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Re: Poor man's PWM

From Don Y <blockedofcourse@foo.invalid>
Newsgroups comp.arch.embedded
Subject Re: Poor man's PWM
Date 2020-01-28 19:58 -0700
Organization A noiseless patient Spider
Message-ID <r0qsdr$u2m$1@dont-email.me> (permalink)
References <r0o0mu$c6l$1@dont-email.me> <r0pkj3$r85$1@dont-email.me>

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Hi Dimiter,

On 1/28/2020 8:39 AM, Dimiter_Popoff wrote:
> whatever PW you do beware the randomness, even small,
> from period to period. I did something like that once many years ago
> (just wanted to have 3 colours out of a 2 colour LED) and the mixed
> colour was quite visibly flickering. I don't remember how much
> randomness there was and how it might compare to yours but using
> IRQ and bit banging it does invite some, I'd test that before
> rolling it out. I think mine used IRQ just to switch tasks and
> measure time on a HC11 back then (some 20 years ago) but I don't
> think I pushed much to get rid of the flicker, it was not a problem.

How were you driving it?  I.e., was it a two-lead (bi-color)
device?  Or, THREE?  In the former case, I could see how
the duty cycle in one direction (terminal 1 to terminal 2)
differing from that of the other direction (terminal 2 to
terminal 1) would alter the relative mix of the two colors
and, thus, the resulting color.  In the latter case,
perhaps driving one (or both) anodes/cathodes with a
particular duty cycle to "tune" the color?

The obvious fix is to double-buffer the drive to the device so
that the same signal that prompts the ISR to change the drive
*also* locks in the previous decision to the actual indicator.
This makes the entire IRQ period available for servicing the
ISR without having visual consequences -- for the cost of (an)
external latch(es).  Latency issues go away (largely).

[But, this either requires a "terminal count output" from an
internal timer/counter -or- an external counter/timer driving
an IRQ input (that can also trigger the latches).]

A related problem is how "nearby" indicators are perceived.
E.g., will two different indicators be seen as having differing
intensities even though *intended* to have the same intensity?
I.e., is the visual output of a device "matched" to others
ACROSS THE FULL RANGE OF DRIVE CONDITIONS?  Or, just at
a particular test criteria?

And, how far apart do such devices need to be in order for the user
NOT to be able to perceive (minor) differences?  Two die in the same
package (your case!) is obviously one extreme!

I'm not keen on having to purchase "matched" devices or any
"select at test".  So, would rather just AVOID issues that
could lead to visual aberrations, etc.

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Thread

Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-27 17:53 -0700
  Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-27 19:29 -0800
    Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-28 20:11 -0700
      Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-28 20:11 -0800
  Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-28 11:29 +0100
    Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-28 20:34 -0700
      Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-28 20:32 -0800
      Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-29 12:53 +0100
  Re: Poor man's PWM Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-28 14:43 +0100
    Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-28 12:03 -0800
    Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-28 20:54 -0700
      Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-28 21:54 -0800
        Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-29 00:17 -0700
          Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-28 23:40 -0800
            Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-29 00:54 -0700
            Re: Poor man's PWM Paul Rubin <no.email@nospam.invalid> - 2020-01-29 00:04 -0800
              Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-29 02:17 -0700
        Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-29 13:21 +0100
          Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-29 09:51 -0800
            Re: Poor man's PWM Paul Rubin <no.email@nospam.invalid> - 2020-01-29 10:07 -0800
              Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-29 10:14 -0800
                Re: Poor man's PWM Paul Rubin <no.email@nospam.invalid> - 2020-01-29 10:28 -0800
                Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-29 11:40 -0800
                Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-30 10:25 +0100
                Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-30 08:06 -0800
                Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-30 21:21 +0100
                Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-30 15:08 -0800
                Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-31 01:17 +0100
                Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-01-30 17:21 -0800
                Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-31 10:59 +0100
                Re: Poor man's PWM Rick C <gnuarm.deletethisbit@gmail.com> - 2020-02-01 10:18 -0800
            Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-30 10:20 +0100
              Re: Poor man's PWM Tom Gardner <spamjunk@blueyonder.co.uk> - 2020-01-30 10:13 +0000
          Re: Poor man's PWM Paul Rubin <no.email@nospam.invalid> - 2020-01-29 09:57 -0800
            Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-30 10:29 +0100
      Re: Poor man's PWM Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2020-01-29 16:34 +0100
  Re: Poor man's PWM Dimiter_Popoff <dp@tgi-sci.com> - 2020-01-28 17:39 +0200
    Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-28 19:58 -0700
      Re: Poor man's PWM Dimiter_Popoff <dp@tgi-sci.com> - 2020-01-29 18:54 +0200
  Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-01-29 00:17 -0700
    Re: Poor man's PWM David Brown <david.brown@hesbynett.no> - 2020-01-29 13:39 +0100
      Re: Poor man's PWM Paul Rubin <no.email@nospam.invalid> - 2020-01-29 10:08 -0800
  Re: Poor man's PWM Don Y <blockedofcourse@foo.invalid> - 2020-04-27 20:13 -0700

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