USB System Architecture

Figure 11-7: SOP Detection

High-Speed Driver/Receiver Compliance Testing

The USB 2.0 specification defines a series of test modes that all high-speed devices (including the host controller and high-speed hubs) must support for compliance testing. These test modes are entered via control transfer requests

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that place port transceivers into the following test modes:

•Test_SE0_NAK — This mode causes the selected port (either upstream or downstream) to enter its high-speed receive state. This enables testing of output impedance, low level output voltage, and loading characteristics.

•Test_J — This mode causes the transceiver to transmit a “J” by switching current to the D+ line. This allows the D+ drive level to be tested.

•Test_K — This mode causes the transceiver to transmit a “K” by switching current to the D- line. This allows the D- drive level to be tested.

•Test_Packet — This mode causes the device to transmit a repeating string of characters. This pattern is used to perform the eye pattern checks to verify proper transmit characteristics and receiver sensitivity.

Activating Test Mode

Test mode may be activated when a device is in its default, addressed, or configured states. A control transfer is used to place the device in one of the test modes. The SetFeature request is delivered to the device during the setup stage of the control transfer, and the device must enter test mode no later than 3ms after the status stage of the transfer completes. Table 11-1 shows the format of the 8 bytes of data sent to the device during the setup transaction. Note that the feature is set to “Port_Test” and the index field contains the “Test_Selector” that defines the test to be performed. To exit test mode, the device’s power must be cycled.

Table 11-1: Device Request Format for Placing Device into Test Mode

The test selector values are defined in Table 11-2 on page 230.

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USB System Architecture

Table 11-2: Test Selector Values

The Test Setup

A test fixture defined by the specification provides an interface to either a differential oscilloscope that can be used to check output waveforms and voltages, or a data generator that sources test waveforms that are used to check receiver sensitivity.

Figure 11-8 on page 231 illustrates two sets of test points that reflect how the device attaches to the cable: 1) devices that have a cable connector and 2) devices that are permanently attached to their cable (i.e., a captive cable). The test points used when the device has a detachable cable are illustrated in the first example.

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