Bendable non-silicon RISC-V microprocessor | Nature

In this section, we describe the microarchitecture of the C-SPI IP block, the ML hardware accelerator, the new instructions to program it, the description of the test benchmarks, the compilation environment, the implementation details of the latch-based RF, the details of the FPGA-based test infrastructure and the OEP assembly method.

Architecture of the C-SPI

To reduce the complexity of assembling Flex-RV onto a FlexPCB in terms of the number of pads, we have developed C-SPI to communicate with external memory by interfacing the 79 RAM ports to 4 SPI ports rather than exposing the 79 ports to the pads of the chip. This results in slower memory access but facilitates the assembly of Flex-RV onto a FlexPCB.

The internal and external ports of the C-SPI are shown in Extended Data Fig. 1. The number of address bits is set to 10 and allows the processor to access 4 kB of external memory, which is sufficient to store the code and data for the largest test bench.

As Serv requires a single-cycle memory access, C-SPI implements a top-level clock gate that halts the rest of the Flex-RV microprocessor when any external memory transaction occurs using a gated clock signal. The gated clock signal is also sent as an output to the external controller such as an FPGA.

There are two internal registers dedicated to transmission in the C-SPI. The transfer register consists of a 46-bit transmit register and a 32-bit receive register. The transmit register serves the purpose of writing to the memory (that is, store operation) and transmitting the designated address for reading, whereas the receive register is used to capture the desired data (that is, load operation). Each write transfer requires a total of 47 clock cycles. The initial clock cycle designates the write mode, followed by 10 clock cycles for sending the memory address, and subsequently, 32 clock cycles for transmitting the write data. The final four cycles are allocated for the write mode type determined by the 4-bit ‘Write En’. The commands are provided in Extended Data Table 1.

Every read transfer involves a total of 46 clock cycles. The initial cycle is dedicated to specifying the reading mode, followed by 10 clock cycles for transmitting the memory address. Subsequently, three cycles are allocated for reading data from the memory and preparing for transmission. Finally, 32 clock cycles are necessary to receive the 32-bit data. The state machine in Extended Data Fig. 2 shows the write and read modes broken down into states.

Microarchitecture of the ML hardware accelerator

The ML hardware accelerator is a tightly coupled CFU²³ extending the data path of the CPU. It follows the RISC-V instruction R-format in which it receives two operands from the RF and writes one result back. Extended Data Fig. 3 shows the microarchitecture of the ML accelerator in relation to the rest of the CPU. The boundary between the CPU and CFU is strictly logical. The current implementation flattens the design and optimizes, places and routes it all together.

The accelerator features a SIMD multiply–accumulate processing engine composed of two 8 × 4 multipliers and two 4 × 4 multipliers as prior art²⁷ has shown sufficient accuracy using 4-bit integer quantization for weights of embedded ML models with 8-bit inputs. The accelerator also has specialized hardware for handling post-processing of activations such as bias addition, applying non-linearity (for example, ReLU) and quantization rescaling. These architectural features can be invoked in software using four custom instructions: (1) use the two 8 × 4 multipliers alone; (2) use all four multipliers with 4 × 4 precision; (3) apply bias addition and non-linearity to activations; and (4) rescale activations for quantization.

To test our accelerator, we developed a Tiny Machine Learning (TinyML) model using Tensorflow²⁸ to perform ECG anomaly detection using the ECG5000 time series classification dataset²⁹. Our tiny neural network consists of a one-dimensional convolution followed by a fully connected layer, an approach commonly used in simple time series classification. We use linear symmetric affine quantization instead of asymmetric quantization to avoid the additional computation of subtracting offsets for each loop iteration.

Using post-training quantization, we quantize our model down to 4-bit precision for weights and activations with minimal accuracy loss. However, the bias must be kept at 32-bit precision to avoid large degradations in accuracy. Finally, we implemented the power of two scaling for our step size so that multiplication during post-processing of a layer could be done using bit shifts. These model optimizations coupled with our hardware co-design enable us to run ML algorithms even in these deeply embedded application settings in which computational resources are limited and constrained.

Test benchmarks and compilation environment

We have written several test benchmarks in C language with some inline RISC-V RV32E assembly code to use in functional testing of the wafers and assembled FlexPCBs. The test benchmarks, as shown in Extended Data Table 2, were developed to exercise many hardware blocks in Flex-RV such as the functional units inside the Serv CPU such as the arithmetic logic unit, RF, ML accelerator and also outside the CPU such as the C-SPI (through memory reads and writes). The core of the test benchmarks is written in C, whereas the GPIO communication code is written in RV32E assembly. At the end of each test benchmark, the results from the test benchmark are sent through the GPIO to the external world. In our test infrastructure, the external world is the FPGA board that reads the data coming out of the GPIO output, and the FPGA board relays the data to a PC through UART to display the results on the PC screen.

The test benchmarks were compiled for bare metal execution using the riscv64-unknown-elf-gcc compiler with the flags of ‘rv32e’, ‘ilp32e’, ‘nostdlib’ and ‘nostartfiles’ to generate an elf file, which was then translated into a hex file using riscv64-unknown-elf-objcopy with the flag ‘-O verilog’. The hex files can be used to test Flex-RV in Verilog simulation environments as well as in wafer and assembled FlexPCB tests when the FPGA memory is loaded with the hex file of each test benchmark.

To invoke and compile the custom instructions for ML workloads, the ‘asm’ directive for inlining assembly is used within the C kernel to directly insert assembly language instructions. This approach bypasses the need for the compiler to understand the custom instruction by manually coding it into the program. The custom instruction is defined using the ‘.word’ directive followed by the opcode and operand specifiers, which are filled in based on the provided C variables holding the operands to be used. When the processor sees the dedicated opcode for the custom instruction, the operands are passed to the accelerator unit.

Implementation of the RF circuitry

The block diagram of the dual-port latch-based memory used as an RF is shown in Extended Data Fig. 4 allowing one register read and one write at the same cycle. Write and read address decoders are synthesized using standard cells. Logic gates in the highlighted region instantiate special standard cells that allow signal connection through the abutment. During implementation, the placement of these cells is controlled through relative cell placement. This achieves much higher cell density and reduces the amount of routing that would otherwise be performed by the EDA tool. All latches use an additional enable signal shown as RWL (read word line), which enables the output RBL (read bit line). To reduce the static power consumption, latches in the bottom word line implement a pull-up resistor on the RBL output, whereas latches in other word lines do not implement such a resistor. Hence, there is only one pull-up resistor per bit column.

FPGA-based test infrastructure

The test infrastructure uses MicroZed FPGA board with Zynq−7000 SOC (ref. ²⁵). Our testing methodology involves two distinct phases: wafer-on-glass testing and FlexPCB testing. These phases serve different purposes and are integral parts of our test infrastructure.

1. 1.

   Wafer-on-glass testing validates the functionality of each Flex-RV on the wafer (or more specifically wafer on glass) using a semi-automatic wafer probe station. The goal of this phase is to identify functional Flex-RVs that are suitable for FlexPCB assembly and testing. The test probe card of the wafer probe station is connected to the FPGA board that sends each test benchmark as an input to a Flex-RV on the wafer, and the output of the executed benchmark from the Flex-RV is sent through the probe card to the FPGA for verification. We also connect a logic analyser to verify that all input and output signals of the Flex-RVs are within specifications. The logic analyser has an input capacitance of 10 pF that has an effect on the performance of the IO drivers inside Flex-RVs. This leads to achieving lower clock frequencies for Flex-RVs.

2. 2.

   The second phase is FlexPCB testing in which the wafers are released from the glass carrier, and the dies are diced. The goal of FlexPCB testing is to evaluate the performance of functional Flex-RVs or known good dies (KGDs) in their intended flexible configuration. The KGDs identified in the first phase are separated from the faulty dies and assembled onto FlexPCBs. The FlexPCB is a polyimide substrate with a thickness of 45 µm onto which a KGD is assembled. The KGD is attached to the FlexPCB using epoxy, whereas the die pads are aligned with the PCB pads. A silver paste is printed onto the aligned pads to bond the pads electrically and mechanically through the OEP assembly method as described below. The PCB traces connect the die pads to an FPC connector through which the FlexPCB is connected directly with the same FPGA board. No logic analyser is used in this phase as functional correctness was already verified in the previous phase. Once the functionality is confirmed, the FlexPCB tests can be conducted without the logic analyser.

Our experiments showed that the highest achievable clock frequencies in wafer-on-glass testing are 25–30% lower than the FlexPCB testing because of high capacitive loading from the logic analyser on the IO drivers of Flex-RVs. The total resistance and capacitive of PCB traces between the test chip and the FPGA board have much less effect compared with the effect of the logic analyser. The total resistance and capacitance from test chips to the FPGA board in wafer-on-glass test setup are 2 Ω and about 2.5 pF, whereas these values are about 1 Ω and 2.5 pF on the FlexPCB test setup. The additional 10 pF capacitance from the logic analyser is the reason for lower clock frequencies in the wafer-on-glass test setup.

The block diagram of FPGA implementation is shown in Extended Data Fig. 5. The Python script running on the processing system controls the test. Flex-RV is held in reset during the initial setup stage, whereas the 4 kB memory is programmed with benchmark binary and the data flow is configured. Although GPIO from Flex-RV could be connected directly to UART Tx pin, as the clock to Serv is halted with every memory transaction, the bitstream is corrupted. Therefore, the GPIO data are captured and re-encoded into the correct UART bitstream using the gated clock signal. Moreover, UART transactions are stored in a separate 1 kB memory and later compared with the expected response. This allows a frequency sweep to be run for each benchmark.

OEP assembly method

With conventional flip-chip assembly approaches—for example, anisotropic conductive paste and anisotropic conductive film, with applying relatively high thermo-compression force—there is a high chance of circuitry damage by conductive particles or thickness difference of the area on which the chip is bonded. An alternative is to attach the flexible chip face-up (non-flip chip) with thermode-free assembly process named OEP assembly method developed by Pragmatic. OEP has the following steps as shown in Extended Data Fig. 6:

1. 1.

   Underfill adhesive application: non-conductive adhesive is applied on the FlexPCB as underfill adhesive to hold the FlexIC. The volume and position of the underfill adhesive must be well controlled for the coverage of the four edges to prevent undesired conductive path formed by infiltration underneath the FlexIC. At the same time, the underfill adhesive must not overflow on top of the FlexIC during the die attachment process, because that could hinder the conductive path formation above the contact pads.

2. 2.

   Die attachment: FlexIC is bonded with the RDL layer facing up. Specially designed ejectors release singulated chips from release tape on the frame, preventing damage to the FlexIC circuitry. The levelling and bond force control must be optimized to achieve 100% coverage of the four edges but no underfill adhesive overflowing on top of the FlexIC.

3. 3.

   Interconnection formation: stretchable isotropic conductive adhesive is applied by a high-precision time-pressure controlled dispensing machine with camera alignment functionality with high placement accuracy. Dispense height measurement is essential to control the isotropic conductive adhesive thickness and its uniformity. The isotropic conductive adhesive material is specially chosen to be stretchable, which can maintain low-resistance electrical connectivity under bending conditions. The coefficient of thermal expansion and elongation should match with the materials such as FlexIC and FlexPCB to keep the devices functional and reliable during and after bending conditions.