PMC:4313588 / 47462-50386
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{"target":"http://pubannotation.org/docs/sourcedb/PMC/sourceid/4313588","sourcedb":"PMC","sourceid":"4313588","source_url":"https://www.ncbi.nlm.nih.gov/pmc/4313588","text":"4.2. Switched-capacitor neuromorphics\nDating back to Carver Mead, subthreshold CMOS has been the mainstay of neuromorphic circuit design, as it offers the advantage of low power consumption, ion-channel like behavior in CMOS devices and currents small enough to reach biological real time operation. However, such a fully analog implementation suffers from mismatch and leakage currents which are increasingly prevalent in deep submicron processes. In addition, the channel-to-transistor design philosophy means that this type of neuromorphic circuit consists largely of handcrafted circuits that depend crucially on the performance of each single transistor. Thus, porting a design between technology nodes essentially means a completely new design.\nSwitched-capacitor neuromorphic circuits move from this device level philosophy to a building block approach, i.e., the required model behavior is achieved with a combination of standard building blocks. SC is used as a mathematical framework to directly translate state-driven models to a mixed-signal realization. This keeps the neuronal states analog for biological veracity, while achieving significantly easier technology porting, as the circuit consists solely of standard building blocks such as amplifiers, switches and charge addition/subtraction. Representation of analog states at block level also eases implementation in deep submicron, as this takes advantage of the available device count for improved signal fidelity, while relying less on the characteristics of individual transistors. This building block approach allows agressive scaling of the active analog components, while the digital part of the SC circuits naturally scales with the technology node. Overall scaling is ultimately limited compared to a purely digital system by the largely invariant capacitor sizes, but is still significantly better than conventional, more device- and analog-centric neuromorphic approaches. As shown, this approach has enabled our SC system to deliver the same computational density as a purely digital neuromorphic system in a deep-submicron technology (Seo et al., 2011), while its power budget is on par with subthreshold circuits (Indiveri et al., 2006). When combined with deep submicron pixel cells (Henker et al., 2007), a sophisticated visual processing pyramid could be implemented (König et al., 2002; Serrano-Gotarredona et al., 2009).\nWhile SC makes neuromorphic circuits possible in principle in deep submicron, one major challenge is still the leakage currents. The leakage completely precludes subthreshold circuits, but it also affects the stored states of capacitors in SC technique, especially for the timescales necessary for biological real time operation. As shown, we have solved this general challenge for SC neuromorphic circuits with our low leakage switch architecture, reaching controllable time constants \u003e100 ms at ambient temperature.","divisions":[{"label":"title","span":{"begin":0,"end":37}},{"label":"p","span":{"begin":38,"end":750}},{"label":"p","span":{"begin":751,"end":2406}}],"tracks":[{"project":"0_colil","denotations":[{"id":"25698914-16526488-299626","span":{"begin":2212,"end":2216},"obj":"16526488"},{"id":"25698914-19635693-299627","span":{"begin":2400,"end":2404},"obj":"19635693"}],"attributes":[{"subj":"25698914-16526488-299626","pred":"source","obj":"0_colil"},{"subj":"25698914-19635693-299627","pred":"source","obj":"0_colil"}]},{"project":"2_test","denotations":[{"id":"25698914-16526488-38235678","span":{"begin":2212,"end":2216},"obj":"16526488"},{"id":"25698914-19635693-38235679","span":{"begin":2400,"end":2404},"obj":"19635693"}],"attributes":[{"subj":"25698914-16526488-38235678","pred":"source","obj":"2_test"},{"subj":"25698914-19635693-38235679","pred":"source","obj":"2_test"}]}],"config":{"attribute types":[{"pred":"source","value type":"selection","values":[{"id":"0_colil","color":"#93ecb8","default":true},{"id":"2_test","color":"#ec9e93"}]}]}}