range for A-Device Session Valid and B-Device Session Valid

comparator. The USB3370 PHY combines the two comparators into one and uses the narrower threshold

range.

TABLE 5-4:

EXTERNAL VBUS INDICATOR LOGIC

Typical

Application

Use External

VBUS Indicator

Indicator Pass

Thru

Indicator

Complement

RXCMD VBUS Valid

Encoding Source

OTG Device

Internal VbusVld comparator (Default)

Fixed 1

Fixed 0

Internal VbusVld comparator.

Fixed 0

Standard Host

Fixed 1

Fixed 0

Standard

Peripheral

Internal VbusVld comparator. This

information should not be used by the Link.

(Note 5-4)

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