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SN74AXC8T245: About wrong output waveform

user4206615
Prodigy 40 points
Part Number: SN74AXC8T245


Hi Team,

This Bus Transceiver is used to perform level conversion from 1.2V to 1.8V.
The signal direction is fixed at the DIR terminal, and is 1.2 V input and 1.8 V output.
Attach 1.2V input (waveform 1) and 1.8V output (waveform 2)

At this time, the output waveform does not swing from 0 V to 1.8 V.
The output is measured by connecting only a 500 MHz band oscilloscope.
What causes the output not to swing?

In addition, the data sheet has the following description.
"Up to 380 Mbps Support When Translating from 1.8 V to 3.3 V"
How many can be supported when converting from 1.2V to 1.8V?

waveform 1

waveform 2

Best Regards,

  • 0
    Guru 318050 points

    Are the probes 1 MΩ or 50 Ω?

    The plateaus do not appear flat. Does the signal eventually reach the rails at a lower frequency?

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points
    Probe is 1 MΩ.
    There is no problem at a frequency of 1 MHz.
  • 0 in reply to user4206615
    Guru 318050 points

    How do the waveforms at 1 MHz look like?

    There does not appear to be documentation about the behaviour, but if the AXC family is similar to the AVC family, then its output drivers use a lower impedance for a short time when switching. I'd guess that the low-impedance phase is stopped before the voltage has reached the rail in order to reduce ringing.

  • 0 in reply to user4206615
    TI__Guru 62550 points

    Hi,

    Since 1Mhz works fine but operating at ~55Mhz is causing issues, I believe that the output loading is large enough to not enable the output to reach its final state.  I do not have information right now about the max supported frequency for 1.2 to 1.8V translation. Do you know the output loading R/C ?

    The tpd( and the speed) is directly related to output loading. The tpd linearly increases with increasing capacitive /resistive loading.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points
    The waveforms for each frequency are shown below
    1MHz
    10MHz
    20MHz
    30MHz
  • 0 in reply to user4206615
    TI__Guru 62550 points

    As I checked back into the characterization data, we see that 1.2 to 1.8V should support up to 150Mbps. Hence, in this application, it should be able to support 110Mbps without issues.

    Do you know the cap loading in this case case? 

  • 0 in reply to ShreyasRao
    TI__Guru 62550 points

    I also see that the high level of the 1.8V reaches 1.5V, and the low level is at about 0.4V. 

    The datasheet Vol and Voh for the 1.65V case is listed as 1.2V and 0.45V respectively. So, the waveform is meeting the electrical char listed in the datasheet.

    The method of qualifying the datarate is most likely if that waveform meets the datasheet characteristics of voh and Vol at that particular Vcc level. 

    Have you seen issues with your receiver devices not able to decipher the logic levels correctly?