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enjoy-digital / litedram

Small footprint and configurable DRAM core
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Add support for TDQS mode. #210

Closed oskirby closed 3 years ago

oskirby commented 3 years ago

This adds an optional argument to add_electrical_settings() that can enable the TDQS mode for x8 DDR3 modules. Normally this would be used when mixing x4 and x8 modules (eg: heterogenous DIMMs), but it can also serve as a workaround boards with missing or broken DM signals.

I had to add this as a workaround for the rev0 Logicbone prototypes because the DM signals on the DDR3 modules wound up floating due to a combination of layout errors. The downside of enabling TDQS mode is that the data mask signals no longer have any effect, and you can only ever write a full burst.

Given that this is a workaround for a prototype board that may not ever see the light of day, it's also okay to reject this PR.

enjoy-digital commented 3 years ago

Thanks @oskirby, this could be useful for other debug cases so happy to merge it.