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Article #85Re: Best way to power array of LEDs?
From: Spehro Pefhany
Date: Fri, 06 Dec 2002 10:58
Date: Fri, 06 Dec 2002 10:58
46 lines
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3773 bytes
On Fri, 06 Dec 2002 08:41:57 +0000 (GMT), the renowned Tony Williams <tonyw@ledelec.demon.co.uk> wrote: > > It's that warning in every PIC data sheet Speff. > An output pin that is driven to 0/1 must be allowed > to reach the correct 0/1 voltage. If not, undefined > things happen during subsequent writes to that o/p. The RMW issue can be gotten around with software- avoid BCF/BSF to the port under conditions like that, since read-modify-write instructions use the values read from the port pins (NOT the latch). If you write the entire latch, the issue doesn't arise (the values may in fact be read, but they are discarded). Sometimes it's convenient to maintain a shadow register, update that, then slap it out to the real port latch. By the way, the 18F series have an alternate PORTx address called LATx that uses latch values rather than pin levels. > In the original software, the sequence of "set port- > pins to logic 0, then set port-pins to outputs" > resulted in the pins being set to logic 1, the LED > lit up like a searchlight, with 130mA drawn off the 5v. Hmm.. so the voltage must have been high enough at the inputs to read as a "1", since most of the PIC inputs are TTL levels, it doesn't take much. I'll bet you wished you put a little transistor in there. ;-) Must have been written as a series of BSFs. > Fortunately that 20mA could be gated-Off for a few uS. > If that was done, then the original sequence worked > correctly. Hardware solutions work too. ;-) Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.co
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X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 112d1e,4ef4c9d4aea85e1f X-Google-Attributes: gid112d1e,public X-Google-Thread: 103407,4ef4c9d4aea85e1f X-Google-Attributes: gid103407,public X-Google-Thread: 1118be,4ef4c9d4aea85e1f X-Google-Attributes: gid1118be,public X-Google-Thread: 10d137,4ef4c9d4aea85e1f X-Google-Attributes: gid10d137,public X-Google-Thread: 1131b0,4ef4c9d4aea85e1f X-Google-Attributes: gid1131b0,public X-Google-ArrivalTime: 2002-12-06 02:58:37 PST Path: arc1-ca.pugleaf.net!archiver1.google.com!news1.google.com!newsfeed.stanford.edu!bloom-beacon.mit.edu!nycmny1-snh1.gtei.net!news.gtei.net!news-out.visi.com!hermes.visi.com!newsfeeds-atl2!news.webusenet.com!news03.bloor.is.net.cable.rogers.com!news04.bloor.is.net.cable.rogers.com.POSTED!not-for-mail From: Spehro Pefhany <speff@interlog.com> Newsgroups: sci.electronics.components,sci.electronics.design,alt.engineering.electrical,aus.electronics,sci.electronics.repair Subject: Re: Best way to power array of LEDs? Organization: Rather Message-ID: <7nu0vukai3nr6l25rh99vjrlhgegbc519a@4ax.com> References: <KVdHjAAjM3w9Ew8l@jmwa.demon.co.uk> <aq17gb$7ut$1@reader1.panix.com> <slrnasg1o7.1t5.Znospam+noZs_0211@hpb13799Z.Zboi.hpZ.com.invalid> <aseln1$jbb$1@reader1.panix.com> <Rm2H9.4457$5P3.213056@newsfeed.slurp.net> <4dqpuugil5jens3iu7uprml0hdjl5e5dqs@4ax.com> <4ba01ef90etonyw@ledelec.demon.co.uk> <c9ruuu87bn6t539n1se6ejueb1u1lia0j6@4ax.com> <4ba0275featonyw@ledelec.demon.co.uk> <h48vuuordbt547u37tb38vpfivfqm2t0at@4ax.com> <4ba080045etonyw@ledelec.demon.co.uk> X-Newsreader: Forte Agent 1.92/32.572 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 46 Date: Fri, 06 Dec 2002 10:58:35 GMT NNTP-Posting-Host: 24.112.151.88 X-Complaints-To: abuse@rogers.com X-Trace: news04.bloor.is.net.cable.rogers.com 1039172315 24.112.151.88 (Fri, 06 Dec 2002 05:58:35 EST) NNTP-Posting-Date: Fri, 06 Dec 2002 05:58:35 EST