14.6 A 28nm 64kb Bit-Rotated Hybrid-CIM Macro with an Embedded Sign-Bit-Processing Array and a Multi-Bit-Fusion Dual-Granularity Cooperative Quantizer

Hybrid-domain CIMs [1]–[3] are attracting increasing attention nowadays as they combine the advantages of both digital CIMs (DCIM) [4]–[6] and analog CIMs (ACIM) [7]–[10], offering a more balanced choice. As depicted in Figure 14.6.1, when designing a hybrid CIM (HCIM), the first challenge to address is (1) the definition of boundary between the digital and analog part, which is determined by feature input mode and weight mapping. Prior HCIMs either utilized a bit-parallel in features to obtain high accuracy at the cost of hardware (HW) overhead [1] or realized a low HW overhead but suffered from accuracy loss due to error pollution problem (error of the analog partial sum makes the high precision digital partial sum of the same bit weight meaningless) with bit-serial scheme and specific weight mapping strategy [2], [5], neither of which provided a perfect solution to the boundary question. Therefore, we propose a bit-rotated feature-in scheme to address this challenge. To compress multiple multiplication results into one MAC result, several levels of shifters and adders are usually required, and the sign-bit needs to be considered when compressing signed and unsigned information. Unlike the bit-parallel/serial approach [1], [2], the bit-rotated scheme considers the sign-bit in the first product-wise adder, which induces (2) more digital overhead for sign-bit processing. Another challenge that continues to plague bit-rotated HCIM is (3) excessive energy wasted on low accuracy-contributed low-bit quantization in analog parts.