Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!us.feeder.erje.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.datemas.de!rt.uk.eu.org!aioe.org!.POSTED!not-for-mail From: Walter Banks Newsgroups: comp.lang.forth,comp.arch.embedded Subject: Re: Parallax Propeller Date: Fri, 25 Jan 2013 11:49:53 -0500 Organization: Aioe.org NNTP Server Lines: 37 Message-ID: <5102B7B1.C1FE8CC2@bytecraft.com> References: <7xwqv9m2ub.fsf@ruckus.brouhaha.com> <9ejmf8hp8uf95n8ot7fqq3tv80tkb2cf7o@4ax.com> <20130124232504.16848.3314@localhost.localdomain> NNTP-Posting-Host: pCNhVbaXXjgaNBqLcbU4og.user.speranza.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org X-Mailer: Mozilla 4.79 [en]C-CCK-MCD NSCPCD479 (Windows NT 5.0; U) X-Accept-Language: en X-Notice: Filtered by postfilter v. 0.8.2 Xref: csiph.com comp.lang.forth:19142 comp.arch.embedded:11199 None wrote: > rickman writes: > > If you read Chuck Moore's blog he spent some time trying to implement a > > video output without a clock. He only gave up that idea when he found > > the pixels jittered on the screen because of the vagaries of async > > processor timing loops. > > I played with the PropTerm and found that the CPU-generated VGA bit stream (a > CPU got dedicated to the task) resulted in displays which always had a little > bit of fuzziness. It worked, and was quite readable, but the sharpness > of a regular PC display really made me aware of the limits of a pure software > approach to analog generation. > I have worked on a couple event driven ISA designs. Jitter is visible on displays but it is equally problematic with control systems. The best solution that I have seen/used is to have the hardware transfer out a precomputed value or latch an input on the event interrupt trigger. Output values are almost always known in advance. This minor change has essentially little impact on the processor silicon complexity. A second important performance issue is to have an easily accesses data area associated with each interrupt source. It means that a lot of common code (pwm, ac phase control... ) can be a single executable. In some cases preloading an index register with the start of data for that interrupt in some hardware has significant performance improvements. Walter Banks Byte Craft Limited