Factor out resource "sevenseg".

[nmigen-issue-migration]

Issue by cr1901 Sunday Jul 07, 2019 at 13:55 GMT Originally opened as https://github.com/m-labs/nmigen-boards/issues/17

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Differences between various 7SDs:

• Common cathode vs anode?
• Is decimal point used?
• Are the digit enable signals one-hot coming out of the FPGA or not?
  • Using a single enable line, where 0 controls one digit, and 1 controls the other seems more common when only two digits are present (fairly common setup). Otherwise, one-hot is used.

Potential Subsignal naming scheme:

• a, b, ... g, dp for decimal point (if present)
• en{0, 1, 2}
  • How should one-hot vs shared en be handled in naming. Should there be a difference?

Boards using it right now

None! But Mercury will (for the baseboard peripheral that is normally attached to it), and in omigen, the naming scheme was inconsistent. So I want to fix this before it becomes a problem.

[nmigen-issue-migration]

Comment by mithro Sunday Jul 07, 2019 at 16:09 GMT

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BTW 7-segment displays are a common learning exercise for beginner HDL coders. It would be good to have good support for them and to not make it too easy....

[nmigen-issue-migration]

Comment by cr1901 Sunday Jul 07, 2019 at 16:12 GMT

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BTW 7-segment displays are a common learning exercise for beginner HDL coders. It would be good to have good support for them and to not make it too easy....

I'm assuming we've left the realm of "not make it too easy" for beginners once PinsN was introduced. For "NMigen for beginners" literature, I'm guessing "active-high" and "active-low" will need to be footnotes. Similar w/ seven seg quirks.

[nmigen-issue-migration]

Comment by whitequark Sunday Jul 07, 2019 at 19:05 GMT

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• Common cathode vs anode?

Logical high means LED is lit.

• Is decimal point used?

Optional, the downstream code can check it with hasattr(seven_seg, "dp").

• Are the digit enable signals one-hot coming out of the FPGA or not?

The enable signal is called sel if it's straight binary, and en if it's one-hot. (No enable signal is also OK for one digit.

I'm assuming we've left the realm of "not make it too easy" for beginners once PinsN was introduced.

Yes.