This kind of change probably belongs in fasm2frames. Can you write up a detailed example of the behavior? I don't think I understand what you are proposing is happening here.

I've done some minitesting today. I've found that the following features are enabled for unused IOB sites when given IOSTANDARD+DRIVE+SLEW is set on the used ones from the same bank. I haven't enforced pullup/pulldown:

LVCMOS12_LVCMOS15_LVCMOS18_SSTL135.STEPDOWN
LVCMOS12_LVCMOS15_LVCMOS18_LVCMOS25_LVCMOS33_LVTTL.SLEW.FAST
PULLTYPE.PULLDOWN

for

• LVCMOS12
• LVCMOS18
• SSTL135

I haven't played with setting pullup/pulldown for all unused IOBs in the design as the result is pretty obvious.

The question is whether we should set the eg. STEPDOWN feature for unused IOBs in SymbiFlow and how not doing it will affect the design operation in hardware.

FYI, SLEW.FAST and PULLTYPE.PULLDOWN are both NULL features (e.g. no bits are set). So only LVCMOS12_LVCMOS15_LVCMOS18_SSTL135.STEPDOWN needs to be propigated to the entire IO bank. I believe the logic is something akin to:

• If STEPDOWN is used anywhere in the bank, all IOB's need to have STEPDOWN set
• If the IOSTANDARD of any IOB requires that STEPDOWN is not set, that is an error (e.g. incompatible IOSTANDARD).

This change should live in fasm2frames.py and/or prjxray/fasm_assembler.py.

I believe STEPDOWN needs to be set for LVCMOS15 too?

Uh, I forgot to use LVCMOS15 in the minitest... I'll check that.

I checked more IOSTANDARDs and the feature LVCMOS12_LVCMOS15_LVCMOS18_SSTL135.STEPDOWN is set for unused IOBs of the same bank when using one of:

• LVCMOS12
• LVCMOS15
• LCVMOS18
• MOBILE_DDR (?)
• SSTL135
• SSTL135_R
• SSTL15
• SSTL15_R
• SSTL18_II
• HSUL_12