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SConstruct edited just to leave group/files definitions close to the platform/groups. Doc content files mirroed to pt, to ease the portuguese version initial migration
209 lines
13 KiB
HTML
209 lines
13 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Strict//EN">
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<html><head>
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<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
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<meta http-equiv="Content-Language" content="en-us"><title>Product</title>
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<link rel="stylesheet" type="text/css" href="../style.css"></head>
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<body style="background-color: rgb(255, 255, 255);">
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<h3>eLua Frequently Asked Questions</h3><br><p>Welcome to the official eLua FAQ!
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It is assumed that you already know <a>what eLua is</a>, so here's a list of questions you might find useful while exploring eLua.</p>
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<ul><li><p><a href="faq.html#learnlua">How can I learn Lua? Is it hard?</a><br></p></li><li><p><a href="faq.html#helpelua">How can I help eLua?</a><br></p></li><li><p><a href="faq.html#comercial">Can I use eLua in my commercial, closed source project?</a><br></p></li><li><p><a href="faq.html#fast">Is eLua fast enough?</a><br></p></li><li><p><a href="faq.html#minimuns">What are the minimum requirements for eLua?</a><br></p></li><li><p><a href="faq.html#portability">Since
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I'm using the Lua platform modules (uart, spi, pwm, tmr...), can I
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trust my peripheral code to run the same on all my platforms?</a><br></p></li><li><p><a href="faq.html#luaversions">What's the deal with floating-point Lua and integer only Lua?</a><br></p></li><li><p><a href="faq.html#windows">All your tutorials give instructions on how to compile eLua under Linux, yet you seem to use a lot of Windows tools. How come?</a><br></p></li><li><p><a href="faq.html#cygwin">Will you ever post instructions about how to compile toolchains under Cygwin in Windows?</a><br></p></li><li><p><a href="faq.html#bytecode">I
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know that Lua can be compiled to bytecode, so I compiled one of the
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eLua examples with luac and tried to run it on my eLua board, but it
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didn't work. Is this a bug in eLua?</a><br></p></li><li><p><a href="faq.html#outofmemory">I get "out of memory" errors when I run my Lua programs, what should I do?</a><br></p></li></ul>
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<p><br></p>
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<hr>
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<p><a name="learnlua"></a>
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<strong>Q: How can I learn Lua? Is it hard?</strong></p>
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<p>A: Lua is a minimalistic language (yet very powerful) which is quite
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easy to learn. Once you understand the basic concepts you'll find
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yourself writing Lua programs in notime. The main resource is the <a target="_top" href="http://www.lua.org/">Lua homepage</a>. In the <a target="_top" href="http://www.lua.org/docs.html">documentation page</a>
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you'll find the reference manual and the first version of the excellent
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"Programming in Lua" book. I recommend purchasing the second version of
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this book, since it's likely that this is all you'll ever need to learn
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Lua. Another very good resource is the <a target="_top" href="http://lua-users.org/wiki/">Lua wiki</a>. If you need more help, check the <a target="_top" href="http://www.lua.org/community.html">community page</a>. Lua has a very friendly and active community.</p><p></p><br><p><a name="helpelua"></a>
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<strong>Q: How can I help eLua?</strong></p>
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<p>A: OK, so I lied, this is NOT a frequently asked question :)
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However, if you really want to help eLua, keep in mind that we're
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looking for developers. eLua has many ambitious goals, so it would be
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great to have more people working on it. Take a look at the <a href="status.html#roadmap">roadmap page</a>, and if you see something there that you'd like to implement, don't hesitate to <a href="overview.html#contacts">contact us</a>.
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Also, if you'd like to make a donation to the project (money, or maybe
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a development board) rest assured that wwe won't say no :) It also
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helps a lot if you test eLua on your own board and you find a bug or an
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incomplete feature. Or if you just thought about a cool feature that
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you'd like to see in eLua. If so, feel free to <a href="overview.html#contacts">contact us</a>.</p><p></p>
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<p><a name="comercial"></a>
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<strong>Q: Can I use eLua in my commercial, closed source project?</strong></p>
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<p>A: ### This needs to be updated for the BSD license .........</p><p></p>
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<p><a name="fast"></a>
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<strong>Q: Is eLua fast enough?</strong></p>
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<p>A: This pretty much depends on what you expect. If you expect your
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Lua code to run as fast as your compiled C code, this won't happen,
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simply because C is a compiled language, while Lua is an interpreted
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language. That said, you'll be happy to know that Lua is one of the
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fastest interpreted languages out there. If you really need both high
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speed and Lua, you can write your speed critical code sections in C and
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export them as a Lua module. This way you get the best of both worlds.
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We don't have any official benchmarks about Lua speed on embedded
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devices, but you might want to check the TV-B-Gone example on the <a href="examples.html">examples page</a>.
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TV-B-Gone is a "software remote control" application coded directly in
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eLua. If you're familiar with the remote control protocols, you'll know
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that this kind of application is quite "real time", and delays in the
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order of milliseconds or even less can make your software remote
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control fail. Yet this sample runs without problems on a 50MHz Cortex
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(Thumb2) CPU. This should give you a fairly intuitive view on the speed
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of eLua.</p><p></p>
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<p><a name="minimuns"></a>
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<strong>Q: What are the minimum requirements for eLua?</strong></p>
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<p>A: It's hard to give a precise answer to this. As a general rule for
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a 32-bit CPU, we recommend at least 256k of Flash (program memory) and
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at least 64k of RAM. However, this isn't a strict requirement. A
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stripped down, integer-only version of eLua can definetely fit in 128k
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of Flash, and depending on your type of application, 32k of RAM might
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prove just fine. It largely depends on your needs.</p><p></p>
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<p><a name="portability"></a>
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<strong>Q: Since I'm using the Lua platform modules (uart, spi, pwm,
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tmr...), can I trust my peripheral code to run the same on all my
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platforms?</strong></p>
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<p>A: Unfortunately, no. While eLua makes it possible to have a common
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code on different platforms using the platform interface
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(docs/platform_interface.txt), it can't possibly provide the same
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functionality on all platforms, since all CPUs are not created equal.
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It is very recommended (and many times imperative) to have an
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understanding of the peripherals on your particular CPU before you
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write your code. This, of course, is not particular to eLua, but it's
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especially important since the platform interface might give the
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impression that it offers an uniform functionality over all platforms,
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when in fact the only thing in common is often just the interface
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itself (that is, the methods and variables you can access in a given
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module). eLua tries to help here by giving you an error when you try to
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access a physical resource that is not available (for example a timer,
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a PWM channel, or a PIO pin/port), but it doesn't try to cover all the
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possible platform-related issues, since this would increase the code
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size and complexity too much. These are some caveats that come to mind
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(note that these are only a few examples, the complete list is much
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longer):</p>
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<ul><li>timers: from all the platforms on which eLua runs, only
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the Luminary Cortex CPUs has rock solid 32-bit timers. You can do
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pretty much everything you need with them. All the other platforms have
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16-bit timers, which imposes some limits on the range of delays you can
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achieve with them. Make sure to use tmr.mindelay(id) and
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tmr.maxdelay(id) to check the actual resolution of your timers, and
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adapt your code accordingly. To 'compensate' for this, it's not
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possible to change the base timer frequency on the Cortex CPUs, but it
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is possible on most other platforms :) So be sure to also check the
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result of tmr.setclock(id)</li><li>also, when using timers,
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remember that if you're using XMODEM and/or the "term" module, TMR0 is
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used by both of them. So, if you change the TMR0 base clock in your
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code, be sure to restore the original setting before returning to the
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shell. You can change this static timer assignment by modifying
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src/main.c. It might also be possible to change it dynamically in the
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future, although I see little use for this.</li><li>PWM: the
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Cortex CPUs have 6 PWM channels, but channels 0/1, 2/3 and 4/5
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respectively share the same base clock setting. So, when you're
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changing the base clock for channel 1, you're also changing the base
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clock for channel 0; if channel 0 was already running, you won't like
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what will happen next. This time no eLua function can save you, you
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simply need you know your CPU architecture.</li><li>GPIO: only
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some platform have internal pullups for the GPIO pins, while Cortex is
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the only platform that also provides pulldowns for its GPIOs. However,
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in this case you're safe, as eLua will signal an error if you try to
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execute a pullup operatin on a platform that does not support it.</li></ul>
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<p>The lesson here is clear: understand your platform first!</p><p></p>
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<p><a name="luaversions"></a>
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<strong>Q: What's the deal with floating-point Lua and integer only Lua?</strong></p>
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<p>A: Lua is build around a number type. Every number in Lua will have
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this type. By default, this number type is a double. This means that
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even if your program only does integer operations, they will still be
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treated as doubles. On embedded platforms this is a problem, since the
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floating point operations are generally emulated in software, thus they
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are very slow. This is why eLua gives you "integer only Lua": a Lua
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with the default number type changed to long. The advantages are
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increased speed and smaller code size (since we can force Newlib to
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"cut" the floating point code from printf/scanf and friends, which has
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quite a strong impact on the code size) and increased speed. The
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downside is that you'll loose the ability to do any floating point
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operations.</p><p></p>
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<p><a name="windows"></a>
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<strong>Q: All your tutorials give instructions on how to compile eLua
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under Linux, yet you seem to use a lot of Windows tools. How come?</strong></p>
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<p>A: It's true that we do all the eLua development under Linux, since we
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find Linux an environment much more suited for development. At the same
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time it's true that most of the tools that come with my development
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boards run under Windows. So we choose to use the best of both world: Bogdan runs Linux under an Virtual Machine Manager (<a target="_top" href="http://www.virtualbox.org/">VirtualBox</a>) and do everything else under Windows. Dado does everything on Linux and runs Windows under <a href="http://www.vmware.com" target="_top">VMWare</a>. Both options are nice if you master your environment. To make everything even more flexible, Bogdan keeps his
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VirtualBox Ubuntu image on an external WD passport disk that he can
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carry with him wherever he goes, so he can work on eLua whenever he has a
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bit of spare time :)</p><p></p>
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<p><a name="cygwin"></a>
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<strong>Q: Will you ever post instructions about how to compile toolchains under Cygwin in Windows?</strong></p>
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<p>A: Bogdan: If I ever have way too much spare time on my hands, yes.
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Otherwise, no. There are many reasons for this. As I already mentioned,
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I favour Linux over Windows when it comes to developing applications.
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Also, I noticed that the GNU based toolchains are noticeable slower on
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Cygwin than on Linux, so experimenting with them can prove frustrating.
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Also, compiling under Linux and Cygwin should be quite similar, so try
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starting from my Linux based tutorials, they might work as well on
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Cygwin.</p><p></p>
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<p><a name="bytecode"></a>
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<strong>Q: I know that Lua can be compiled to bytecode, so I compiled
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one of the eLua examples with luac and tried to run it on my eLua
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board, but it didn't work. Is this a bug in eLua?</strong></p>
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<p>A: This is not a bug in eLua, it's a bit more subtle than that. It's
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true that ARM and i386 are very similar when it comes to data types:
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all the fundamental data types have the same length, and they are both
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little endian. So, in theory, if you compile a Lua source file on PC
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you should be able to run the compiled bytecode on your eLua board
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without any modifications. But there's a problem here: the default
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double precision floating point representation is different on ARM and
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PC. So, while the two data types have the same endianess and size, they
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are represented differently in memory. This means that you can't use
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the "regular" luac compiler for this task. However, starting with
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version 0.5, you can cross-compile Lua code on PC to run on target. </p><p></p>
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<p><a name="outofmemory"></a>
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<strong>Q: I get "out of memory" errors when I run my Lua programs, what should I do?</strong></p>
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<p>A: There are a number of things you can try to overcome this:</p>
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<ul><li>precompile your source to bytecode: If you use bytecode instead of
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source code Lua won't need to compile your source, so you save some RAM.</li><li>try
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to avoid using too many strings: strings are immutable in Lua. That
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means that a statement like s = s .. "\n" (where s is a string) will
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create a new string each time it's called. If this happens a lot (for
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example in a loop), your memory will quickly run out because of all the
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strings. If you really need to do frequent string operations, put them
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in a table and then use <a href="http://www.lua.org/manual/5.1/manual.html#5.5">table.concat</a> to make a string from your table.</li><li>controll Lua's garbage collection manually: if you're still running out of memory, try
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calling collectgarbage('collect') from your code, which will force a
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garbage collection and thus might free some memory.</li></ul><br><br></body></html> |