demux component bargraph mode never turns on highest output #3519

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1. BsAtHome commented on Jul 19, 2025

   

   BsAtHome

   on Jul 19, 2025

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   With personality=8, you get pins out-00 through out-07, which are 8 pins. It should read (MM=00..personality-1) in the man-page.

   That the last bit is not turned on is a logic error in the code.

   The bargraph has personality+1 states ("0=all off" through "personality=all on") whereas the demux version only has personality states. In your test case: 0 for none activated and 8 for all activated, thus 0...8 and that equals 9 possible values.

   That also makes the use of sel-bit-XX problematic with personality=32 because it cannot represent that case. You can use the sel-u32 pin to set the value.

   This is the snippet:

       if (bit >= personality) bit = personality - 1;
       for (i = 0; i < personality ; i++) {
           out(i) = (bargraph) ? (bit > i) : (bit == i);
       }

   It may be changed into:

     if(bargraph) {
       for (i = 0; i < personality; i++) {
         out(i) = bit > i;
       }
     } else {
       if (bit >= personality) bit = personality - 1;
       for (i = 0; i < personality; i++) {
         out(i) = bit == i;
       }
     }

   Another reason for removing the ternary inner test on bargraph is because it is a shared memory variable (parameter), which must be physically read each time in the loop. But that 1) creates a race because the variable can change in the execution of the loop and 2) if it doesn't change, it is ineffective because it cannot be optimized by the compiler.
2. rmu75

   added

   hal component

   bug

   on Dec 8, 2025
3. rmu75 commented on Jan 23, 2026

   

   rmu75

   on Jan 23, 2026

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   Fixing the manpage is a no-brainer. The question is: can something go wrong if we fix the bargraph mode? Or leave it like that and just document the shortcoming?
4. andypugh commented on Jan 23, 2026

   

   andypugh

   on Jan 23, 2026

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   I think it is probably safe to change it. Whilst we can't know what the component is being used for (ie, can't assume that it only drives LEDs and not guillotines, for example) allowing the (presumably unused in existing systems) top output to set is unlikely to have any affect at all.
5. hansu commented on Apr 11, 2026

   

   hansu

   on Apr 11, 2026

   Last edited by hansu

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   What I'd like to mention here is that the name doesn't really match the function.
   It is more a bit shift than a demultiplexer.
   It could be interpreted as bit demultiplexer with fixed input.
   I would like to rename this component but this will probably break a lot of configs.
   We could add an optional input pin, so that it is a real demultiplexer but stays compatible. But a bit demultiplexer makes not much sense compared to the current function. Hmm...
6. tzuohann

   added 3 commits that reference this issue on Oct 16, 2026

   demux: bargraph turns on its highest output; selection cannot wrap (L…

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   demux: bargraph turns on its highest output; selection cannot wrap (L…

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   demux: bargraph turns on its highest output; selection cannot wrap (L…

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