IEEE Circuits and Systems Magazine - Q4 2020 - 40

(L3) compressed DWT coefficients. The proposed classification algorithm is depicted in Fig. 7(c). When a new
vector ( V L3 ) becomes available, the average vector of
each cluster is loaded from the SRAM. Then, the L1 distance is computed between V L3 and the cluster average
values. If the minimal distance ( D min1 ) is greater than
the threshold T1, a new cluster is created and initialized
with V L3 . Otherwise, V L3 is associated to the closest
cluster, and its average vector is updated using a moving
average calculation. In the latter case, the L1 distance is
computed between the updated cluster average vector
and the other clusters average vectors. If the minimal

Closed-Loop
Algorithm

distance ( D min2 ) is less than the threshold T2, the clusters are merged together.
VI. CL FPGA Controller
A CL link can be established between the stimulator and
the neural recording circuit of the mixed-signal chip
through the Neural DSP chip. In this system, the FPGA
counts the number of detected APs within a sliding time
window in real time, and with respect to their respective cluster associations, established by the sorting
core of the Neural DSP chip. Each AP-to-cluster association can be set to trigger a stimulation pattern after a

Wait for Cluster and
Time Stamp (TNEW)
FIFO Empty?
Yes

Get the Cluster FIFO
Memory Pointer

Push the New Time
Stamp in the FIFO

No
Read the Time Stamp (TOLD)
at the Top of the Cluster FIFO

Trigger Stimulation

TOLD -TNEW > Sliding Time Window?
Yes

No

FIFO Size > Number of
Occurrences for That Cluster?
Yes

Enqueued the Time
Stamp in the FIFO

Dequeued the FIFO

Clear the
FIFO

No

Parameters
Sliding Window: 25 ms
Cluster 2: 3 Occurrences
Cluster 1: 3 Occurrence Cluster 3-5: Deactivated
Sliding Window

Stimulation
Trigger

3 AP From Neuron 1 Within 25 ms,
Stimulation Triggered

Cluster 1

Ex:

Cluster 2
Sliding Window

Stimulation
Trigger

3 AP From Neuron 2 Within 25 ms,
Stimulation Triggered

Sliding Window

1 AP for Neuron 1, NO
Stimulation Triggered

Figure 8. Diagram of the CL algorithm (top). This algorithm counts the number of detected AP and their cluster association within
a sliding time window. For example, the CL algorithm can trigger a feedback stimulation pattern if 3 similar AP i.e. from a same
cluster, are within a 25-ms time window (bottom).

40â

IEEE CIRCUITS AND SYSTEMS MAGAZINE

FOURTH QUARTER 2020

IEEE Circuits and Systems Magazine - Q4 2020

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