Field Programmable Gate Array with Integrated Optics

1
Field Programmable Gate Array with Integrated
Optics

• Jason F. Cantin and Fred R. Beyette Jr.
• Photonic Systems Development Laboratory
• University of Cincinnati
• 8/16/2000

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Light

• provides massive parallelism and transfer rates
  not possible with electricity
• can carry lots of information long distances with
  little attenuation and noise
• has interesting properties that can be exploited
  for computing purposes

3
Light is even more useful with circuits

• We can combine optics with circuits
• Enhance the performance of optics
• Improve bandwidth and communication latency
  between electronic systems
• On a single chip

Optics
Logic
Memory
Pads
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Some Applications

• Single-chip cameras
• Image processing systems
• Capture and compress images in real time
• Optical Memories
• Disk read heads, database filtering
• Retinal Prosthesis

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Problem Motivation

• Wed like to do research with novel
  optoelectronic chips
• Chips are hard to develop
• Expensive ()
• Long lead time
• CAD tools are unavailable or expensive
• Typically the experimental optics are placed on a
  separate chip

Wires
Logic
Optics
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Custom chips can be prototyped using Field
Programmable Gate Arrays

• FPGAs are chips with uncommitted logic that are
  programmable
• A string of bits is loaded onto the chip to
  define its behavior
• Once, or at power-up
• Software defined hardware
• Dense enough to implement entire digital systems
• Processors, I/O controllers, etc.

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Designing with FPGAs

• Fast and cheap because the FPGA does the job of a
  custom chip
• Designs can be specified in a high-level
  language, and a compiler used to generate
  low-level configuration data
• FPGAs are available in all shapes, sizes, and
  colors

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Simple FPGA
I/O Blocks
Routing channel (Wires that can be Connected to
blocks)
Logic Block (Function can be defined by user)
Die
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Our Idea
I/O Blocks
Routing channel
Structure same as a conventional FPGA, with
sensors and emitters inserted
Logic Block
Reconfigurable Sensor
Reconfigurable Emitter
Embedded RAM
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Lots of sensors and emitters can be fabricated
with logic on a chip

• Optical detectors and temperature sensors are
  made from reverse-biased diodes
• Optical emitters made from suspended polysilicon
  filaments (micro-heaters)
• Pressure sensors and strain-guages are leveraged
  from parallel-plate capacitors
• Measure capacitance to determine displacement

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Our first chip
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Our first chip
Logic Block
2.2 mm
Detector
Sensor Block
Wires
Clock Driver
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Crowbar

• Small 34 array of configurable elements
• Full-custom design
• 5,000 CMOS transistors
• 8 logic-blocks
• Look-up tables used for logic functions
• 4 sensor-blocks
• Each with a photodiode

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Crowbar Logic-Block Characteristics

• 32 Possible 3-input logic functions
• Inputs can be selected from the 4 nearest
  neighbors, and past outputs
• One flip-flop with reset
• Maximum Delay of 19.3nS

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Crowbar Logic-Block Structure
INPUTS (NS, NEWS, NEWS)
Configuration in
Clocks
Configure
3-input lookup table
Scan chain in
Test Mode
FF
User Reset
Scan chain out
OUTPUT
Configuration Out
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Crowbar Logic-Block Layout
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Crowbar Logic-Block Layout
Look-up table
Multiplexers
Flip-Flop
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Crowbar Sensor-Block Characteristics

• 8 Selectable thresholds for the detector
• One flip-flop with reset
• Detector and Receiver
• 1Mhz maximum frequency

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Photodiode
Photons

• PN Junction between the well and substrate
• Illumination creates electron-hole pairs in
  semiconductor
• In the depletion region, this creates a drift
  current

5 volts
I-photo
P
N
N-type well
P-type Silicon Substrate
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Optical Receiver Schematic
VDD
IN0
IN1
IN2
I_OPT
VREF
IREF
VDD
Programmable Voltage Reference
Photodiode Amplifier
Comparator
Light goes here
OUT_H
OUT_L
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Crowbar Sensor-Block Structure
Configuration in
Clocks
Configure
Config. Optical
Receiver Detector
(3 bits ? 8 possible thresholds)
Scan chain in
Test
N, E, W, S, ClkA, ClkB
FF
User Reset
Scan chain out
OUTPUT
Configuration Out
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Crowbar Sensor-Block Layout
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Crowbar Sensor-Block Layout
Receiver
Photodiode
Flip-flop
Multiplexer
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Added Bonus

• The Crowbar chip can be configured optically
• Entire pages of configuration data received at
  once
• Changing the configuration takes microseconds!

Spatial Light Modulator
Configuration Data
Crowbar chip
Board
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Project Status

• Project began 2/25/2000
• Prototype design sent to factory on 5/30/2000
• First-silicon expected this month
• Concept paper in progress

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Future Work

• CAD tool support for prototype
• Larger version with more sensors, more logic,
  emitters and embedded RAM
• Evaluating other types of sensors and emitters
  (not just optical)