Chapter 11: The High-Speed Signaling Environment

Figure 11-8: Test Points

Eye Pattern Tests

The specification defines the operational characteristics of High-speed drivers and receivers with a series of eye diagrams. These diagrams are used when testing high-speed driver and receiver operation under dynamic signaling conditions. These eye patterns define minimum:

•output waveforms requirements for hubs and devices measured at the nearest connector. The driver output waveforms define the minimum rise and fall times and output strength of the driver.

•input waveform requirements for hubs and devices when test waveforms are applied at the nearest connector. The receiver input waveforms define the minimum input sensitivity.

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USB System Architecture

The following measurements are made:

•hub transmits test waveforms that are checked at TP2.

•device transmits test waveforms that are checked at TP3.

•hub receiver sensitivity is checked with test waveforms applied to TP2.

•device receiver sensitivity is checked with test waveforms applied to TP3.

The tests performed when a device has a captive cable as illustrated in the second example:

•device transmits test waveforms that are checked at TP2.

•device receiver sensitivity is checked with test waveforms applied to TP2.

Transmit Eye Pattern Tests. These tests require that the device under test be placed into the Test_Packet mode, which causes the device to transmit the test packet repetitively. See test packet definition in Figure 11-9. Note that the test pattern specified in the illustration is the NRZI bit pattern driven across the bus and includes stuffed bits. The oscilloscope used to verify transmission characteristics must be placed in the infinite persistence mode to capture waveform behavior of the transmitter during the test. A successful test requires that the resulting waveforms captured by the scope fall outside the eye of the waveform template (e.g., eye diagram).

Figure 11-9: Test Packet Contents

Figure 11-10 on page 233 is a replication of one of the eye diagrams from the specification. This diagram shows the minimum waveform requirements for a hub measured at TP2 or a device with a detachable cable measured at TP3. Note that the template specifies minimum rise and fall times and minimum output voltages.

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Chapter 11: The High-Speed Signaling Environment

Figure 11-10: Example Eye Diagram for Transmit Test

/HYHO

P9 'LIIHUHQWLDO

9ROWV 'LIIHUHQWLDO

P9 'LIIHUHQWLDO

/HYHO

Receiver Eye Pattern Tests. These tests require a data generator that is set up to deliver the test pattern illustrated in Figure 11-9 on page 232. The waveform must meet the transmit characteristics defined by the eye diagrams in the specification. The test packet is applied to the receiver to verify that its input sensitivity meets the specification. The device is placed into the Test_SE0_NAK mode, which causes the receiver to enable its high-speed receiver. When the packet is delivered, the device should respond with a NAK handshake if the packet was received correctly.

Figure 11-11 on page 234 is a replication of the eye diagram for a device with a captive cable and with the test packet applied at TP2. The eye diagram in this case defines the waveform that must be delivered at TP2 by the data generator to verify that the minimum input sensitivity of the receiver is within tolerance.

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