In-cell Differential Read-out Circuitry for Signed Weight Values
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Solution Overview
Problem
Resistive processing unit (RPU) systems struggle to read and process signed weight values, as existing analog weight storage elements only store values without signs, requiring additional processing to determine positive, negative, or zero weight values, which hampers efficient deep neural network (DNN) training.
Innovation Solution
In-cell differential read-out circuits and methods are implemented to directly read signed weight values from RPU cells, using configurations such as reference current sources, complementary current integration circuits, and current mirror circuits to generate read currents that represent the magnitude and sign of weight values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If conventional weight storage elements are used, then weight values can be stored, but signed weight values (positive, negative, zero) cannot be directly read out
Solution Approach 1:
The weight value representation is segmented into two independent components: magnitude stored in the analog weight storage element and sign stored in a separate digital bit in the control circuitry. This segmentation allows the magnitude and sign to be handled independently, enabling direct reading of signed weight values without requiring complex additional circuitry to encode or decode sign information from the analog element alone.
Solution Approach 2:
A control circuitry intermediary is introduced that receives the analog weight value from the storage element and the digital sign bit, then combines them to generate the final signed weight output. This intermediary circuitry simplifies the overall system by centralizing the sign-magnitude combination logic in a dedicated control unit rather than distributing complexity throughout the read-out path.
2Loss of information
If additional processing is added to determine weight signs, then signed weight values can be read, but processing time and power consumption increase
Solution Approach 1:
The sign information is prepared in advance and stored in the control circuitry alongside the magnitude information from the weight storage element. By having the sign bit readily available before the read operation completes, the system eliminates the need for time-consuming post-read processing to determine weight signs, as both magnitude and sign are simultaneously ready for output.
Solution Approach 2:
The weight storage system serves its own sign information requirement through the integrated control circuitry that maintains the sign bit locally. This self-service approach eliminates the need for external processing units or additional memory accesses to retrieve sign information, thereby reducing processing time and allowing parallel execution of forward, backward, and weight update cycles in DNN training.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient reading of signed weight values within RPU cells, reducing the need for external processing and enhancing DNN training by allowing parallel execution of forward, backward, and weight update cycles with lower power consumption and computation resources.
Implementation Method 1
a weight storage device configured to store a weight voltage which corresponds to a weight value of the RPU device
Implementation Method 2
The read transistor is configured to generate a weight current in response to the weight voltage applied to the gate terminal of the read transistor
Implementation Method 3
The current source is configured to generate a fixed reference current
Implementation Method 4
The magnitude of the read current is equal to a difference between the weight current generated by the read transistor and the fixed reference current of the current source
Data Source
AI summary
A resistive processing unit (RPU) device includes a weight storage device to store a weight voltage which corresponds to a weight value of the RPU device, and a read transistor having a gate connected to the weight storage device, and first and second source/drain terminals connected to first and second control ports, respectively. A current source connected to the second source/drain terminal generates a fixed reference current. The read transistor generates a weight current in response to the weight voltage. A read current output from the second control port represents a signed weight value of the RPU device. A magnitude of the read current is equal to a difference between the weight current and the fixed reference current. The sign of the read current is positive when the weight current is greater than the fixed reference current, and negative when the weight current is less than the fixed reference current.