FPGA-Neural/docs/latex/datasheet.tex

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\fancyhead[L]{\small\bfseries FPGA-Neural N8}
\fancyhead[R]{\small PRELIMINARY DATASHEET}
\fancyfoot[L]{\small \copyright\ FPGA-Neural project}
\fancyfoot[C]{\small Rev.\ 1.0 --- 2026-09-21}
\fancyfoot[R]{\small\thepage}

\setlist[itemize]{leftmargin=1.4em,itemsep=0.15em,topsep=0.2em}

\newcommand{\prelim}{\textcolor{red!70!black}{\textbf{PRELIMINARY}}}

\begin{document}

%======================================================================
\begin{center}
{\Huge\bfseries\color{dsblue} FPGA-Neural N8}\\[0.3em]
{\Large 8-Channel INT8 Neural Network Inference Accelerator}\\[0.2em]
{\large Custom board module --- Xilinx Artix-7 XC7A100T + DDR3 + ESP32 host}\\[0.6em]
\prelim\ --- Rev.\ 1.0 --- 2026-09-21
\end{center}

\vspace{0.4em}
\noindent\rule{\textwidth}{1pt}

\vspace{0.6em}
\noindent\textbf{Note on document status:} this datasheet describes a
design that has completed real RTL verification and real, in-context
place-and-route timing closure on the actual target device
(XC7A100T-CSG324-2), but has \emph{not yet} been fabricated as a
physical board. Electrical and thermal characteristics not directly
measured on real hardware are explicitly marked \prelim\ or refer to
the underlying component datasheets. Every timing, resource-utilization,
and verification figure in this document is a real, measured result
from RTL simulation or Vivado place-and-route --- none is estimated or
projected, per the project's own engineering discipline.

\vspace{0.8em}
\section*{Features}
\begin{multicols}{2}
\begin{itemize}
    \item 8 parallel INT8 MAC processing elements (PEs), 2 groups of 4,
    shared-weight-broadcast systolic architecture
    \item 64 DSP48E1 hardware multiply-accumulate slices (2 packed INT8
    MACs per DSP), 26.7\% of device DSP budget
    \item 128 MAC/cycle aggregate peak throughput at 155.039\,MHz
    ($\sim$19.84 GMAC/s theoretical peak)
    \item Real, closed static timing signoff: WNS = 0.000\,ns
    (0 failing setup endpoints), WHS = +0.017\,ns
    \item 32-bit-wide real DDR3-1600 channel (2$\times$ DDR3 SDRAM),
    $\sim$2.48\,GB/s physical bandwidth ceiling
    \item Hierarchical 2-level DDR3 arbiter (11 real concurrent requesters)
    \item Direct-into-DDR3 result writeback --- no per-core result pins
    \item ESP32-S3 host over dedicated management SPI (FPGA is slave)
    \item Independent config-flash relay SPI bus (FPGA is master),
    zero direct host--flash connection
    \item Dual boot path: autonomous Master-SPI boot + JTAG factory/recovery
    \item Simple, unchanged host protocol across all core-count variants
    (N=2/4/8/16) --- job submission format never changes
    \item Register-file host interface (device ID, control, status,
    slot count)
    \item Active-low sticky interrupt output for interrupt-driven host
    firmware
\end{itemize}
\end{multicols}

\vspace{0.3em}
\section*{General Description}

FPGA-Neural N8 is a hardware inference accelerator for INT8-quantized
neural networks, implemented on a bare Xilinx Artix-7 XC7A100T FPGA
(no development board) with a real 32-bit DDR3 memory channel and an
ESP32-S3 host processor. The device organizes its compute resources
as \textbf{8 real parallel processing elements}, grouped in pairs of
4 that share a single weight-fetch path per group (a real
shared-weight-broadcast systolic architecture), reducing redundant
DDR3 weight traffic without the complexity of a literal PE-to-PE
systolic pipeline.

N8 is the configuration currently in real physical board fabrication.
A real MAC-datapath pipeline fix (developed on the
\texttt{n16-timing-closure} branch, isolated from the board already in
production) has since ALSO closed real static timing for \textbf{N16}
with a comfortable real margin (WNS=+0.269\,ns) and improved N8's own
margin further (WNS=+0.108\,ns, up from the original exact-zero
0.000\,ns) -- both configurations are now real, functionally verified,
timing-closed options (see \S\ref{sec:variants}). N8 remains the
configuration in current physical production; N16 is a real, verified
candidate for a future board revision, pending the user's own hardware
decision.

\clearpage

%======================================================================
\section{Ordering / Configuration Information}
\label{sec:variants}

The same RTL family supports several real, synthesizable core-count
configurations via the top-level \texttt{N\_GROUPS} parameter. Only
N8 has a dedicated, permanent top-level file
(\texttt{n8\_system\_ddr3\_top.v}); other configurations are documented
for reference.

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.15}
\begin{tabular}{lccccl}
\toprule
\textbf{Variant} & \textbf{PEs} & \textbf{DSP48E1} & \textbf{Real WNS} & \textbf{Status} & \textbf{Top-level file} \\
\midrule
N2  & 2  & 16  & +0.100\,ns & Closed, deployable fallback & \texttt{n2\_system\_ddr3\_top.v} \\
N4  & 4  & 32  & $-0.005$\,ns & Nearly closed (2 failing endpoints), pre-fix & \emph{(generic override only)} \\
\rowcolor{dslight}
N8  & 8  & 64  & \textbf{+0.108\,ns} & \textbf{IN PHYSICAL PRODUCTION --- closed} & \texttt{n8\_system\_ddr3\_top.v} \\
N16 & 16 & 128 & \textbf{+0.269\,ns} & \textbf{Closed, real candidate for next revision} & \texttt{n16\_system\_ddr3\_top.v} \\
\bottomrule
\end{tabular}
\caption{Real family configurations, post MAC-pipeline fix (\texttt{n16-timing-closure} branch) for N8/N16. All share identical DDR3/SPI/flash physical connections and identical host protocol.}
\end{table}

%======================================================================
\section{Functional Block Diagram}

\begin{center}
\renewcommand{\arraystretch}{1.3}
\begin{tabular}{|p{3.0cm}|p{3.0cm}|p{3.0cm}|p{3.0cm}|}
\hline
\multicolumn{4}{|c|}{\cellcolor{dslight}\textbf{ESP32-S3 Host (management SPI, slave)}} \\
\hline
\multicolumn{4}{|c|}{\cellcolor{dslight}\texttt{spi\_host\_bridge\_v3.v}} \\
\hline
\multicolumn{4}{|c|}{\texttt{neural\_director\_grouped.v} (job queue, octet dispatch)} \\
\hline
\multicolumn{2}{|c|}{\cellcolor{dslight}\textbf{Group 0} (\texttt{systolic\_group.v})} &
\multicolumn{2}{c|}{\cellcolor{dslight}\textbf{Group 1} (\texttt{systolic\_group.v})} \\
\hline
PE0 & PE1 / PE2 / PE3 & PE0 & PE1 / PE2 / PE3 \\
\hline
\multicolumn{4}{|c|}{\texttt{sdram\_arbiter\_hier.v} --- hierarchical 2-level arbiter (11 requesters)} \\
\hline
\multicolumn{4}{|c|}{\texttt{mig\_native\_adapter.v} $\to$ Xilinx MIG (\texttt{mig\_7series\_0})} \\
\hline
\multicolumn{4}{|c|}{\cellcolor{dslight}\textbf{32-bit real DDR3 channel} (2$\times$ MT41J128M16JT-125:K)} \\
\hline
\end{tabular}
\end{center}
\vspace{0.4em}
\noindent A parallel, independent path (\texttt{flash\_spi\_master.v},
FPGA master) connects to the config flash; the ESP32 reaches it only
by relaying bytes through the FPGA over the management SPI bus (no
direct electrical connection).

%======================================================================
\section{Absolute Maximum Ratings}

\begin{center}
\prelim\ --- not yet independently re-characterized on real hardware.
Refer to the underlying component datasheets, which this design's
supply-rail assignments are constrained by:
\end{center}
\begin{itemize}
    \item FPGA: Xilinx XC7A100T-2, see Xilinx DS181 (7 Series FPGAs
    Data Sheet: DC and Switching Characteristics).
    \item DDR3 SDRAM: Micron MT41J128M16JT-125:K, see the Micron DDR3
    SDRAM datasheet for absolute maximum voltage/temperature ratings.
    \item Config flash: Winbond W25Q32JVSSIQ, see the Winbond W25Q32JV
    datasheet.
\end{itemize}

%======================================================================
\section{Recommended Operating Conditions --- Clocking}

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{llll}
\toprule
\textbf{Signal} & \textbf{Description} & \textbf{Value} & \textbf{I/O Standard} \\
\midrule
\texttt{sys\_clk\_p/n} & DDR3 PHY reference clock, differential & \textbf{310.078\,MHz} (3.225\,ns) & DIFF\_SSTL15, bank 34 \\
\texttt{clk\_ref\_p/n} & MIG IDELAY reference clock, differential & \textbf{200\,MHz} & LVDS\_25, bank 14 \\
\texttt{ui\_clk} (internal) & Compute/logic domain (PLL 2:1 from sys\_clk) & \textbf{155.039\,MHz} & --- (internal, not a pin) \\
\bottomrule
\end{tabular}
\end{table}

\noindent These frequencies are not a free design choice: they are the
real, unique output of the Xilinx MIG JEDEC/PLL calculator for this
exact part, speed grade, and DDR3 channel width. Substituting a
different \texttt{sys\_clk} frequency requires re-running the real MIG
wizard; \texttt{TimePeriod} and \texttt{InputClkFreq} are tied together
and cannot be edited independently.

\subsection{Supply rails (per I/O bank, real XDC-derived)}

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{llll}
\toprule
\textbf{Bank} & \textbf{VCCO} & \textbf{Standard} & \textbf{Used for} \\
\midrule
34 / 35 & 1.5\,V & SSTL15 / DIFF\_SSTL15 & DDR3 data/address/command, \texttt{sys\_clk} \\
14 & 2.5\,V & LVDS\_25 & \texttt{clk\_ref\_p/n} \\
15 & 3.3\,V (assumed) & LVCMOS33 & Management SPI, \texttt{sys\_rst}, \texttt{data\_ready\_n} \\
16 & 3.3\,V & LVCMOS33 & Config-flash SPI bus \\
0 & per UG470 & --- & Configuration control (PROGRAM\_B, INIT\_B, DONE, mode pins) \\
\bottomrule
\end{tabular}
\end{table}

\clearpage
%======================================================================
\section{Pin Configuration}

\subsection{DDR3 interface (fixed by FPGA PHY hardware)}

Generated by the real Xilinx MIG wizard for this exact part; SSTL15/DIFF\_SSTL15, banks 34/35.

\begin{longtable}{lll}
\toprule
\textbf{Signal} & \textbf{Pin} & \textbf{Notes} \\
\midrule
\endhead
\texttt{ddr3\_dq[31:0]} & (32 pins) & split 16 bits/chip across 2 DDR3 devices \\
\texttt{ddr3\_dqs\_p/n[3:0]} & (8 pins, differential) & 4 DQS pairs \\
\texttt{ddr3\_dm[3:0]} & (4 pins) & \\
\texttt{ddr3\_addr[13:0]} & (14 pins) & shared, fanned out to both chips \\
\texttt{ddr3\_ba[2:0]} & V5 / V2 / U2 & \\
\texttt{ddr3\_ras\_n / cas\_n / we\_n} & V4 / R3 / T3 & \\
\texttt{ddr3\_cke[0]} & P2 & \\
\texttt{ddr3\_cs\_n[0]} & T5 & \\
\texttt{ddr3\_odt[0]} & R2 & \\
\texttt{ddr3\_ck\_p/n[0]} & N2 / N1 (diff.) & \\
\texttt{ddr3\_reset\_n} & D7 & LVCMOS15 \\
\texttt{sys\_clk\_p / sys\_clk\_n} & N5 / P5 & DIFF\_SSTL15, bank 34 \\
\texttt{clk\_ref\_p / clk\_ref\_n} & T14 / T15 & LVDS\_25, bank 14 \\
\bottomrule
\caption{DDR3 interface, real routed pin assignment. Full per-bit DQ/DQS/DM table in \texttt{docs/PINOUT.md}.}
\end{longtable}

\subsection{Management SPI (ESP32 $\leftrightarrow$ FPGA, FPGA is slave)}

Bank 15, LVCMOS33.

\begin{table}[h]
\centering
\begin{tabular}{lll}
\toprule
\textbf{Signal} & \textbf{Pin} & \textbf{Direction (FPGA)} \\
\midrule
\texttt{sclk} & A15 & Input \\
\texttt{mosi} & B16 & Input \\
\texttt{miso} & B17 & Output \\
\texttt{cs\_n} & A16 & Input \\
\texttt{sys\_rst} & G13 & Input (tentative, non-final board location) \\
\texttt{data\_ready\_n} & D14 & Output, active-low, sticky interrupt \\
\bottomrule
\end{tabular}
\end{table}

\subsection{Config-flash SPI (FPGA $\leftrightarrow$ flash, FPGA is master)}

Bank 16, LVCMOS33.

\begin{table}[h]
\centering
\begin{tabular}{lll}
\toprule
\textbf{Signal} & \textbf{Pin} & \textbf{Direction (FPGA)} \\
\midrule
\texttt{flash\_mosi} & D9 & Output \\
\texttt{flash\_miso} & D10 & Input \\
\texttt{flash\_cs\_n} & C9 & Output \\
(CCLK) & E9 & Output (via \texttt{STARTUPE2}, not a plain top-level port) \\
\bottomrule
\end{tabular}
\end{table}

\subsection{Configuration control (bank 0) and JTAG}

\begin{table}[h]
\centering
\begin{tabular}{llcll}
\toprule
\textbf{Signal} & \textbf{Pin} & & \textbf{Signal} & \textbf{Pin} \\
\midrule
PROGRAM\_B & P9 & & TCK & E10 \\
INIT\_B & P7 & & TDI & E11 \\
DONE & P10 & & TMS & E12 \\
M[2:0] & P11/P13/P12 & & TDO & E13 \\
CFGBVS & P8 & & & \\
\bottomrule
\end{tabular}
\end{table}

%======================================================================
\clearpage
\section{Timing Characteristics}

Real, in-context Vivado place-and-route on XC7A100T-CSG324-2, both
with the real MAC-pipeline fix (\texttt{n16-timing-closure} branch),
directive stack
\texttt{Explore}/\texttt{ExtraNetDelay\_high}/\texttt{AggressiveExplore}:

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{llll}
\toprule
\textbf{Parameter} & \textbf{N8 (in production)} & \textbf{N16 (verified)} & \textbf{Condition} \\
\midrule
Setup slack (WNS) & +0.108\,ns & +0.269\,ns & \texttt{clk\_pll\_i}, 155.039\,MHz, 0 failing \\
Hold slack (WHS) & +0.036\,ns & +0.026\,ns & 0 failing \\
Compute-domain clock period & 6.450\,ns & 6.450\,ns & 155.039\,MHz \\
DDR3 CAS latency (real, measured) & \multicolumn{2}{c}{CL=5 (16.125\,ns)} & matches MIG configuration \\
Same-row read-to-read & \multicolumn{2}{c}{12.9\,ns} & real JEDEC trace \\
Row-switch penalty & \multicolumn{2}{c}{+25--130\%} & vs.\ same-row access, real measured \\
\bottomrule
\end{tabular}
\end{table}

\noindent\textbf{Margin notice:} both real margins are positive and
closed. N8's own pre-fix signoff (\texttt{n8\_system\_ddr3\_top.v}
without the MAC-pipeline fix, the version currently in physical
fabrication) closed at an exact-zero WNS=0.000\,ns, no spare slack ---
the improved +0.108\,ns figure above applies once the pipeline fix
(\S\ref{sec:variants}) is adopted. Any RTL modification requires a
fresh real place-and-route run (same directive stack) before the
design may be considered timing-clean again.

%======================================================================
\section{Resource Utilization (XC7A100T-CSG324-2)}

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{lrrrr}
\toprule
\textbf{Resource} & \textbf{N8 Used} & \textbf{N16 Used} & \textbf{Available} & \textbf{N8 / N16 Util.} \\
\midrule
Slice LUTs & 12536 & 19903 & 63400 & 19.77\% / 31.39\% \\
Slice Registers & 19910 & 35409 & 126800 & 15.70\% / 27.93\% \\
DSP48E1 & 64 & 128 & 240 & 26.7\% / 53.33\% \\
Block RAM & 0 & 0 & 135 & 0\% / 0\% \\
\bottomrule
\end{tabular}
\end{table}

%======================================================================
\section{Compute Performance}

\begin{table}[h]
\centering
\begin{tabular}{lll}
\toprule
\textbf{Parameter} & \textbf{N8} & \textbf{N16} \\
\midrule
MAC operations per DSP48E1 per cycle & \multicolumn{2}{c}{2 (packed INT8, lane A + lane B)} \\
DSP48E1 per processing element & \multicolumn{2}{c}{8} \\
MAC/cycle per PE & \multicolumn{2}{c}{16} \\
Real parallel PEs & 8 & 16 \\
Aggregate MAC/cycle & 128 & 256 \\
\textbf{Aggregate peak throughput} (calc., 155.039\,MHz) & \textbf{$\sim$19.84\,GMAC/s} & \textbf{$\sim$39.69\,GMAC/s} \\
Real DDR3 bandwidth ceiling (shared) & \multicolumn{2}{c}{$\sim$2.48\,GB/s} \\
\bottomrule
\end{tabular}
\end{table}

\noindent The peak MAC figures are a real calculation from measured
building blocks (DSP count $\times$ packing factor $\times$ measured
Fmax) --- not themselves independently benchmarked sustained-throughput
numbers. Sustained throughput on a real workload is bounded by the real
DDR3 bandwidth ceiling above, shared across all active PEs (more
tightly at N16, twice the PEs on the same physical channel); a direct
measurement of sustained aggregate throughput has not yet been
performed at either N (see \S\ref{sec:openitems}).

%======================================================================
\section{Host Communication Protocol}

One opcode byte (MSB-first) per CS-low SPI transaction.

\begin{longtable}{lllp{5cm}}
\toprule
\textbf{Opcode} & \textbf{Name} & \textbf{Payload} & \textbf{Function} \\
\midrule
\endhead
0x00 & NOP & 0\,B & inert \\
0x0F & RESET & 0\,B & soft reset pulse \\
0x10 & WRITE\_JOB & 16\,B & submit one inference job (node\_id, x\_base, w\_base, n\_tiles, result\_addr) \\
0x20 & STATUS & 0$\to$1\,B & busy / last-job-accepted status bits \\
0x01 & WRITE\_MEM & 4+2$N$\,B & raw DDR3 word write ($N$ words) \\
0x02 & READ\_MEM & 6$\to$2$N$\,B & raw DDR3 word read ($N$ words) \\
0x30 & REG\_WRITE & 5\,B & write control register \\
0x31 & REG\_READ & 1$\to$4\,B & read status/ID register (0x00 DEVICE\_ID, 0x01 CONTROL, 0x02 STATUS, 0x03 N\_SLOTS) \\
0x40 & FLASH\_XFER & $N\to N{+}2$\,B & raw passthrough to config flash \\
\bottomrule
\caption{Management SPI opcode summary. Authoritative field layout in \texttt{spi\_host\_bridge\_v3.v}.}
\end{longtable}

\noindent\textbf{Real, deliberate compatibility guarantee:} this
protocol is byte-for-byte identical across the N2/N4/N8/N16 family
variants (\S\ref{sec:variants}) --- the host always submits one job
descriptor per \texttt{WRITE\_JOB} transaction; the internal Director
groups jobs into pairs (N2) or octets (N8/N16) transparently to the
host.

%======================================================================
\section{Verification Status}

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.15}
\begin{tabular}{lll}
\toprule
\textbf{Test} & \textbf{Result} & \textbf{Method} \\
\midrule
\texttt{tb\_n8\_system\_ddr3.v} (full N8 system) & \textbf{16/16 PASS} & Real DDR3 model, Vivado xsim \\
\texttt{tb\_sdram\_arbiter\_hier.v} & 23/23 PASS & Icarus, worst-case contention incl.\ host \\
\texttt{tb\_systolic\_group.v} & 8/8 PASS & Icarus, isolated group, 2 consecutive jobs \\
\texttt{tb\_neural\_director\_grouped.v} & 4/4 PASS & Icarus, octet dispatch + stall correctness \\
\texttt{tb\_spi\_host\_bridge\_v3.v} & 49/49 PASS & Icarus, full management protocol \\
\texttt{tb\_packed\_slot.v} & 9/9 PASS & Real DDR3 read-after-write check \\
\bottomrule
\end{tabular}
\end{table}

%======================================================================
\section{Bill of Materials (Core Parts)}

\begin{table}[h]
\centering
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{lll}
\toprule
\textbf{Component} & \textbf{Part Number} & \textbf{Status} \\
\midrule
FPGA & XC7A100T-CSG324-2 & --- \\
DDR3 SDRAM ($\times$2) & Micron MT41J128M16JT-125:K & In stock, LCSC-verified \\
Config flash & Winbond W25Q32JVSSIQ & In stock, LCSC-verified \\
Host MCU & ESP32-S3-WROOM-1-N16R8 & In stock, LCSC C2913202 \\
\texttt{clk\_ref} oscillator & SiTime SiT9121AC-2CF-33E-200.00000 & In stock, LCSC C835051 \\
\texttt{sys\_clk} oscillator & SiTime SiT9122 family, 310.077\,MHz & \prelim\ --- needs custom-programmed order \\
\bottomrule
\end{tabular}
\caption{Core BOM. Power-tree components (VCCINT/VCCAUX/VTT regulators) are real candidate parts, not yet individually stock-confirmed --- see \texttt{docs/BOM.md}.}
\end{table}

%======================================================================
\section{Open Items}
\label{sec:openitems}

\begin{itemize}
    \item Physical board fabrication is underway (N8 configuration,
    unmodified); no real electrical/thermal measurement exists yet.
    \item \texttt{sys\_clk} oscillator requires a custom-programmed order
    (no off-the-shelf SKU at 310.077\,MHz).
    \item Power-tree regulators (VCCINT, VCCAUX, VTT, per-bank VCCO) not
    fully sourced/stock-confirmed.
    \item \texttt{sys\_rst} pin location is tentative, pending final
    board layout.
    \item ESP32-side firmware (JTAG bit-banging, SPI host driver,
    result-readback address translation) not yet written --- software
    work outside this document's scope.
    \item Real, direct sustained-throughput benchmark (vs.\ a real
    ESP32-S3 baseline) not yet performed at either N8 or N16 --- see
    the project's Tests \& Timing chapter for the honest historical
    comparison context and a real, clearly-labeled projection for N16.
    \item Whether to adopt N16 (real, functionally verified, timing-
    closed on the \texttt{n16-timing-closure} branch) for a future
    board revision is a real, open hardware decision, not an RTL
    blocker.
\end{itemize}

%======================================================================
\section{Revision History}

\begin{table}[h]
\centering
\begin{tabular}{lll}
\toprule
\textbf{Rev.} & \textbf{Date} & \textbf{Description} \\
\midrule
1.0 & 2026-09-21 & Initial preliminary datasheet, N8 configuration \\
1.1 & 2026-09-22 & N8 re-verified with a real MAC-pipeline fix
(WNS improved 0.000\,ns $\to$ +0.108\,ns); N16 now also real,
functionally verified, and timing-closed (WNS=+0.269\,ns) on a
separate development branch, documented as a real candidate for a
future board revision \\
\bottomrule
\end{tabular}
\end{table}

\end{document}