Design Article
PCM Progress Report No 4: Brains
Ron Neale
7/22/2011 2:32 PM EDT
Resistance discrimination of 100 levels
Realistically emulating the characteristics in Figure 2 requires accuracy of 1%, so this is the level of performance demanded of a PCM. This appears to have been achieved by the team at Stanford in a repeatable manner. Figure 3 illustrates how the PCM synapse emulation fits into the more familiar PCM characteristics. The left side of Figure 3 shows the conventional two-state resistance range of the PCM, in this case 500 Ohms to 2 M-Ohms.
To add to the challenge, the required 1% or 100 level discrimination of resistance uses only a fraction of the possible resistance range of the PCM. The two bar graphs alongside the normal PCM characteristics in Figure 3 indicate the resistance values used for the demonstration of the 100 levels of resistance and for the synapse emulation experiments. Ignoring stability, it has always been possible to obtain any value of resistance for a PCM by using an incremental write-read verify-write technique. Outside of this work Ferdinando Bedeschi and team [2] attempt to make multi-bit, 2-bit PCM devices, appear to have fallen by the wayside in terms of the stability required for a commercial PCM array. Although more recently IBM [3 EEtimes] has taken another look at the problems of instability in multi-level 2-bit PCM cell. Compared with those efforts, each PCM in the emulation work of the Stanford group is in effect operating in a repeatable manner as a multi-level > 6 bit memory device albeit, this is under less rigorous conditions than would be required for a commercial device.
The method used by Stanford's team to obtain the 100 levels is shown in Figure 3. As shown, to reset the PCM device a sequence of 100 pulses progressively increasing in value is used, while to set the device a 100 step staircase waveform is used. In the latter case, each individual step of the set pulse consists of 20 pulses. The initial set pulse will consist of 20 pulses of the lowest voltage; the next set pulse will consist of 19 pulses of the lowest voltage and one from the next highest step; then 18 and 2; 17 and 3; and so on for the required 100 pulses. In Figure 3, the two curves of resistance were obtained as the result of successive pulses without reset. The resistance value is, in effect, the integration of the total number of set pulses. The resistance characteristics were proven to be repeatable over many cycles.
Next: Synapse Emulation
Realistically emulating the characteristics in Figure 2 requires accuracy of 1%, so this is the level of performance demanded of a PCM. This appears to have been achieved by the team at Stanford in a repeatable manner. Figure 3 illustrates how the PCM synapse emulation fits into the more familiar PCM characteristics. The left side of Figure 3 shows the conventional two-state resistance range of the PCM, in this case 500 Ohms to 2 M-Ohms.
To add to the challenge, the required 1% or 100 level discrimination of resistance uses only a fraction of the possible resistance range of the PCM. The two bar graphs alongside the normal PCM characteristics in Figure 3 indicate the resistance values used for the demonstration of the 100 levels of resistance and for the synapse emulation experiments. Ignoring stability, it has always been possible to obtain any value of resistance for a PCM by using an incremental write-read verify-write technique. Outside of this work Ferdinando Bedeschi and team [2] attempt to make multi-bit, 2-bit PCM devices, appear to have fallen by the wayside in terms of the stability required for a commercial PCM array. Although more recently IBM [3 EEtimes] has taken another look at the problems of instability in multi-level 2-bit PCM cell. Compared with those efforts, each PCM in the emulation work of the Stanford group is in effect operating in a repeatable manner as a multi-level > 6 bit memory device albeit, this is under less rigorous conditions than would be required for a commercial device.
The method used by Stanford's team to obtain the 100 levels is shown in Figure 3. As shown, to reset the PCM device a sequence of 100 pulses progressively increasing in value is used, while to set the device a 100 step staircase waveform is used. In the latter case, each individual step of the set pulse consists of 20 pulses. The initial set pulse will consist of 20 pulses of the lowest voltage; the next set pulse will consist of 19 pulses of the lowest voltage and one from the next highest step; then 18 and 2; 17 and 3; and so on for the required 100 pulses. In Figure 3, the two curves of resistance were obtained as the result of successive pulses without reset. The resistance value is, in effect, the integration of the total number of set pulses. The resistance characteristics were proven to be repeatable over many cycles.
Next: Synapse Emulation
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janine.love
7/22/2011 2:53 PM EDT
I asked Ron Neale to take a look at some of the current work being done in PCM and he took the time to analyze some work being done by Stanford University. Look for more on PCM, with a new progress report coming in a week or two.
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iniewski
8/2/2011 6:35 PM EDT
Could you let us know where the report is published/posted? Kris
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resistion
7/22/2011 9:10 PM EDT
I think the desired outcome will be foiled by resistance drift. I also wonder if something similar could be accomplished by Flash? If not, the reason(s) would also be important.
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R G.Neale
7/23/2011 6:38 AM EDT
Resistion-You could be right, I guess it's up to someone to prove the point experimentally, arm waving solutions and claims of brain functions or any other aspect of PCM developments are no longer acceptable.
The 100 level resolution (1%)for Flash might result in some drift. In a modern scaled Flash how many electrons does that involve for each step? It is even possible if all devices in an emulated synapse network drift together the relative learned experience of the neural network is the same, others will have to answer that question. Drift tolerance is the feature that IBM use in their MLC-PCM. Both IBM-MLC and PCM-Onyx were originally part of this Progress Report I understand that material will now appear in the next PCM Progress Report, #5 in the near future.
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resistion
7/24/2011 12:02 AM EDT
The drift can be compensated with a reference, but each new data input needs a new reference, so things (costs, power, delays) add up that way.
100 levels in flash I think is not doable, even 8 levels or 3 bits is borderline. Too few random electrons to tell the difference.
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janine.love
7/23/2011 4:44 PM EDT
I have hidden some of the comments on this article because of their unproductive nature. My thanks to those readers who pointed them out. I understand and am glad that we are passionate about our work, but let's keep it professional. Thanks.
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Volatile Memory
7/25/2011 10:58 AM EDT
Dear RF/Memory Editor:
Yes, let's keep it professional! When pseudo-research is touted as some kind of breakthrough in a respected publication, the duty of the editor is to notice, not to silence the whistleblower. The fact is, Mr. Neale dropped the ball on this one. He knew or should have known that Mr. Ovshinsky has claimed similar "results" for at least 25 years. Those claims and results turned out to be fraudulent. As will the latest "results" from the "researchers" at Stanford University.
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janine.love
7/25/2011 1:18 PM EDT
Debate and conversation is welcome. In fact, it is the intent of the comments section. As long as you refrain from name calling, I'm happy to let the comments stand and the debate ensue.
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Solster
7/25/2011 5:55 PM EDT
The fact remains that this is an academic paper peer-reviewed by credentialed research scientists and a professor. Constructive dialogue is the cornerstone of academic research, while "commentators" to web-articles expressing opinions with no apparent personal credibility whatsoever, don't contribute much to the debate, really. There's hopefully a reason why R.G. Neale (and not a certain "commentator") was invited to write this review article and any real constructive debate on this paper could really just be a rebuttal paper in Nano Letters. Anything less, especially those without any technical discussions and instead full of dubious accusations, is worth little more than idle chit-chat for mere entertainment.
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Volatile Memory
7/25/2011 9:05 PM EDT
Solster: Care to explain how exactly Mr. Ovshinsky managed to publish a fraudulent paper, describing the 16-level magic neuron device, on the pages of the peer-reviewed Japanese Journal of Applied Physics in 2004:
http://jjap.jsap.jp/link?JJAP/43/4695/
I wonder what Mr. Neale has to say about it.
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rbtbob
7/25/2011 12:44 AM EDT
Some of the readers that are not current on the research being done on phase change materials and devices in the last few years might like to read some of the papers presented at the European Phase Change and Ovonics Science Symposium. Regarding the subject of Mr. Neale's analysis, I recommend the paper presented by Stan Ovshinsky at the 2004 Symposium.
http://www.epcos.org/library/library2010.htm
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Volatile Memory
7/25/2011 11:10 AM EDT
Here is the 10x Microsofts quote, accompanied with the "results:"
http://goo.gl/WCxQm
The document was created in December of 2004 (and published in early 2005) when Mr. Ovshinsky was still at the helm of Ovonic Cognitive Computer and its parent.
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R G.Neale
7/25/2011 5:23 AM EDT
rbtbob-I was careful to put the precedence claim in quotes in case I had missed a paper. My brief was to explore what the Stanford team had been able to get the PCM to do based on their real experimental data, not to research the whole field of bio-science for claims and counter claims. The word "promising" in the title of the paper you recommend gives cause for concern. I think to date the whole field of phase change memory has been beset and damaged by too many unfulfilled promises.
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R G.Neale
7/26/2011 10:57 AM EDT
Solster: Outside of the subject of this paper, and your comment""The fact remains that this is an academic paper peer-reviewed by credentialed research scientists and a professor. Constructive dialogue is the cornerstone of academic research, while "commentators" to web-articles expressing opinions with no apparent personal credibility whatsoever, don't contribute much to the debate, really.""
I think as the web takes over paper publishing peer group review will change and become web based with an opportunity for peer group review to come quickly from all quarters, including your professors etc. With the editors responsible for the removal of any offensive material. I am convinced that is the future of peer group review.
Also would you care to explain your words regarding myself and my reputation in parenthesis and quotes.
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R G.Neale
7/26/2011 12:01 PM EDT
Solster- On the subject of peer group review there is an interesting article "Putting Peer Review on Trial", by Raoul Franklin,in PhysicsWorld, December 2010,p17.Published by the Institute of Physics (IoP)
Key quote "The system we use to judge our peers work must be made more transparent" He points out a number of problems and defects with the present method and offers some possible changes.
If you read it it may shake your faith a little in the present system.I will try and assess if it is on the web.
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