80 lines
4.1 KiB
Markdown
80 lines
4.1 KiB
Markdown
# Cacao - Multi-Channel Signal Acquisition Board
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Cacao is a compact 12-channel audio-frequency signal-acquisition board built around the RP2350. It explores how modern, accessible components can consolidate multi-channel acquisition, buffering, storage, and Ethernet communication onto a single 80 x 50 mm PCB.
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<!-- Replace docs/images/cacao-render.png with the final board render. -->
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## Overview
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Cacao was developed independently as a proof of concept alongside an industrial R&D internship. The project acts as a technical calling card: a complete architecture, schematic, and dense PCB layout designed to demonstrate what can be achieved with a modern microcontroller and cost-effective audio-conversion hardware.
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Twelve differential audio inputs are digitized by three TLV320ADC5140 ADCs at 44.1 kHz. The RP2350 collects and processes the resulting data before sending it over 100BASE-TX Ethernet through a W5500 or storing it locally on a microSD card.
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## Key Features
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- RP2350 in the 80-pin package with internal flash
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- 12 differential audio-input channels
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- Three TLV320ADC5140 four-channel ADCs synchronized by a 2.5 ppm clock
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- 44.1 kHz acquisition with planned 12- or 16-bit sample handling
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- W5500-based 100BASE-TX Ethernet output
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- 8 MB external RAM for acquisition buffering
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- MicroSD storage for standalone data capture
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- USB-C 5 V power input
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- 3.3 V buck supply for the digital electronics
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- Separate low-noise 3.3 V analog rail using an LP5907MFX-3.3/NOPB LDO
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- Four-layer, 80 x 50 mm PCB with more than 200 components
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- Components on both sides, with most passive components placed on the back
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The Micro-USB connectors are used as compact keyed connectors for the differential analog channels and their 5 V amplifier supply. They are **not USB data ports**.
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## Engineering Focus
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The principal layout challenge was integrating 12 analog channels, three synchronized ADCs, local buffering, storage, Ethernet, and the required power architecture within an 80 x 50 mm footprint. Particular attention was given to separating low-noise analog supplies from the general digital rail, maintaining clean reference and return paths, and placing a large number of passive components without increasing the board area.
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An alternative future configuration could replace the W5500 and external RAM with a W6300 connected through QSPI, reducing the number of separate devices while retaining high-speed Ethernet communication.
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The firmware will have to be as efficient as possible to allow 10-12 channels at the same time to be sampled and transferred to the W5500.
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## Project Status
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V1.0 is currently being assembled by myself and testing will soon start.
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## known V1 issues
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There are a few problems with the V1 of this PCB and they are pretty serious so I would advise not to send any of these files to be produced and to either modify these mistakes or to wait for a V1.1 or V2 of this board.
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### Bad selection of Rp2350 ports
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This is the most embarassing problem of them all. I put a bit too much faith in the MUX capabilities of the rp MCU and choose the SPI and QSPI pins incorrectly.
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The RAM needs to be totally rerouted, 5/6 pins are wrong and need to be changed :
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```
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RAM_QSD3 not to GPIO 16 but to QSPI_SD3
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RAM_QSD2 not to GPIO 17 but to QSPI_SD2
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RAM_QSD1 not to GPIO 18 but to QSPI_SD1
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RAM_QSD0 not to GPIO 19 but to QSPI_SD0
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RAM_QSCLK not to GPIO 20 but to QSPI_SCLK
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```
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This is extremely embarassing as the pins are even labeled, I should have seen this before sending the files to production.
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The W5500 and SD CARD are wired wrong too as even if the selected pins are correct, not all pins of a SPI bank can do every job.
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So you have to do these swaps :
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```
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ETH_CS GPIO28 to GPIO29
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ETH_SCLK GPIO29 to GPIO30
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ETH_MISO GPIO30 to GPIO28
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```
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And for the SD card its all wrong as I did not plan for SPI0 and SPI1 differences.
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I would advise not to botch anything but rather use a PIO SPI that then can be used on pretty much any pins.
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Those mistakes are extremely embarassing but they exist and will have to be dealt with when assembling the boards.
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