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DS00002103A-page 17

USB3500

5.2

TX Logic

This block receives parallel data bytes placed on the DATA bus and performs the necessary transmit operations. These

operations include parallel to serial conversion, bit stuffing and NRZI encoding. Upon valid assertion of the proper TX

control lines by the Link and TX State Machine, the TX LOGIC block will synchronously shift, at either the FS or HS rate,

the data to the FS/HS TX block to be transmitted on the USB cable. Data transmit timing is shown in Figure 5-3.

The behavior of the Transmit State Machine is described below.

• The Link asserts TXVALID to begin a transmission.

• After the Link asserts TXVALID it can assume that the transmission has started when it detects TXREADY has

been asserted.

• The Link must assume that the USB3500 has consumed a data byte if TXREADY and TXVALID are asserted on

the rising edge of CLKOUT.

• The Link must have valid packet information (PID) asserted on the DATA bus coincident with the assertion of

TXVALID.

• TXREADY is sampled by the Link on the rising edge of CLKOUT.

• The Link negates TXVALID to complete a packet. Once negated, the transmit logic will never reassert TXREADY

until after the EOP has been generated. (TXREADY will not re-assert until TXVALD asserts again.)

• The USB3500 is ready to transmit another packet immediately. However, the Link must conform to the minimum

inter-packet delays identified in the USB 2.0 specification.

• Supports high speed disconnect detect through the HOSTDISC pin. In Host mode the USB3500 will sample the

disconnect comparator at the 32nd bit of the 40 bit long EOP during SOF packets.

• Supports FS pre-amble for FS hubs with a LS device.

• Supports LS keep alive by receiving the SOF PID.

• Supports Host mode resume K which ends with two low speed times of SE0 followed by 1 FS “J”.

FIGURE 5-3:

TRANSMIT TIMING FOR A DATA PACKET