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Adventurer
Adventurer
8,802 Views
Registered: ‎03-15-2012

GTP: 8B/10B encoding

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hi,

 

i've a design which transmits data via MGT with enabled 8B/10B encoder. I use 2 GTP with the same input data but different speeds.

 

The GTP with 940MBit/s (Refclk 94 MHz) seems to work as expected, whereas the 2nd GTP with 1.25GBit/s or 2.5GBit/s (Refclk 100MHz) has running disparity, although both GTPs are equal configured (verified via file compare at the generated VHD-files from the coregen, only the PLL-settings are different). But if i look at the waveform (Modelsim or ISim), the waveforms are different.

 

I see, that the 2 GTP inserts for example 2 negative symbols back-to-back, which isn't allowed. The first one does it correct.

 

Does any one have a hint?

 

I didn't found any AR about a problem of the 8B/10B encoder.

 

thanks

both.bmp
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Adventurer
Adventurer
14,801 Views
Registered: ‎03-15-2012

none of the RESETDONE got assigned within 2 ms

 

but nevertheless problem solved:

 

i used the wrong clock-source: TILE0_GTPCLKOUT0_OUT(0) instead of TILE0_TXOUTCLK0_OUT

 

thanks

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Moderator
Moderator
8,792 Views
Registered: ‎02-16-2010
What are the signals shown in the waveform?

How do you simulate the design? Do you find GTRESETDONE, RXBYTEISALIGNED from GT to be behaving correctly?

It will be good if you can provide the snapshot of the signals you find incorrect.
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Adventurer
Adventurer
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Registered: ‎03-15-2012

sorry for posting too fast...

 

the waveforms show the TX_P of the both GTPs. The upper waveform is the working (slower) GTP and the lower the faster one (it is scaled to have the same bit-width for comparing). It is just a part, but the interesting one. It shows first the IDLE-symbols (waveforms in sync, because the symbol is balanced) and later the real data at with the waveforms then differs.

 

I simulate the sender against a provided Altera Stratix receiver design and saw receiving disparity errors.

 

RXBYTEISALIGNED is not used since i only transmit, but GTPRESETDONE will be checked.

 

 

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Moderator
Moderator
8,780 Views
Registered: ‎02-16-2010
What did you set with TXCHARDISPMODE, TXCHARDISPVAL? If you have set them to "0", then the encoder will take care of the running disparity.
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Highlighted
Adventurer
Adventurer
14,802 Views
Registered: ‎03-15-2012

none of the RESETDONE got assigned within 2 ms

 

but nevertheless problem solved:

 

i used the wrong clock-source: TILE0_GTPCLKOUT0_OUT(0) instead of TILE0_TXOUTCLK0_OUT

 

thanks

View solution in original post

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