USB3320 datasheet(44/70 Pages) MICROCHIP

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USB3320

DS00001792B-page 44

 2014-2015 Microchip Technology Inc.

6.3.2

EXITING LOW POWER MODE

To exit Low Power Mode, the Link will assert STP. Upon the assertion of STP, the USB3320 will begin its start-up pro-

cedure. After the transceiver start-up is complete, the transceiver will start the clock on CLKOUT and de-assert DIR.

After DIR has been de-asserted, the Link can de-assert STP when ready and start operating in Synchronous Mode. The

transceiver will automatically set the SuspendM bit to a 1 in the Function Control register.

The value for TSTART is given in Table 4-2.

Should the Link de-assert STP before DIR is de-asserted, the USB3320 will detect this as a false resume request and

return to Low Power Mode. This is detailed in section 3.9.4 of the ULPI 1.1 specification.

6.3.3

INTERFACE PROTECTION

ULPI protocol assumes that both the Link and transceiver will keep the ULPI data bus driven by either the Link when

DIR

is low or the transceiver when DIR is high. The only exception is when DIR has changed state and a turn around

cycle occurs for 1 clock period.

In the design of a USB system, there can be cases where the Link may not be driving the ULPI bus to a known state

while DIR is low. Two examples where this can happen is because of a slow Link start-up or a hardware reset.

6.3.3.1

Start up Protection

Upon start-up, when the transceiver de-asserts DIR, the Link must be ready to receive commands and drive Idle on the

data bus. If the Link is not ready to receive commands or drive Idle, it must assert STP before DIR is de-asserted. The

Link can then de-assert STP when it has completed its start-up. If the Link doesn’t assert STP before it can receive com-

mands, the transceiver may interpret the data bus state as a TX CMD and transmit invalid data onto the USB bus, or

make invalid register writes.

When the USB3320 sends a RXCMD the Link is required to drive the data bus back to idle at the end of the turn around

cycle. If the Link does not drive the databus to idle the USB3320 may take the information on the data bus as a TXCMD

and transmit data on DP and DM until the Link asserts stop. If the ID pin is floated the last RXCMD from the USB3320

will remain on the bus after DIR is de-asserted and the USB3320 will take this in as a TXCMD.

A Link should be designed to have the default POR state of the STP output high and the data bus tri-stated. The

USB3320 has weak pull-downs on the data bus to prevent these inputs from floating when not driven. These resistors

are only used to prevent the ULPI interface from floating during events when the link ULPI pins may be tri-stated. The

strength of the pull down resistors can be found in Table 4-4. The pull downs are not strong enough to pull the data bus

low after a ULPI RXCMD, the Link must drive the data bus to idle after DIR is de-asserted.

FIGURE 6-10:

EXITING LOW POWER MODE

DIR

CLK

DATA[7:0]

STP

TURN

AROUND

LOW

POWER MODE

DATA BUS IGNORED (SLOW LINK)

IDLE (FAST LINK)

IDLE

Slow Link Drives Bus

Idle and STP low

Fast Link Drives Bus

Idle and STP low

...

Note: Not to Scale

TSTART

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