HDSoC: High Density System-on-Chip

• HDSoC is a system-on-chip architecture designed for high-density integration, supporting 32–64 channels per die and scalable modular tiling for O(10^3) channels.
• It employs aggressive digitization with 1–3 GSa/s 12-bit ADCs and global low-jitter clocks to achieve sub-100 ps timing resolution and efficient data throughput.
• Advanced 3D integration and hybrid interconnects, including copper-tin pillar bonding and TSVs, deliver reduced latency, enhanced bandwidth (>5 Tb/s·mm²), and energy efficiency.

A High Density System on Chip (HDSoC) refers to a System-on-Chip architecture explicitly engineered to maximize integration density of analog, digital, and photonic functionality, often to accommodate extremely high channel counts, bandwidths, or memory/processing core counts within a constrained footprint or power envelope. HDSoC technology is a central enabler in data-intensive domains such as photon-timing instrumentation, heterogeneous multiprocessor arrays, and next-generation AI/ML accelerators. Its evolution reflects a convergence of advances in 3D integration, hybrid interconnects, advanced memory hierarchies, and ultra-scaled mixed-signal design flows.

1. Architectural Foundations and Channel Integration

HDSoC architectures are defined by their ability to instantiate large numbers of parallel channels or tiles with full waveform digitization, memory, and processing capabilities within a monolithic or vertically-stacked die. For example, in photon-timing applications, the Nalu Scientific HDSoC integrates 32 fully independent analog front-end channels—each comprising a 50 Ω input, GHz-bandwidth preamplifier, and a 2,048-cell switched-capacitor array (SCA)—alongside digital conversion and buffering logic (Li et al., 27 Nov 2025). All channels digitize waveforms in parallel via on-chip 12-bit SAR-type ADCs, with data distributed into per-channel FIFO buffers for downstream event-building and serialization.

Channel scaling is addressed via two strategies:

• Mono-die channel multiplexing, with up to 64 independent analog channels integrated in the latest HDSoC revisions;
• Modular tiling, enabling $O(10^3)$ channels per crate through synchronized chip arrays and hierarchical data aggregation.

These principles also inform multiprocessor SoC design, where clusters of processor elements and shared cache slices are partitioned across separate 3D tiers, tightly coupled by low-latency crossbar fabrics or high-bandwidth packet-based Network-on-Chip (NoC) meshes (Cataldo et al., 28 Apr 2025).

2. Data Acquisition, Digitization, and Bandwidth

Digitization pipelines in HDSoC environments are characterized by aggressive sampling and resolution targets, with the Nalu Scientific HDSoC achieving 1–3 GSa/s with 12-bit resolution and analog bandwidths near 1 GHz. All channels are synchronized to a global low-jitter DLL clock, supporting tight channel-to-channel timing alignment (Li et al., 27 Nov 2025). Quantization noise and signal-to-noise ratios conform to:

$$Q_{\mathrm{noise}} = \frac{\Delta}{\sqrt{12}},\quad \Delta = \frac{V_{\mathrm{FS}}}{2^{N}},$$

$\mathrm{SNR}_q = 6.02N + 1.76~\mathrm{dB}$

where $N$ is ADC resolution and $V_{\mathrm{FS}}$ the input swing.

Memory depth and throughput are tuned for zero-deadtime operation, with per-channel FIFO buffers providing ≈2 μs capture at 1 GSa/s. Event rates of ≳23 kHz/channel (≈200 kHz/chip) are supported, capped by ADC conversion and serialization logic. Data is transmitted via high-speed LVDS to FPGAs and further to Ethernet, with typical per-chip throughput set at ≈100 MB/s (Li et al., 27 Nov 2025).

Table 1: HDSoC and AARDVARC Digitizer Comparison (Li et al., 27 Nov 2025)

Architecture	Channels	Max Sample Rate	Input BW	ADC Bits	Buffer Depth
HDSoC	32/64	1–3 GSa/s	1 GHz	12	2k SCA
AARDVARC	4/8	10–14 GSa/s	>1.6 GHz	12	32k SCA

The recent demonstration of 3D-integrated photonic-electronic transceivers achieves 5.3 Tb/s·mm⁻² bandwidth density and an energy per communicated bit of 120 fJ/bit, using 80-channel microresonator-based optical links bonded to advanced CMOS logic. These links, operating at 10 Gb/s per channel, provide a two- to three-fold improvement in bandwidth density relative to prior 3D approaches (Daudlin et al., 2023).

3. Signal Timing, Jitter, and Calibration

HDSoC performance in time-resolved photon detection and other latency-sensitive applications depends critically on sub-nanosecond timing. The Nalu Scientific HDSoC achieves per-channel timing resolution below 100 ps rms, with dominant contributions from:

• SCA aperture jitter ($t_{\mathrm{ap}} \approx 50$ ps rms);
• DLL clock distribution skew (<20 ps rms);
• Readout chain jitter ($\sigma_{\mathrm{readout}} \approx 30$ ps in AARDVARC proxy measurements).

System-level timing jitter is evaluated via quadrature subtraction:

$$\sigma_{\rm LAPPD+HDSoC} = \sqrt{\sigma_{\rm total}^2 - \sigma_{\rm laser}^2 - \sigma_{\rm readout}^2}$$

This timing precision supports Cherenkov/scintillation photon separation in advanced detectors, as the HDSoC floor is commensurate with or below the time interval between signal types in water-based neutrino detectors (Li et al., 27 Nov 2025).

Calibration encompasses per-channel gain matching (measured via SPE scans) and fine timing offset correction (common fast test pulse alignment). Digital delay offsets programmed into the chip permit per-channel skew minimization.

4. Interconnect Structures and 3D Integration

HDSoC architectures increasingly leverage 3D vertical integration to overcome interconnect density, area, and bandwidth limitations. Recent methods include copper–tin pillar flip-chip bonding between thinned CMOS logic and silicon photonic dies, at sub-30 μm pitch and with parasitic capacitance under 10 fF per signal—enabling ultra-high bandwidth connections without degrading signal integrity (Daudlin et al., 2023). For memory and compute arrays, similar schemes apply using TSVs and interposers, with die area scaling modeled as

$$A_{\mathrm{total}}(n) = A_0\cdot n + A_1(n),\quad A_1(n) \approx N_{\mathrm{TSV}}(n)\cdot a_{\mathrm{TSV}} + N_{\mathrm{interconnect}}\cdot a_{\mathrm{link}}$$

Hybrid interconnects include:

• 3D mesh packet-based NoCs; e.g., $8 \times 8 \times n$ topologies supporting 16 GB/s-per-direction links with aggregate one-way latencies predicted from $$L_{\mathrm{noc}} = \sum_i L_{\mathrm{hop}_i} + L_{\mathrm{router}}$$;
• Local crossbar fabrics for intra-cluster coherence, with arbitration delays scaling logarithmically in port count.

5. Memory Hierarchies and Emerging Technologies

High-density SoC scaling is contingent on integrating advanced memory technologies. 3D-stacked multiprocessor designs employ multilevel caches distributed across tiers, for instance:

• Top tier: clusters of ARMv7 cores each with private 32 KB I/D caches;
• Middle tiers: shared 512 KB L2 per cluster (MOESI protocol);
• Lower tier: unified 4 MB L3;
• Innermost tier: option for non-volatile PCRAM or STT-MRAM caches or direct DRAM interface (Cataldo et al., 28 Apr 2025).

Table 2: Emerging Memory Properties in HDSoC Cache Design (Cataldo et al., 28 Apr 2025)

Technology	Read Latency	Write Latency	Energy (R/W)	Relative Density
PCRAM	45–100 ns	40–110 ns	0.1/6–13 nJ	~4× SRAM
STT-MRAM	5–10 ns	5–10 ns	0.06/0.6 nJ	~2× SRAM
SRAM	1 ns	1 ns	0.1/0.1 nJ	1× (baseline)

PCRAM offers high density and zero leakage but shows high write energy and limited endurance; STT-MRAM balances moderate density, low write energy, and zero leakage with high endurance.

6. Experimental Outcomes and Application Implications

Quantitative system-level evaluation using Gem5 (with PARSEC 2.1 and SPLASH-2 benchmarks) demonstrates that 3D HDSoC cache architectures achieve a 38% reduction in average L2 miss latency, 38% uplift in sustained throughput, 36% reduction in dynamic cache energy per access, and 28% savings in total cache area relative to 2D baselines, even accounting for TSV/interposer overhead. Thermal analyses show ≈10 °C peak reductions due to vertical cache redistribution (Cataldo et al., 28 Apr 2025).

In photon-timing systems, integration with LAPPDs via HDSoC delivers sub-100 ps timing, high per-channel throughput, and direct event-based data flows optimized for massive Cherenkov and scintillation detectors (Li et al., 27 Nov 2025).

The deployment of 3D photonic interconnects further positions HDSoC to overcome the “computation-communication gap” in AI and large-scale scientific instrumentation, supporting >5 Tb/s·mm⁻² at 120 fJ/bit, compared to electrical and monolithic photonic baselines (typically 1–2 pJ/bit and 0.5 Tb/s·mm⁻², respectively) (Daudlin et al., 2023).

7. Scalability, Power, and Future Trajectories

HDSoC designs scale via both channel multiplication (32–64+ channels per die, tiling into $O(10^3)$-channel systems) and 3D stacking (layered compute-memory-coupling), while maintaining low per-channel power—20–40 mW/chip in present photon-timing digitizers (Li et al., 27 Nov 2025). Fast, energy-efficient optical links enable decoupling of on-chip and inter-chip locality constraints, vital for heterogeneous platforms and for scaling AI fabrics.

Evolutionary directions include migration to advanced CMOS nodes (e.g., 130 nm and below), deeper on-chip buffering, hierarchical timing/skew management, per-cell timebase calibration, and full heterogenous (electrical, photonic, analog, and memory) vertical integration. Such advancements aim to sustain momentum in area, energy, and communication efficiency gains beyond the limits of planar scaling, directly serving the memory- and bandwidth-centric workflows that define next-generation high-density system-on-chip deployments.