133 lines
7.8 KiB
HTML
133 lines
7.8 KiB
HTML
<html>
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<head>
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<title>RingLock for AVR Microcontrollers</title>
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</head>
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<body>
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<h1>RingLock</h1>
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<h2>How it started</h2>
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After reading about <a href="http://piclock.wordpress.com/2010/10/08/picloc/">PIClock4</a>
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a inter-microcontroller mutex on <a href="http://dangerousprototypes.com/2011/09/26/piclock4-inter-microcontroller-mutex/">dangerousprototypes.com</a>,
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I got curious about the actual implementation.<br/>
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It uses shift registers to pass a token in form of just a single bit around.<br/>
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Once the token arrives at a mcu that needs access to the resource, the mcu can stop the shifting.<br/>
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After having done whatever it needs the resource for, it can start the shifting again.<br/>
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Unfortunately the schematic files were down, so I had to ask for them.<br/>
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The marker of piclock told me that his project was just for fun and never intended for actual usage.<br/>
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Because of the huge amount of logic chips needed he suggested rather using a arduino/atmega328 to emulate the hardware in software.<br/>
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That's what got me thinking initially.<br/>
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Using a 32k flash mcu to emulate 9 logic ics, sounded terribly wasteful to me.<br/>
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<i>First idea :</i> I could use a Attiny instead.<br/>
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<i>Advantages :</i> I would not waste that much flash space. <br/>
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<i>Disadvantage :</i> Attinys have only few IO-pins and therefore the amount of mcus that can be attached to the lock is limited.<br/>
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<br/>
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So I was looking for a solution that allows a huge number of mcus, while needing no/not much additional hardware.<br/>
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At some point I got reminded of <a href="http://en.wikipedia.org/wiki/Token_ring">token ring networks</a>, something I've never seen myself,<br/>
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because it's a rather old technique and at least for computer networks totally replaced by Ethernet.<br/>
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It's basic ideas is that the hosts pass a token from one to the next in a ring like topology.<br/>
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Once a host gets a token he is allowed to attach data to it while passing it on.<br/>
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This is very similar to the bit that gets shifted around in the piclock system,<br/>
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but it does not require a central piece of hardware controlling the whole process.<br/>
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That sounded just like the thing I wanted.<br/>
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<br/>
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After googling for a while I found a few implementations for token passing on microcontrollers,<br>
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but the majority of them were integrated in other projects and designed for a single usage scenario.<br/>
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At that point I started experimenting with an own implementation.<br/>
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It had easily configurable for different AVR mcus and applicable to various scenarios where multiple mcus access the same resource.<br/>
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<br/>
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<b>
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It's important to notice that I don't claim this to be a full featured library.<br/>
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Let's be honest, there not even enough lines of code to call this a program.<br/>
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I rather wanted to present the idea behind it, as well as providing a little reference implementation.<br/>
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So just think of this as a request for comments.<br/>
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</b>
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<h2>How it works</h2>
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The basic idea is to implement a decentralized <a href="http://en.wikipedia.org/wiki/Token_passing">token passing mechanism</a>,<br>
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allowing multiple mcus to share access to a resource like some kind of memory, a sensor or a communication interface of some kind.<br>
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Each mcu has one of its IO Pins connected to the external interrupt pin e.g. INT0 of the next mcu.<br>
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The last mcu in this chain has one of its IOs connected to the interrupt pin of the first one.<br>
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This results in a ring topology as shown below.<br/>
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<br/>
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<img src="ringlock_idea.png"/>
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<br/>
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When everything is powered up somebody has to have the token and the access initially.<br/>
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I called this special mcu master.<br/>
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It does whatever it needs to do with the resource and then pulls his IO-pin (called PYX in the drawing) high for 1us.<br/>
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Slave 1 will get interrupted because of the rising signal at his INT0.<br/>
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In his ISR he will check whether the rest of his program has requested access to the resource.<br/>
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This is done by simply checking a flag. If the flag is not set it will pass the token to slave 2 immediately in his ISR.<br/>
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Otherwise it will set a flag, signalling the program that it can now access the resource and leave the ISR.<br/>
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It's now up to the program to work with the shared resource for a while and to pass on the token.<br/>
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The next mcus will do exactly the same thing and the token will circulate in the ring infinitely.<br/>
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<br/>
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At least theory.<br/>
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In praxis you'll sooner or later lose the token, simply because of murphys law.<br/>
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Imagine a mcu crashes, there is a power failure or a bad connection.<br/>
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Having lost the token, the system will get stuck since no one is allowed to access the shared resource.<br/>
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Every mcu waits for the token, that will never arrive.<br/>
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That's the point where a time out might come handy.<br/>
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Using a timer of the master mcu it's easy to implement one.<br/>
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The timer gets reset every time the master gets the token back from the last mcu in the ring.<br/>
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If the token doesn't arrive in time, the master assumes that it was lost and pretends to have the token.<br/>
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The new token will be passed on and everything is hopefully working again.<br>
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Still there is one pitfall in this approach, the timeout has to be chosen carefully.<br/>
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If the master sends a new token while another is still on the ring, to mcu will try to work with the resource, <br/>
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which is in fact the situation we are trying to avoid using lock.<br/>
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The best idea is to use a timeout that is at least 1,5 times as long, as the longest time a token need back to the master.<br/>
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<br/>
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<h2>How to use it</h2>
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There are two ways of getting the source code :<br/>
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<ol>
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<li>Get the tarball <a href="#">here</a></li>
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<li>Clone the hg repository: <b>hg clone <a href="#">hg repo url here</a></b></li>
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</ol>
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<br/>
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Once you got the source, you'll want to adjust the configuration in <i>include/ringlock.h</i>.<br/>
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The config is documented there and changing it should be pretty straight forward.<br/>
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<br/>
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Using the makefile is a bit more complex then the usual <i>make all</i>.<br/>
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There are some variables that need to be set :<br/>
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<ul>
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<li><b>LOCKROLE</b> sets role of the mcu in the ring. Can be master or slave.</li>
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<li><b>AVRMCU</b> sets the mcu. Can be atmega8 or atmega32. If you want an other mcu add it in <i>include/ringlock.h</i>.</li>
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<li><b>F_CPU</b> set the frequency of the mcu in Hz.</li>
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</ul>
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So if you want to build a slave for an Atmega8 with 8mHz call <i>make ROLE=slave AVRMCU=atmega8 F_CPU=8000000 all</i>.<br/>
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Currently the source only supports Atmega32 as master or slave and Atmega8 as slave.<br/>
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This can be changed by extending the configs in <i>include/ringlock.h</i>.<br/>
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Once you have added a new mcu, it would be nice to send me patch so I can add it to the source.<br/>
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<br/>
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Using the ringlock in your source code is simple, it boils down to 4 functions :<br/>
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<ul>
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<li>rl_init() sets up timers and ports</li>
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<li>rl_request_lock() tells the ISR to keep the token</li>
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<li>rl_have_lock() retuns 1 if the mcu has the lock, else 0</li>
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<li>rl_release_lock() releases the lock again</li>
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</ul>
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<h2>Example</h2>
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Since this was a solution to which I didn't have a Problem I made one up :<br/>
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One ATmega32 (Master) and one Atmega8 (Slave) share the TXD line of a RS232 connection.<br/>
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I used the OpenBench LogicSniffer to measure the signals.<br/>
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<br/>
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<img src="ringlock_test.png">
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<br/>
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<br/>
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The and gate was in a HCF4081.<br/>
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If you set the TXD of the passive mcu to high-Z you won't need it, but I wanted the unmixed signals as well.<br/>
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The program used in this test is bundled with the sourcecode as <i>main.c</i>.<br/>
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<br/>
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Here are all the signals in the jawis OLS client:<br/>
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<a href="test.png"/><img src="test_short.png"></a>
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<br/>
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and a photo of my setup :<br/>
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<img src="photo.jpg"/><br/>
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<br/>
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<br/>
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So that's all so far.<br/>
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Have fun with this stuff and <i>please</i> don't waste an entire Atmega32 on locks.<br/>
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</body>
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</html>
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