Kit Cheung1 , Simon R. Schultz2 , Wayne Luk1
Department of Computing1 , Department of Bioengineering2
Imperial College London
email: {k.cheung11, s.schultz, w.luk}@imperial.ac.uk
Spiking neural networks (SNN) aim to mimic membrane potential dynamics of biological neurons. They have been used
widely in neuromorphic applications and neuroscience modelling studies. We design a parallel SNN accelerator for producing large-scale cortical simulation targeting an off-theshelf Field-Programmable Gate Array (FPGA)-based system. The accelerator parallelizes synaptic processing with
run time proportional to the firing rate of the network. Using
only one FPGA, this accelerator is estimated to support simulation of 64K neurons 2.48 times real-time, and achieves a
spike delivery rate which is at least 1.45 times faster than a
recent GPU accelerator with a benchmark toroidal network.
Despite the vast amount of anatomical and functional
knowledge of the brain, the complete picture of how higher
cognitive function emerges from neuronal and synaptic
dynamics still eludes us. Large-scale simulation is useful
in this regard, since we can investigate how such functions
emerge from deterministic simulation.
There are a number of previous attempts to simulate
large number of neurons from the scale of 106 neurons to
109 neurons in various level of biological details [1][2].
However, the availability, cost and energy consumption
of such supercomputers can be a concern. The interest in
customized platforms for neural network simulation has
therefore been growing, resulting in platforms based on
microprocessor chips [3], GPUs [4], and silicon wafers [5].
This work, building on an earlier proof of principle [6],
involves designing a large-scale SNN accelerator targeting
FPGA platforms. One contribution of this work is a module capable of efficient parallel weight distribution. We utilize on-chip memory to store frequently accessed variables
in this module, such that most of the memory bandwidth is
used to access neuronal parameters and synaptic data.
A major challenge of devising an efficient SNN
accelerator concerns the memory storage and access
patterns. When simulating a cortical neural network, the
main memory holds primarily the synaptic data since
one neuron typically receives 1000 to 10000 synapses
from other neurons. As a result, SNN calculation is a
memory-intensive task and the efficiency of an SNN
accelerator heavily depends on the time spent on accessing
neuronal parameters and synaptic weight data. We design a
memory storage and access scheme for synaptic data using
a simple yet effective method.
A prototype node with a single FPGA is implemented to
investigate the proposed design with axonal delay support.
In short, the novelty and merits demonstrated in this work
• Accumulation of synaptic weights in parallel
• Simple and efficient memory data storage and access
• Event-driven and fully pipelined architecture
SNN is a type of neural network widely adopted in studying
neural computation using spikes, an abstracted form of action potential, as the means of communication between neurons. This work adopts the Izhikevich model [7] for modelling neurons which is computationally efficient, while sufficiently precise to produce various dynamics. In each time
step, the membrane potential variable v and recovery variable u are changed according to equation (2.1), (2.2) and
v = 0.04v 2 + 5v + 140 − u + I + sN
u = a(bv − u)
if (v > 30) then v = c, else u = u + d
In the above equations, a, b, c and d are the parameters
describing the dynamics of the neuron. I is the incoming
postsynaptic current into the neuron due to the spiking from
other neurons. N is a normally distributed random number
to mimic random fluctuation in synapse, and s represents the
magnitude of the fluctuation.
To simulate a network of Izhikevich neurons, the platform iterates in time steps, each step requires calculation of
neuron states according to equation above and accumulation
Indices of fired
3. Accumulators
in parallel
weights to
current I stored
in BRAMs
Synaptic data
2. Send address
to DRAM to
retrieve postsynaptic weights
Copy It+1 to It
after each 1ms step
Weight Distribution
Synaptic data
m synaptic data
packets per cycle
Fig.1 shows the overall architecture of the design. The two
main modules, neuron module and weight distribution module, correspond to neuron state update and weight accumulation respectively. They are connected together by the weight
distribution controller which manages synaptic weight access and scheduling of address streaming. The design is
fully pipelined hence processing time is reduced.
The neuron module is a pipelined implementation of the
equations in the Izikevich model. It takes the postsynaptic
current I accumulated from the previous time step as input
and outputs indices of the fired neurons. The equation parameters are streamed into the module every cycle from external memory, whereas fired neuron addresses stream out
from the module and are buffered in on-chip memory. Several neuron modules can be employed to parallelize neuron
state update should the updating becomes bottleneck of the
Using a look-up table, the weight distribution controller
then translates indices of fired neurons buffered in on-chip
memory into memory addresses and data size required one
at a time. The controller retrieves synaptic data whenever
the fired neuron index buffer is not empty, and is thus event-
3.1. Processing in a Simulation Step
1. Update
neuron states
in a pipeline
of synapse weight when a neuron fires. The weights are
then accumulated in a on-chip memory slot according to the
postsynaptic neuron index of the synaptic weight.
FPGA and GPUs are two popular platforms for implementation of SNN accelerators due to their inherent parallel
computation architecture. We suggest that FPGA technology is a better choice for SNN accelerator than microprocessors and GPUs. SpiNNaker, a microprocessor-based architecture, adopts a distributed approach such that a single
microprocessor is responsible for processing a number of
neurons. The distributed synaptic data means a large amount
of data has to be sent across the microprocessor network
which complicates the architecture and communication. In
contrast, a centralized approach is used in a number of GPU
[4] and FPGA [8] implementations where data are stored in
a centralized storage of main memory, reducing the communication required between processors.
Although GPUs have a high memory bandwidth to offchip memory which is an advantage in memory-intensive
SNN calculation, their on-chip memory to store frequently
accessed data is small in size. Contemporary FPGAs have
several megabytes of on-chip memory, which we have utilized to store accumulated postsynaptic current. The main
memory can then be used mainly for accessing neuronal parameters and synaptic weights. We design a module capable
of parallelizing the access, distribution and accumulation of
synaptic weight.
Synaptic Neuron
(16 bits) (12 bits)
(4 bits)
A synaptic data packet
Weight Distribution Module
Fig. 1. Overall architecture of accelerator showing the connection between the two main modules (neuron module and
weight distribution module), the weight distribution controller and off-chip DRAM (double-bordered).
driven with running time proportional to the firing rate. The
synaptic weights retrieved correspond to synapses outgoing
from the fired neurons, and for each fired neuron it takes a
number of sequential access to the external memory to fetch
all the synaptic packets. The controller then continues to the
next fired neuron when the data retrieval ends.
In the next stage, the weight distribution module then accumulates the synaptic weights in parallel. The module consists of a number of branches, each of which corresponds to
a synapse. The number of branches in the module is equal to
the number of synaptic data packets received from DRAM in
one cycle. Fig.2 shows the process of weight accumulation
with synapse axonal delay. Axonal delay is implemented by
using a looping counter which keeps track of the slot for the
current time. The counter counts to the maximum allowed
delay and then counts back to zero, and effectively utilizing
the BRAMs as circular buffers. The delay value is added
to the current time to obtain the slot the incoming synaptic
weight should store in. The previous value is read from the
BRAM, added to the new incoming weight using an adder,
and is stored back to the same address. The delay of the input to the write port is to account for the latency of the read
and add operations. At the end of a 1ms iteration, the contents of BRAMs storing It+1 is transferred to It at the start
of the pipeline.
Using the current design, the network size is constrained
by the available BRAM size on the FPGA. For a maximum
axonal delay 16 ms, the platform is able to simulate a network of size of approximately 100K neurons. For larger network to be simulated, we can employ multiple FPGAs and
Synaptic Neuron Axonal
weight Index
Synaptic packet
from DRAM
(current time)
memory address
Neuron 1
Neuron 2
current I stored
in BRAMs
Fig. 2. Architecture of a synaptic branch supporting axonal
connect the on-chip memory holding fired neuron indices to
the weight distribution controller in the neighbouring FPGA
nodes in the form of systolic array. This design requires
direct communication bus between FPGAs to ensure speed
of communication does not become the bottleneck of the design. Alternatively, it is possible to increase the network size
by swapping the buffer holding I to the external memory at
the end of each cycle, at the cost of additional communication latency between external memory and the FPGA.
3.2. Memory Layout of Synaptic Data
Synaptic data packets are arranged in consecutive RAM locations with the packet location specially arranged in prior
of simulation (Fig.3). Each packet has a total of 32 bits:
the synaptic weight occupies 16 bits, the postsynaptic neuron index uses 12 bits and the axonal delay value which is
16 ms at maximum uses the remaining 4 bits. For a row
of packets to be sent to the FPGA in each cycle from the
DRAM, the packets are arranged according to its destination
branch in prior during the accelerator configuration stage,
with packets having smaller postsynaptic neuron index on
the top. The synaptic data packets is then transferred to the
correct branch with no extra routing required when its presynaptic neuron fires. This method contributes to a considerable saving in the packet data size and resources.
The proportion of the empty area in Fig.3 causes
overhead for synaptic weight accumulation since the
empty packets are not used. The packets assignment to the
branches is almost random and the efficiency of the transfer
scheme is estimated to be 65% for a 48-branch scheme
and 76% for a 24-branch scheme. The higher efficiency
property of lower-branch designs would be utilized to boost
Neuron 3
index of fired neuron
Cycles to process
equal to the
entries in the
most occupied
target branch
Data_in Addr
Write Port
Read Port
I1t+1 I2t+1 I3t+1 …
I1t+2 I2t+2 I3t+2 …
I1t+3 I2t+3 I3t+3 …
Weight distribution controller
48 synaptic packets (1 row
of data) per cycle
Weight distribution module
Fig. 3. Memory layout for synaptic data. Regions in
black represent occupied memory space storing synaptic
data packets. The packets are arranged according to its target branch index in each row.
the performance in future work.
3.3. Platform
This work targets an off-the-shelf computing node provided
by Maxeler, with 4 Virtex6 SX475T FPGA (40 nm process)
as the main computing element. The platform has 96 GB
external DDR3 DRAM with 38.4 GB/s shared memory
bandwidth and synchronous on-chip BRAM of 4.6MB
in each FPGA. For the current implementation only one
FPGA is used, but the work can be extended to use all
the 4 FPGAs. The platform is configured as a stream
processor thus allows us to implement a fully pipelined
design. The vendor also provides high-level design tools
which translate Java description of hardware resource
and functions into low level VHDL, thus simplifying
the implementation work. The platform has designated
data paths connecting all the FPGAs in a ring to provide
high-speed inter-FPGA connections, thus suits our need to
implement a multi-FPGA network.
We build a prototype of the accelerator at 100MHz to estimate the resource utilization (Table 1) and latency. Since
the task is primarily a memory-intensive task and is bound
by memory bandwidth, there will not be a large performance
gain if higher clock rate is used once the maximum memory
bandwidth is reached.
To evaluate the performance, we use a toroidal network
[4] with various sizes (p) and various degrees of synaptic
Table 1. Resource utilization of prototype with 48 branches
simulating 64K-neuron network
Neuron Modules 1
199421 (67.0%) 205985 (69.2%)
135032 (22.6%) 143805 (24.1%)
886 (83.3%)
901 (84.7%)
Our system
This paper describes a spiking neural network accelerator
capable of supporting large-scale simulation using an
FPGA-based system. We propose a scheme to efficiently
parallelize the access and accumulation of synaptic
weights in SNN. The accelerator shows high spike delivery
throughput and outperforms a GPU accelerator.
To achieve a more biologically realistic simulation, the
current work needs to be extended to support spike-timing
dependent plasticity. Future improvements of the current
design include optimizing the memory to reduce overhead,
increasing the parallelism of the design by using more hardware resources, and enhancing the efficiency of the memory
Fig. 4. Estimation of spike delivery rate vs. network size,
using 48 branches and 2 neuron modules under localized (σ
= 32) and uniform synaptic connectivity conditions.
The research leading to these results has received funding
from European Union Seventh Framework Programme
under grant agreement number 287804, 248976 and
257906. The support by the Croucher Foundation, UK
EPSRC, HiPEAC NoE, Maxeler University Program, and
Xilinx is gratefully acknowledged.
connectivity sparseness (σ). The network has 1024 × p neurons, each neuron making 1000 synaptic connections to its
neighbouring neurons. The network fires at approximately
7Hz under all conditions. The propagation delay is proportional to the distance between the neurons in the grid, and
the maximum propagation delay is 16 ms in this example.
We estimate the spike delivery rate (Fig.4), suggested
as a measure of performance [4], using a 48-branch and
2-neuron-module design using one FPGA. The measure is
used since the firing rate and synapses vary for different
networks and this measure is relatively similar across
various conditions. Unlike related work [4], the sparseness
of synaptic connection does not affect the performance
of our architecture. The accelerator achieves throughput
of 1.30G/1.11G spikes/s for spike delivery rate, enabling
5.79 times/2.48 times speedup with respect to real time for
32K/64K neuron simulation. The drop of performance for
the 64K network is due to bottleneck at the neuron modules,
which can be solved by employing more neuron modules
running in parallel.
Our accelerator is 1.45 times (localized connectivity)
to 5.27 times (uniform connectivity) faster than the GPU
NeMo accelerator (Tesla C1060 65 nm process) in terms
of spike delivery rate. Comparing to NeMo however, our
current accelerator does not support synaptic plasticity at
the moment which requires additional hardware resources.
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