bodybytes-hardware

Hardware design files and reference material for the Bodybytes implantable WiFi router, a custom flexible PCB built around the MediaTek MT7628AN.

Firmware, OS, and provisioning documentation live in the sibling bodybytes-firmware repository, see that repo if you're looking for anything software-related rather than hardware design.

• A WiFi-capable microprocessor manages the operating system.
• On boot, a sensor can trigger the device to enter a read-only recovery bootloader, which lets the user roll back the OS and configure the device for use cases beyond a simple WiFi file server.
• User-accessibl memory is the primary storage medium for both the OS and user data (documents, pictures, video).
• Bodybytes uses wireless power delivery, with capacitive energy storage buffering brown-outs from power-draw spikes and temporary coil misalignment. A battery-powered option is possible but not planned at this time.

Bend radius

The only copper traces transiting the bend are the two for the Qi receiver coil, both on layer 2. The routing in this area incorporates an accordion strain-relief topology. PCBWay's flexible PCB specifications indicate that a polyimide flex with a single copper layer can safely bend at a radius 3-6 times the PCB thickness - with a 0.3mm thick PCB, a 1mm bend radius should be safe to let the coil adhere to the ferrite sheet.

MT7628 variants

Bootstrapping pins (MT7628AN datasheet section 2.4)

DDR2 RAM: DDR2 does not support write leveling to accommodate the skew introduced by the fly-by routing topology.

eMMC: the reset pin is connected to MDI_TN_P1 (pin 42) to overcome fault conditions. Vccq IO voltage is 3.3V, which does not allow HS200/HS400 speeds - but the MT7628 doesn't support those speeds either, so this isn't a limitation in practice.

Antenova SR4W035 Serica, 2.4 GHz SMD chip antenna (see Specifications). See bodybytes-firmware/docs/wifi.md for the RF path and calibration profile.

Coil configuration

The wireless charging coil is a 4-layer continuous PCB trace, on the same flexible PCB as the rest of the components. Assembly folds the coil section over the main PCB; a ferrite sheet is adhered to the bottom of the main PCB, and the coil is adhered to the top of the ferrite sheet.

Coil properties (Bodybytes v1.0)

Coil capacitor calculations (BQ51013C datasheet, page 30)

C1 = 1 / ((fS * 2pi)^2 * Ls')
C1 = 1 / ((100kHz * 2pi)^2 * 66.5uH) = 38.1 nF

Selected: ~39nF

C2 = 1 / ((fD * 2pi)^2 * Ls - (1 / C1))
C2 = 1 / ((1MHz * 2pi)^2 * 21.15uH - (1 / 39nF)) = 1.20 nF

Selected: ~1.2nF

Page 31 of the BQ51013C datasheet specifies 47nF, 470nF, and 10nF for the COMM/CLAMP/BOOT capacitors respectively. The COMM capacitor could be bumped down to 22nF to weaken power-contract negotiation fidelity while increasing power-transfer efficiency.

Power supply system

The BQ51013C Qi v2.0 receiver negotiates a power contract with a Qi-compliant transmitter and generates a 5VDC output. Designs with similar coil configurations have supplied 500mA continuously without significant voltage droop, though transmit/receive coil alignment is critical for consistent operation. Qi charging losses manifest as heat in the receiver, which is a concern in the hermetically sealed environment of an implant.

The TPS62442 dual step-down converter is configured for a 3.3VDC output powering most of the system, plus a 1.8VDC output for several subsystems. The MT7628's integrated power supply provides 1.2VDC for the digital core. The COMP/FSET resistor (45kOhm) selects compensation setting 2 with spread-spectrum clocking disabled, weighting the operating frequency toward the maximum (4MHz) to increase efficiency and reduce ripple/heat in the low-value switching inductor (0.47uH). SSC was deemed unnecessary since output noise isn't a significant concern and dithering could introduce unforeseeable complications with surrounding circuitry.

MT7628AN current consumption

Two external sensor types are redundantly connected to a single GPIO on the MT7628 (MDI_TP_P1, pin 40) so the user can force the device into the recovery bootloader on power-up.

IR photodiode: human skin has a light-absorption window around 1000nm. Würth's WL-SDCB series photodiodes have a good spectral response around 940nm (1.8uA/mW/cm^2 photocurrent) over a roughly 250nm window (800-1050nm). Solar spectra from NREL put average irradiance in that window at 0.07 mW/cm^2/nm, or 17.5 mW/cm^2 across the window. Published transmission data shows ~16.5% of light passing through 5mm of skin at 870nm; assuming thinner skin and higher transmittance at 940nm, ~30% is assumed to reach the sensor (5.25mW/cm^2).

Candidate IR LEDs emit 5-6 mW/sr at 20mA, equivalent to 5-6 mW/cm^2 at 1cm. The MT7628's 3.3V GPIO needs to read below 0.8V for low and above 2V for high. Sunlight would drive the photodiode's I_P to 9.5uA; targeting 3.1V output in sunlight needs a 22kOhm resistor with a 3.3V supply, and reaching 0.8V through that resistor needs 120uA, requiring 66.6mW/cm^2 at the photodiode - accounting for skin attenuation, that means ~200 mW/sr from the IR LED. 15412094A3060, 15412094A2070, and 15435394A9050 had the highest mW/sr/mA among candidates evaluated.

Hall-effect sensor: a surface-mount omnipolar Hall-effect sensor sits at the board edge near the WiFi antenna. A magnetic field strength calculator determined that a 3x1mm N52 magnet generates 13mT at 5mm from the sensor. The DRV5032FC Hall-effect sensor (Texas Instruments) was selected to avoid erroneous triggering from environmental magnets or the Qi wireless-charging field.

Background and design-journey notes that shaped v1.0 but aren't part of its current spec.

Bodybytes is an implantable WiFi router with software the user can configure and update. The Pegleg was a previous implant with similar goals, but it ran the now-defunct PirateBox software, couldn't be updated or configured over the air, and was built from off-the-shelf development boards like the Raspberry Pi Zero W. Bodybytes is a custom PCB designed from the ground up for implant performance and usability.

Before the custom PCB existed, a VoCore2 development board - which uses the same MT7628AN SoC - served as the hardware reference and software bring-up platform. Early bring-up work on VoCore2 also characterized power consumption and file-transfer throughput as a baseline for the custom board.

VoCore2 remains useful today as a lower-risk development board for firmware bring-up; see bodybytes-firmware/docs/vocore2.md for how it's used as a development proxy, and the Reference Docs/vocore2/ directory in this repo for its schematics and PCB files.

Proposed changes and open considerations for later hardware revisions, none of these are part of the current v1.0 board.

• Companion charging device: a separate device, worn on a strap enclosing the install location, incorporating a wireless-power transmitter with support for multiple charging coils and a sensor trigger to initiate the recovery bootloader. Intended to improve charging efficiency/reliability over a hand-aligned transmitter and reduce receiver heat in the sealed implant environment.
• Add a pull-up resistor to SPI_MOSI for reliable 4-byte addressing of the NOR flash.
• Remove the pull-down resistor R9 on UART_TXD1, because the eMMC SDIO bus does not work when JTAG is enabled by this strapping pin. Instead, expose it as a testpoint to be driven during flashing.
• Possible hardware write-protect on the NOR flash.
• Increase the size of C42 (BQ51013A rectifier capacitor) to decrease derating at the 20V limit.

PJabbs, Mayhem, mei&, SilentPotato, Satur9

• Design-for-test guidelines (XJTAG)
• Choosing power supply ICs for DDR memory
• DDR memory layout design rules and considerations
• Board design guidelines for DDR2 memory (Analog Devices)