8.3.19.1 BIST Configuration and Status

The BIST mode is enabled at the deserializer by either the BISTEN pin or the BIST configuration register. The test may select either an external TMDS clock or the internal Oscillator Clock (OSC) frequency. In the absence of the TMDS clock, the user can select the internal OSC frequency at the deserializer through the BISTC pin or BIST configuration register.

When BIST is activated at the deserializer, a BIST enable signal is sent to the serializer through the back channel. The serializer outputs a test pattern and drives the link at speed. The deserializer detects the test pattern and monitors it for errors. The deserializer PASS output pin toggles to flag each frame received that contained one or more errors. The serializer also tracks errors indicated by the CRC fields in each back channel frame.

The BIST status can be monitored real time on the deserializer PASS pin, with each detected error resulting in a half pixel clock period toggled LOW. After BIST is deactivated, the result of the last test is held on the PASS output until a reset (through either a new BIST test or during power down). A high on PASS indicates that no errors were detected. A Low on PASS indicates that one or more errors were detected. The duration of the test is controlled by the pulse width applied to the deserializer BISTEN pin. LOCK is valid throughout the entire duration of BIST.

See Figure 18 for the BIST mode flow diagram.

Step 1: The Serializer is paired with another FPD-Link III deserializer and BIST Mode is enabled through the BISTEN pin or through the register on the deserializer. Right after BIST is enabled, part of the BIST sequence requires that bit 0x04[5] is toggled locally on the serializer (set 0x04[5]=1, then set 0x04[5]=0). The desired clock source is selected either through the deserializer BISTC pin or through register on the deserializer.

Step 2: An all-zeros pattern is balanced, scrambled, randomized, and sent through the FPD-Link III interface to the deserializer. When the serializer and the deserializer are in BIST mode and the deserializer acquires a lock state, the PASS pin of the deserializer goes high and BIST starts checking the data stream. If an error in the payload (1 to 35) is detected, the PASS pin will switch low for one half of the clock period. During the BIST test, the PASS output can be monitored and counted to determine the payload error rate.

Step 3: To stop the BIST mode, the deserializer BISTEN pin is set low. The deserializer stops checking the data. The final test result is held on the PASS pin. If the test ran error-free, the PASS output will remain high. If one or more errors were detected, the PASS output will output a constant low. The PASS output state is held until a new BISTEN toggle, the device is RESET, or the device is powered down. The BIST duration is user-controlled by the duration of the BISTEN signal.

Step 4: The link returns to normal operation after the deserializer BISTEN pin is low. Figure 19 shows the waveform diagram of a typical BIST test for two cases: Case 1 is error-free, and Case 2 shows one with multiple errors. In most cases, it is difficult to generate errors due to the robustness of the link (differential data transmission and so forth), thus they may be introduced by greatly extending the cable length, faulting the interconnect medium, or reducing signal condition enhancements (Rx Equalization).

Figure 18. BIST Mode Flow Diagram