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Unknown bits for IOBs when using SSTL135 #1182

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mkurc-ant opened this issue Dec 16, 2019 · 3 comments
Closed

Unknown bits for IOBs when using SSTL135 #1182

mkurc-ant opened this issue Dec 16, 2019 · 3 comments

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@mkurc-ant
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Unknown bits were found during running of the minitest/iostandards/features test. Didn't make more detailed tests to which mode if IOB (input/output/inout) those bits apply to. Here are some findings:

Part: xc7a50tfgg484-1

For SSTL135 single ended in region X0Y1

{ unknown_bit = "00000027_23_7", unknown_segment = "0x00000000", unknown_segbit = "39_743" }
{ unknown_bit = "00000027_76_7", unknown_segment = "0x00000000", unknown_segbit = "39_2439" }

For SSTL135 differential SLEW=SLOW in region X1Y0:

{ unknown_bit = "004015a6_23_8", unknown_segment = "0x00401580", unknown_segbit = "38_744" }
{ unknown_bit = "004015a6_39_8", unknown_segment = "0x00401580", unknown_segbit = "38_1256" }
{ unknown_bit = "004015a6_47_8", unknown_segment = "0x00401580", unknown_segbit = "38_1512" }
{ unknown_bit = "004015a7_23_9", unknown_segment = "0x00401580", unknown_segbit = "39_745" }
{ unknown_bit = "004015a7_39_9", unknown_segment = "0x00401580", unknown_segbit = "39_1257" }
{ unknown_bit = "004015a7_47_9", unknown_segment = "0x00401580", unknown_segbit = "39_1513" }

For SSTL135 differential SLEW=FAST in region X0Y1:

{ unknown_bit = "00000026_39_8", unknown_segment = "0x00000000", unknown_segbit = "38_1256" }
{ unknown_bit = "00000026_52_8", unknown_segment = "0x00000000", unknown_segbit = "38_1672" }
{ unknown_bit = "00000026_96_8", unknown_segment = "0x00000000", unknown_segbit = "38_3080" }
{ unknown_bit = "00000027_39_9", unknown_segment = "0x00000000", unknown_segbit = "39_1257" }
{ unknown_bit = "00000027_52_9", unknown_segment = "0x00000000", unknown_segbit = "39_1673" }
{ unknown_bit = "00000027_96_9", unknown_segment = "0x00000000", unknown_segbit = "39_3081" }
@mkurc-ant mkurc-ant mentioned this issue Dec 16, 2019
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@mkurc-ant mkurc-ant mentioned this issue Jan 14, 2020
Merged
@litghost
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I believe this has been fixed?

@mkurc-ant
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I believe that it is still relevant. These bits pop up when running the minitest/iostandards/features for SSTL135. I'm surprised that they don't show for eg. LiteX which uses SSTL135

@mkurc-ant
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It appears that once I've made the minitest to set INTERNAL_VREF in #1324 the missing bits disappeared. These bits may not be related to SSTL135 itself but may mean something like "use external VREF" on a given IOB.

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@litghost @mkurc-ant