From: Spehro Pefhany
Subject: Re: Driving Seven-Segment displays
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
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Date: Fri, 10 Jan 2003 17:59:40 GMT
NNTP-Posting-Date: Fri, 10 Jan 2003 12:59:40 EST
On Fri, 10 Jan 2003 09:46:17 -0800, the renowned "RP Henry"
>Scanning displays introduces additional complications.
Such is life.
>The rate must be
>fast enough to avoid flicker or descanning caused by eye movement or
>vibration. The rate must be relatively even on the average, which implies a
>high-priority timing interrupt.
True. Things like beepers and LED scanning often determine the basic
timing of programs, which does seem kind of bass-ackwards but there
you go... though 10% or less jitter probably won't be too
objectionable. It might also consume 10% of the throughput of a slow
processor. That leaves 90% for other things, so it's like having a
3.6MHz processor instead of a 4MHz one, no big deal.
> The process must avoid any lockup that will
>burn out LED segments which are dependent on low duty cycle to limit device
>heating, or must have some added external safeguard.
I've never found such things necessary. In this case (2:1 mux) you'd
have to have a VERY marginal design to begin with for double the
current to cause failure under typical conditions. If the things I
design are locking up at random, there are typically much more serious
things to worry about than the display. Good thing they don't.
>Personally, I avoid it.
Not to mention any names, but there have been some brain-dead
processors produced that lack interrupts and/or don't support
interruptable interrupts very well that might leave a bad taste like
this in your mouth. However, many modern displays such as these ones:
are internally multiplexed, so you have no choice. 12 leads does
beat 32 in my books.
"it's the network..." "The Journey is the reward"
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