AER Building Blocks for Multi-Layer Multi-Chip Neuromorphic Vision Systems

AER Building Blocks for Multi-Layer Multi-Chip Neuromorphic Vision Systems
AER Building Blocks for Multi-Layer Multi-Chip
Neuromorphic Vision Systems
R. Serrano-Gotarredona1, M. Oster2, P. Lichtsteiner2, A. Linares-Barranco4, R. PazVicente4, F. Gómez-Rodríguez4, H. Kolle Riis3, T. Delbrück2, S. C. Liu2, S. Zahnd2,
A. M. Whatley2, R. Douglas2, P. Häfliger3, G. Jimenez-Moreno4, A. Civit4, T.
Serrano-Gotarredona1, A. Acosta-Jiménez1, B. Linares-Barranco1
1
Instituto de Microelectrónica de Sevilla (IMSE-CNM-CSIC) Sevilla Spain, 2Institute of
Neuroinformatics (INI-ETHZ) Zurich Switzerland, 3University of Oslo Norway (UIO),
4
University of Sevilla Spain (USE).
Abstract
A 5-layer neuromorphic vision processor whose components
communicate spike events asychronously using the address-eventrepresentation (AER) is demonstrated. The system includes a retina
chip, two convolution chips, a 2D winner-take-all chip, a delay line
chip, a learning classifier chip, and a set of PCBs for computer
interfacing and address space remappings. The components use a
mixture of analog and digital computation and will learn to classify
trajectories of a moving object. A complete experimental setup and
measurements results are shown.
1 Introduction
The Address-Event-Representation (AER) is an event-driven asynchronous inter-chip
communication technology for neuromorphic systems [1][2]. Senders (e.g. pixels or
neurons) asynchronously generate events that are represented on the AER bus by the
source addresses. AER systems can be easily expanded. The events can be merged with
events from other senders and broadcast to multiple receivers [3]. Arbitrary connections,
remappings and transformations can be easily performed on these digital addresses.
A potentially huge advantage of AER systems is that computation is event driven and thus
can be very fast and efficient. Here we describe a set of AER building blocks and how we
assembled them into a prototype vision system that learns to classify trajectories of a
moving object. All modules communicate asynchronously using AER. The building
blocks and demonstration system have been developed in the EU funded research project
CAVIAR (Convolution AER VIsion Architecture for Real-time). The building blocks
(Fig. 1) consist of: (1) a retina loosely modeled on the magnocellular pathway that
responds to brightness changes, (2) a convolution chip with programmable convolution
kernel of arbitrary shape and size, (3) a multi-neuron 2D competition chip, (4) a spatiotemporal pattern classification learning module, and (5) a set of FPGA-based PCBs for
address remapping and computer interfaces.
Using these AER building blocks and tools we built the demonstration vision system
shown schematically in Fig. 1, that detects a moving object and learns to classify its
Fig. 1: Demonstration AER vision system
trajectories. It has a front end retina, followed by an array of convolution chips, each
programmed to detect a specific feature with a given spatial scale. The competition or
‘object’ chip selects the most salient feature and scale. A spatio-temporal pattern
classification module categorizes trajectories of the object chip outputs.
2 Retina
Biological vision uses asynchronous events (spikes) delivered from the retina. The stream
of events encodes dynamic scene contrast. Retinas are optimized to deliver relevant
information and to discard redundancy. CAVIAR’s input is a dynamic visual scene. We
developed an AER silicon retina chip ‘TMPDIFF’ that generates events corresponding to
relative changes in image intensity [8]. These address-events are broadcast
asynchronously on a shared digital bus to the convolution chips. Static scenes produce no
output. The events generated by TMPDIFF represent relative changes in intensity that
exceed a user-defined threshold and are ON or OFF type depending on the sign of the
change since the last event. This silicon retina loosely models the magnocellular retinal
pathway.
The front-end of the pixel core (see Fig. 2a) is an active unity-gain logarithmic
photoreceptor that can be self-biased by the average photocurrent [7]. The active feedback
speeds up the response compared to a passive log photoreceptor and greatly increases
bandwidth at low illumination. The photoreceptor output is buffered to a voltage-mode
capacitive-feedback amplifier with closed-loop gain set by a well-matched capacitor ratio.
The amplifier is balanced after transmission of each event by the AER handshake. ON and
OFF events are detected by the comparators that follow. Mismatch of the event threshold
is determined by only 5 transistors and is effectively further reduced by the gain of the
amplifier. Much higher contrast resolution than in previous work [6] is obtained by using
the excellent matching between capacitors to form a self-clocked switched-capacitor
change amplifier, allowing for operation with scene contrast down to about 20%. A chip
photo is shown in Fig. 2b.
(a)
(b)
Fig. 2. Retina. a) core of pixel circuit, b) chip photograph.
(a)
(x,y)
AER
out
(c)
Row−Arbiter
y−Decoder
Line Buffer & Column Arbiter
pixel array
(d)
Pixel Array
Monostable
High
Speed
Clock
Address
Rqst
Ack
x−neighbourhood
Control
Control
Block
X-neighb.
kernel-RAM
(b)
I/O
Kernel−RAM
Fig. 3. Convolution chip (a) architecture of the convolution chip. (b) microphotograph of fabricated
chip. (c) kernel for detecting circumferences of radius close to 4 pixels and (d) close to 9 pixels.
TMPDIFF has 64x64 pixels, each with 2 outputs (ON and OFF), which are communicated
off-chip on a 16-bit AER bus. It is fabricated in a 0.35µm process. Each pixel is
40x40 µm2 and has 28 transistors and 3 capacitors. The operating range is at least 5
decades and minimum scene illumination with f/1.4 lens is less than 10 lux.
3 Convolution Chip
The convolution chip is an AER transceiver with an array of event integrators. For each
incoming event, integrators within a projection field around the addressed pixel sum a
weighted event integration. The weight of this integration is defined by the convolution
kernel . This event-driven computation splats the kernel onto the integrators [4].
Fig. 3a shows the block diagram of the convolution chip. The main parts of the chip are:
(1) An array of 32x32 pixels. Each pixel contains a binary weighted signed current source
and an integrate-and-fire signed integrator [5]. The current source is controlled by the
kernel weight read from the RAM and stored in a dynamic register. (2) A 32x32 kernel
RAM. Each kernel weight value is stored with signed 4-bit resolution. (3) A digital
controller handles all sequence of operations. (4) A monostable. For each incoming event,
it generates a pulse of fixed duration that enables the integration simultaneously in all the
pixels. (5) X-Neighborhood Block. This block performs a displacement of the kernel in
the x direction. (6) Arbitration and decoding circuitry that generate the output address
events. It uses Boahen’s burst mode fully parallel AER [2].
The chip operation sequence is as follows: (1) Each time an input address event is
received, the digital control block stores the (x,y) address and acknowledges reception of
the event. (2) The control block computes the x-displacement that has to be applied to the
kernel and the limits in the y addresses where the kernel has to be copied. (3) The
Afterwards, the control block generates signals that control on a row-by-row basis the
copy of the kernel to the corresponding rows in the pixel array. (4) Once the kernel copy is
finished, the control block activates the generation of a monostable pulse. This way, in
each pixel a current weighted by the corresponding kernel weight is integrated during a
fixed time interval. Afterwards, kernel weights in the pixels are erased. (5) When the
integrator voltage in a pixel reaches a threshold, that pixel asynchronously sends an event,
which is arbitrated and decoded in the periphery of the array. The pixel voltage is reset
upon reception of the acknowledge from the periphery. The integration is ‘perfect’ to the
limit of switch leakage; there is no leak conductance.
A prototype convolution chip has been fabricated in a CMOS 0.35µm process. Both the
size of the pixel array and the size of the kernel storage RAM are 32x32. The input address
space can be up to 128x128. In the experimental setup of Section 7, the 64x64 retina
output is fed to the convolution chip, whose pixel array addresses are centered on that of
the retina. The pixel size is 92.5µm x 95µm. The total chip area is 5.4x4.2 mm2. Fig. 3b
shows the microphotograph of the fabricated chip. AER events can be fed-in up to a peak
rate of 50 Mevent/s. Output event rate depends on kernel lines nk. The measured output
AER peak delay is (40 + 20 x nk) ns/event.
4 Competition ‘Object’ Chip
This AER transceiver chip consists of a group of VLSI integrate-and-fire neurons with
various types of synapses [9]. It reduces the dimensionality of the input space by
preserving the strongest input and suppressing all other inputs. The strongest input is
determined by configuring the architecture on the ’Object’ chip as a spiking winner-takeall network. Each convolution chip convolves the output spikes of the retina with its
preprogrammed feature kernel (in our example, this kernel consists of a ring filter of a
particular resolution). The ’Object’ chip receives the outputs of several convolution chips
and computes the winner (strongest input) in two dimensions. First, it determines the
strongest input in each feature map and in addition, it determines the strongest feature.
The computation to determine the strongest input in each feature map is carried out using
a two-dimensional winner-take-all circuit as shown in Fig. 4. The network is configured so
that it implements a hard winner-take-all, that is, only one neuron is active at a time. The
activity of the winner is proportional to the winner’s input activity.
The winner-take-all circuit can reliably select the winner given a difference of input firing
rate of only 10% assuming that it receives input spike trains having a regular firing rate
[10]. Each excitatory input spike charges the membrane of the post-synaptic neuron until
one neuron in the array--the winner--reaches threshold and is reset. All other neurons are
then inhibited via a global inhibitory neuron which is driven by all the excitatory neurons.
Self-excitation provides hysteresis for the winning neuron by facilitating the selection of
this neuron as the next winner.
Because of the moving stimulus, the network has to determine the winner using an
estimate of the instantaneous input firing rates. The number of spikes that the neuron must
integrate before eliciting an output spike can be adjusted by varying the efficacies of the
input synapses.
To determine the winning feature, we use the activity of the global inhibitory neuron
(which reflects the activity of the strongest input within a feature map) of each feature
map in a second layer of competition. By adding a second global inhibitory neuron to each
feature map and by driving this neuron through the outputs of the first global inhibitory
neurons of all feature maps, only the strongest feature map will survive. The output of the
object chip will be events encoding the spatial location of the stimulus and the identity of
the winning feature. (In the characterization shown in Section 7, the global competition
was disabled, so both objects could be simutaneously localized by the object chip.)
We integrated the winner-take-all circuits for four feature maps on a single chip with a
total of 16x16 neurons; each feature uses an 8x8 array. The chip was fabricated in a
0.35 µm CMOS process with an area of 8.5 mm2.
5 Learning Spatio-Temporal Pattern Classification
The last step of data reduction in the CAVIAR demonstrator is a subsystem that learns to
classify the spatio-temporal patterns provided by the object chip. It consists of three
Fig. 4: Architecture of ’Object’ chip configured
for competition within two feature maps and
competition across the two feature maps.
Fig. 5: System setup for learning direction of
motion
components: a delay line chip, a competitive Hebbian learning chip [11], and an AER
mapper that connects the two. The task of the delay line chip is to project the temporal
dimension into a spatial dimension. The competitive Hebbian learning chip will then learn
to classify the resulting patterns. The delay line chip consists of one cascade of 880 delay
elements. 16 monostables in series form one delay element. The output of every delay
element produces an output address event. A pulse can be inserted at every delay-element
by an input address event. The cascade can be programmed to be interrupted or connected
between any two subsequent delay-elements. The associative Hebbian learning chip
consists of 32 neurons with 64 learning synapses each. Each synapse includes learning
circuitry with a weak multi-level memory cell for spike-based learning [11].
A simple example of how this system may be configured is depicted in Fig. 5: the mapper
between the object chip and the delay line chip is programmed to project all activity from
the left half of the field of vision onto the input of one delay line, and from the right half of
vision onto another. The mapper between the delay line chip and the competitive Hebbian
learning chip taps these two delay lines at three different delays and maps these 6 outputs
onto 6 synapses of each of the 32 neurons in the competitive Hebbian learning chip. This
configuration lets the system learn the direction of motion.
(a)
(b)
(c)
(d)
Fig. 6: Developed AER interfacing PCBs. (a) PCI-AER, (b) USB-AER, (c) AER-switch, (d) mini-USB
2
3
AER
USB
AER
mapper
motion
retina
(a)
4
AER
Splitter
7 PCI−AER
USB
AER
monitor
Monitor
5
8
AER
Convolution
chip
10
11
12
13
14
15
16
AER
USB
AER
mapper
AER
object
USB
AER
monitor
USB
AER
mapper
AER
delay
line
chip
USB
AER
mapper
Merger
USB
AER
monitor
AER
6 Convolution
chip
17
AER
learning
chip
9
chip
7
(b)
1
5
11
17
16
14
13
15
10 8
9
12
3
4
2
6
Fig. 7: Experimental setup of multi-layered AER vision system for ball tracking (white boxes include
custom designed chips, blue boxes are interfacing PCBs). (a) block diagram, (b) photograph of setup.
6 Computer Interfaces
When developing and tuning complex hierarchical multi-chips AER systems it is crucial
to have available proper computer interfaces for (a) reading AER traffic and visualizing it,
and (b) for injecting synthesized or recorded AER traffic into AER buses. We developed
several solutions. Fig. 6(a) shows a PCI-AER interfacing PCB capable of transmitting
AER streams from within the computer or, vice versa, capturing them from an AER bus
and into computer memory. It uses a Spartan-II FPGA, and can achieve a peak rate of
15 Mevent/s using PCI mastering. Fig. 5(b) shows a USB-AER board that does not require
a PCI slot and can be controlled through a USB port. It uses a Spartan II 200 FPGA with a
Silicon Labs C8051F320 microcontroller. Depending on the FPGA firmware, it can be
used to perform five different functions: (a) transform sequence of frames into AER in real
time [13], (b) histogram AER events into sequences of frames in real time, (c) do
remappings of addresses based on look-up-tables, (d) capture timestamped events for offline analysis, (e) reproduce time-stamped sequences of events in real time. This board can
also work without a USB connection (stand-alone mode) by loading the firmware through
MMC/SD cards, used in commercial digital cameras. This PCB can handle AER traffic of
up to 25 Mevent/s. It also includes a VGA output for visualizing histogrammed frames.
The third PCB, based on a simple CPLD, is shown in Fig. 6(c). It splits one AER bus into
2, 3 or 4 buses, and vice versa, merges 2, 3 or 4 buses into a single bus, with proper
handling of handshaking signals. The last board in Fig. 6(d) is a lower performance but
more compact single-chip bus-powered USB interface based on a C8051F320
microcontroller. It captures timestamped events to a computer at rates of up to 100 kevent/
s and is particularly useful for demonstrations and field capture of retina output.
7 Demonstration Vision System
To test CAVIAR’s capabilities, we built a demonstration system that could simultaneously
track two objects of different size. A block diagram of the complete system is shown in
Fig. 7(a), and a photograph of the complete experimental setup is given in Fig. 7(b). The
(a)
(b)
(b)
(c)
Fig. 8: Captured AER outputs
at different stages of processing
chain. (a) at the retina output,
(b) at the output of the 2
convolution chips, (c) at the
output of the object chip. ‘I’
labels the activity of the
inhibitory neurons.
complete chain consisted of 17 pieces (chips and PCBs), all numbered in Fig. 7: (1) The
rotating wheel stimulus. (2) The retina. The retina looked at a rotating disc with two solid
circles on it of two different radii. (3) A USB-AER board as mapper to reassign addresses
and eliminate the polarity of brightness change. (4) A 1-to-3 splitter (one output for the
PCI-AER board (7) to visualize the retina output, as shown in Fig. 8(a), and two outputs
for two convolution chips). (5-6) Two convolution chips programmed with the kernels in
Fig. 3c-d, to detect circumferences of radius 4 pixels and 9 pixels, respectively. They see
the complete 64x64 retina image (with rectified activity; polarity is ignored) but provide a
32x32 output for only the central part of the retina image. This eliminates convolution
edge effects. The output of each convolution chip is fed to a USB-AER board working as
a monitor (8-9) to visualize their outputs (Fig. 8b). The left half is for the 4-radius kernel
and the right half for the 9-radius kernel. The outputs of the convolution chips provide the
center of the circumferences only if they have radius close to 4 pixels or 9 pixels,
respectively. As can be seen, each convolution chip detects correctly the center of its
corresponding circumference, but not the other. Both chips are tuned for the same feature
but with different spatial scale. Both convolution chips outputs are merged onto a single
AER bus using a merger (10) and then fed to a mapper (11) to properly reassign the
address and bit signs for the winner-take-all ‘object’ chip (12), which correctly decides the
centers of the convolution chip outputs. The object chip output is fed to a monitor (13) for
visualization purposes. This output is shown in Fig. 8(c). The output of this chip is
transformed using a mapper (14) and fed to the delay line chip (15), the outputs of which
are fed through a mapper (16) to the learning (17) chip. The system as characterized can
simultaneously track two objects of different shape; we have connected but not yet studied
trajectory learning and classification.
8 Conclusions
In terms of the number of independent components, CAVIAR demonstrates the largest
AER system yet assembled. It consists of 5 custom neuromorphic AER chips and at least 6
custom AER digital boards. Its functioning shows that AER can be used for assembling
complex real time sensory processing systems and that relevant information about object
size and location can be extracted and restored through a chain of feedforward stages. The
CAVIAR system is a useful environment to develop reusable AER infrastructure and is
capable of fast visual computation that is not limited by normal imager frame rate. Its
continued development will result in insights about spike coding and representation.
Acknowledgements
This work was sponsored by EU grant IST-2001-34124 (CAVIAR), and Spanish grant
TIC-2003-08164-C03 (SAMANTA). We thank K. Boahen for sharing AER interface
technology and the EU project ALAVLSI for sharing chip development and other AER
computer interfaces [14].
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