PRBS PASS Latch Circuits
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PRBS PASS Latch Circuits
The TLK6002EVM has a Pass/Fail Latch circuit to aid in PRBS testing. The PRBS_PASSA and
PRBS_PASSB status signals have been routed out to Header JMP43 for monitoring with an oscilloscope.
Additionally, these signals are routed to the base of an NPN transistor that drives an LED on or off
depending on whether the PRBS_PASSA/B signals are High or Low. The blue LEDs D51 and D52 are a
good indication of general passing or general failing depending on whether the LED is on or off. However,
it is not a good method to easily monitor individual bit failures. An error counter can be read through the
MDIO register interface that contains the actual error count should the counter not have overflowed;
however, it requires the counter to be read in order to determine whether an error has occurred. With
proper setup and operation, the PRBS_PASS Latch circuits can quickly indicate whether an error has
occurred or not.
To properly operate the PRBS_PASS latches, configure the TLK6002 for PRBS operation either by pulling
the PRBS_EN line high or through the appropriate MDIO register settings. PRBS data is generated and
output on the Serial Transmit and the Serial Receive monitors for valid PRBS data and counts the number
of errors received. In addition to counting the errors, the PRBS_PASS line is high if the line is error free,
and transitions low for the duration of time that an error, or errors, are detected. Once the PRBS link is
established and running error free, pushing the PRBS_PASS RESET button resets the latch circuit, which
consists of a J/K flip-flop and a red and green LED to indicate the state of the flip-flop. When the J/K
flip-flop is reset, the green LED is lit until a high-to-low transition on the PRBS_PASS signal is observed.
Because the PRBS_PASS signal is connected to the clock input of the J/K flip-flop, a high-to-low transition
causes the flip-flop to toggle to the next state which turns OFF the green LED and turns ON the red LED.
The circuit remains in this state until the RESET button is pushed again, which resets the flip-flop to its
initial state.
NOTE:
If the PRBS_PASS lines are low, indicating constant PRBS failure and the PRBS_PASS
RESET buttons are pushed, the green LEDs lights and remain lit, which might be falsely
interpreted as a passing result. This is incorrect as the line was not ever passing and as long
as it is failing the PRBS_PASS line will be low and never have the high-to-low transition
required to toggle the state and light the red LED. This is not the intended operation of the
circuit, and this latch circuit must ONLY be used on a passing and correctly established
channel with valid PRBS data. An easy indication of whether the PRBS Latch may be used
is whether the blue PRBSA/B LEDs are on as well, because they can only be on if the
PRBS_PASS signals are high indicating a passing status. If the blue LEDs are not on, then
the green and red PRBS_PASS/Fail LEDs are not a valid indication of the status of the test.
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TLK6002 Dual-Channel, 0.47-Gbps to 6.25-Gbps, Multi-Rate Transceiver
SLLU132 – October 2010
Evaluation Module
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