325 lines
12 KiB
TeX
325 lines
12 KiB
TeX
\chapter{Hardware Overview}
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\label{ch:hardware}
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This chapter describes the board in detail: the design choices behind it,
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each integrated circuit and how it is controlled, the bus architecture,
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and the inter-chip connections. The complete hardware design ---
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schematics, PCB, Gerbers, and bill of materials --- is published in the
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repository under the CERN-OHL-S v2 licence.
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\section{Design philosophy}
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\label{sec:hw-philosophy}
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Every major choice on the board serves audio quality and
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reproducibility.
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\begin{drkey}[The four-chip split]
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Rather than a single all-in-one radio IC, DigiRadio separates concerns:
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a dedicated tuner for reception, a dedicated DSP for audio conditioning,
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a dedicated Bluetooth module for the wireless codec, and a general-purpose
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host to coordinate them. Each stage can be understood, measured, and
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improved on its own.
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\end{drkey}
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The consequences of that split run through the rest of the board: a
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control host that speaks several buses, a clean power tree that keeps the
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tuner's analogue supplies away from digital noise, and careful RF layout
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so the two 2.4\,GHz radios do not interfere.
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\section{System block diagram}
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\label{sec:hw-block}
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Four devices are coordinated by the ESP32-S3. The tuner produces the
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audio stream, the DSP conditions and mixes it, and the Bluetooth module
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transmits it; the host manages all three and provides the network
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interface.
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}llll@{}}
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\drhead Device & Role & Control bus & Firmware \\
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\midrule
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Si4684 & DAB+/FM tuner & SPI & host-loaded image \\
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ADAU1701 & Audio DSP (EQ + mixer) & I\textsuperscript{2}C & host-loaded RAM \\
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ESP32-S3-WROOM-1 & Host, Wi-Fi/BLE & --- & application FW \\
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FSC-BT1035 & Bluetooth 5.2 (aptX Ad.) & UART (AT) & vendor FW + config \\
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\bottomrule
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\end{tabular}
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\caption{Principal integrated circuits, their control bus, and how each
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receives its firmware or configuration.}
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\label{tab:hw-ics}
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\end{table}
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\section{The tuner: Si4684}
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\label{sec:hw-si4684}
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The Si4684 is a single-chip DAB+/FM receiver front-end. It delivers the
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decoded audio stream to the DSP and reports signal-quality metrics to the
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host.
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\subsection{Control interface}
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The device is controlled through a command/response protocol over its
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host bus. The host issues a command, then polls a clear-to-send (CTS)
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status bit before reading the response; the driver validates that status
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byte before trusting any returned payload.
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\begin{drkey}[SPI for the tuner]
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The tuner needs a large firmware image loaded at every power-up
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(Section~\ref{sec:hw-si4684-fw}). SPI offers substantially higher
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throughput than I\textsuperscript{2}C, so on DigiRadio the tuner uses
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\textbf{SPI}, keeping boot time short, while the low-rate DSP control uses
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a separate I\textsuperscript{2}C bus.
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\end{drkey}
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\subsection{Firmware image upload}
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\label{sec:hw-si4684-fw}
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The Si4684 holds no application firmware of its own at reset. At power-up
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the host loads a bootloader/patch and then the firmware image for the
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desired mode (FM or DAB). These images are large, so the driver streams
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them to the chip in bounded chunks read from the ESP32's flash, rather
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than buffering a whole image in RAM.
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\begin{drref}[Datasheet]
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The exact power-up, patch, image-load, and boot command sequence follows
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the Si468x programming guide (AN649). No command byte is issued without a
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documented source, and each step in the driver cites its section.
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\end{drref}
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\section{The audio DSP: ADAU1701}
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\label{sec:hw-adau}
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The ADAU1701 is a SigmaDSP audio processor. In DigiRadio it performs
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equalisation and mixes the tuner audio with the ESP32 audio source before
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the signal reaches the Bluetooth stage.
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\subsection{Control and program loading}
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The DSP is controlled over I\textsuperscript{2}C. Its signal-processing
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program is designed in SigmaStudio and exported as a sequence of register
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writes (control settings, program RAM, and parameter RAM).
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\begin{drkey}[RAM boot, no EEPROM]
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The board deliberately omits the ADAU1701 self-boot EEPROM. Instead, the
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ESP32 writes the exported program into the DSP's RAM at \emph{every} boot.
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This keeps the audio processing under host control and field-updatable ---
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a new EQ or mixer configuration is a firmware update, not a chip
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re-programming.
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\end{drkey}
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\subsection{Click-free parameter updates}
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Runtime changes --- adjusting an EQ band, changing a mix level --- are
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applied through the ADAU1701 safeload mechanism, which updates parameter
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cells atomically between audio samples. Writing parameter cells directly
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while audio plays would produce audible clicks and is not done.
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\section{The Bluetooth module: FSC-BT1035}
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\label{sec:hw-bt1035}
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The FSC-BT1035 (Qualcomm QCC3056) is a Bluetooth~5.2 audio transmitter
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with aptX, aptX~HD, and aptX~Adaptive support --- the high-resolution
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codecs that make the wireless output hi-fi rather than merely convenient.
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\subsection{Control interface}
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The module is controlled with AT commands over UART, with hardware flow
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control (RTS/CTS). The host sends commands and parses the module's
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responses explicitly, treating timeouts as errors. Some settings are held
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in the module's own non-volatile memory.
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\begin{drcaution}[Line-In mode]
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The initialisation sequence must enable Line-In mode (\texttt{AT+AUXCFG=1})
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so the module accepts the wired audio coming from the DSP. Omitting it
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silently breaks the audio path.
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\end{drcaution}
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\section{The host: ESP32-S3}
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\label{sec:hw-esp32}
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The ESP32-S3-WROOM-1 coordinates the whole system. It loads the tuner and
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DSP firmware at boot, controls all three companion chips over their
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respective buses, serves the configuration web interface, and provides
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Wi-Fi connectivity. Its dual-core processor and generous RAM comfortably
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handle the firmware images and the network stack alongside the control
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tasks.
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\section{Bus architecture}
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\label{sec:hw-buses}
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The host speaks several interfaces, each chosen for its traffic:
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}lll@{}}
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\drhead Bus & Devices & Purpose \\
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\midrule
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SPI & Si4684 & fast firmware-image upload, tuner control \\
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I\textsuperscript{2}C & ADAU1701, 24AA025E48 EEPROM & DSP control, device identity \\
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UART & FSC-BT1035 & AT command / response (with RTS/CTS) \\
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I\textsuperscript{2}S & Si4684, ESP32, ADAU1701, BT1035 & audio (ADAU is master) \\
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\bottomrule
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\end{tabular}
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\caption{Interface assignment and rationale.}
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\label{tab:hw-buses}
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\end{table}
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\section{Power}
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\label{sec:hw-power}
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The board is powered from USB-C. An AP63203 buck converter generates the
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main rail from the USB input, feeding the digital and analogue sections.
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The tuner's sensitive supplies are derived and filtered separately to keep
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RF performance clean.
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\section{RF and antennas}
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\label{sec:hw-rf}
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Two 2.4\,GHz radios are present --- the ESP32-S3 and the Bluetooth module.
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Their antennas are placed on opposite diagonal corners of the board to
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maximise separation and minimise mutual desense. Each module keeps its
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antenna keep-out clear of copper on all layers. The PCB is a six-layer
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stack-up with controlled impedance on the USB and RF sections.
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\section{Inter-chip connections (GPIO map)}
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\label{sec:hw-gpio}
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This section is the authoritative wiring reference between the ESP32-S3
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and the companion chips. The firmware pin definitions
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(\texttt{board\_pins.hpp}) must match these tables exactly.
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\subsection{System and control lines}
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}lll@{}}
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\drhead Signal & ESP32-S3 GPIO & Notes \\
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\midrule
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USB D$-$ & GPIO19 & native USB \\
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USB D$+$ & GPIO20 & native USB \\
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Reset / boot button & GPIO0 & strapping pin \\
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\bottomrule
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\end{tabular}
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\caption{System and control lines.}
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\label{tab:hw-sys}
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\end{table}
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\subsection{Si4684 tuner (SPI)}
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}llll@{}}
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\drhead Signal & Si4684 pin & ESP32-S3 GPIO & Notes \\
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\midrule
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SCLK & pin 30 & GPIO13 & \\
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MOSI & pin 31 & GPIO12 & \\
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MISO & pin 32 & GPIO9 & \\
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SSB (CS) & pin 29 & GPIO8 & chip select, active-low \\
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RSTB\# & pin 4 & GPIO38 & active-low, ext.\ pull-down \\
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INTB\# & pin 3 & GPIO39 & active-low, ext.\ pull-up \\
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\bottomrule
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\end{tabular}
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\caption{Si4684 control and SPI lines.}
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\label{tab:hw-si4684}
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\end{table}
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\subsection{ADAU1701 DSP (I\textsuperscript{2}C)}
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}lll@{}}
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\drhead Signal & ADAU1701 pin & ESP32-S3 GPIO \\
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\midrule
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SDA & --- & GPIO4 \\
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SCL & --- & GPIO5 \\
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RESET\# & pin 5 & GPIO47 (active-low, ext.\ pull-up) \\
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\bottomrule
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\end{tabular}
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\caption{ADAU1701 control (I\textsuperscript{2}C) and reset. The 7-bit
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I\textsuperscript{2}C address is 0x34 (ADDR0 and ADDR1 tied to GND).}
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\label{tab:hw-adau}
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\end{table}
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\subsection{Device-identity EEPROM (24AA025E48)}
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A 24AA025E48 EEPROM shares the DSP's I\textsuperscript{2}C bus. Besides a
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small general-purpose memory, it carries a factory-programmed, globally
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unique EUI-48 identifier, which the firmware can read to give each unit a
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stable identity --- for example a unique Bluetooth name or a device
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serial number. Its 7-bit address is 0x52 (A0 to GND, A1 to 3V3).
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}lll@{}}
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\drhead Device & 7-bit address & Set by \\
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\midrule
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ADAU1701 & 0x34 & ADDR0, ADDR1 = GND \\
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24AA025E48 EEPROM & 0x52 & A0 = GND, A1 = 3V3 (no A2 pin) \\
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\bottomrule
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\end{tabular}
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\caption{I\textsuperscript{2}C address map (shared bus).}
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\label{tab:hw-i2c-addr}
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\end{table}
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\subsection{FSC-BT1035 Bluetooth}
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}llll@{}}
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\drhead Signal & BT1035 pin & ESP32-S3 GPIO & Direction \\
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\midrule
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SYS\_CTL & pin 34 & GPIO15 & ESP32 $\rightarrow$ BT \\
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BT\_CTS & pin 15 & GPIO21 & flow control \\
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BT\_RTS & pin 16 & GPIO14 & flow control \\
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BT\_RX & pin 14 & GPIO40 & ESP32 TX $\rightarrow$ BT RX \\
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BT\_TX & pin 13 & GPIO41 & BT TX $\rightarrow$ ESP32 RX \\
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RESET & pin 8 & GPIO17 & ESP32 $\rightarrow$ BT \\
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\bottomrule
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\end{tabular}
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\caption{FSC-BT1035 UART, control, and reset lines.}
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\label{tab:hw-bt1035}
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\end{table}
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\subsection{Audio bus (I\textsuperscript{2}S)}
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The audio path is a chip-to-chip I\textsuperscript{2}S bus with the
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ADAU1701 acting as master. Table~\ref{tab:hw-i2s} lists the chip-to-chip
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routing; Table~\ref{tab:hw-i2s-esp} lists the ESP32's own
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I\textsuperscript{2}S source lines that feed the mixer.
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\begin{table}[htbp]
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\centering
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\small
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\begin{tabular}{@{}lll@{}}
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\drhead Signal & Path & ADAU1701 / BT1035 pins \\
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\midrule
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SDATA\_IN0 & Si4684 $\rightarrow$ ADAU & MP0 (pin 11) \\
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SDATA\_IN1 & ESP32 $\rightarrow$ ADAU & MP1 (pin 10) \\
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SDATA\_OUT0 & ADAU $\rightarrow$ BT1035 & MP6 (pin 15) $\rightarrow$ pin 5 \\
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LRCLK & ADAU $\rightarrow$ BT1035 & MP4 (pin 8) + MP10 (pin 16) $\rightarrow$ pin 7 \\
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BCLK & ADAU $\rightarrow$ BT1035 & MP5 (pin 9) + MP11 (pin 19) $\rightarrow$ pin 4 \\
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\bottomrule
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\end{tabular}
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\caption{I\textsuperscript{2}S audio routing (chip-to-chip). The
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MP4/MP10 and MP5/MP11 pairs are electrically connected on the board.}
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\label{tab:hw-i2s}
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\end{table}
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\begin{table}[htbp]
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\centering
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\begin{tabular}{@{}lll@{}}
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\drhead Signal & ESP32-S3 GPIO & Direction \\
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\midrule
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BCLK & GPIO6 & in from ADAU (master) \\
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LRCLK & GPIO7 & in from ADAU (master) \\
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SDATA\_IN1 & GPIO16 & out $\rightarrow$ ADAU MP1 \\
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\bottomrule
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\end{tabular}
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\caption{ESP32 I\textsuperscript{2}S source lines feeding the mixer. The
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ESP32 is an I\textsuperscript{2}S slave (clocks in), transmitting
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audio data. SDATA\_IN1 lands on ADAU MP1 (pin 10); GPIO16 is the
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ESP32 module's physical pin 9.}
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\label{tab:hw-i2s-esp}
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\end{table}
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\subsection{Completeness}
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\label{sec:hw-gpio-open}
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\begin{drnote}[Map complete]
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The GPIO and I\textsuperscript{2}C maps are complete and match
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\texttt{board\_pins.hpp}. No values remain open.
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\end{drnote}
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This section is the authoritative wiring reference for firmware bring-up:
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the pin definitions in the source code must match it exactly.
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