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Prying Eyes: Design trade-offs in Moto 360

09 Jun 2015  | Brian Dipert

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Let's now look more closely at the rear housing. We already knew that it encompasses the photoplethysmography sensor. But as you may also already be aware, the Moto 360 is a wireless-only charged device, specifically via the Qi inductive power transfer standard. Peel away the sticker which shields the circuit board from any misbehaving energy, and the wireless receiver coil underneath is exposed for your perusal.

Here's its transmitter coil companion, in the charger dock. The Moto 360 fits snugly against it and it's micro-USB interface-equipped. Motorola included a wall wart to power it, but you can always alternatively make do with a micro-USB to USB cable in combination with an USB-output wall wart, multi-port charger, or sufficiently powerful computer USB port.

After separating the battery from the system board, it's finally time to examine the latter in greater detail. In the upper right corner is Texas Instruments' TMS320C5545 fixed-point DSP, which I suspect is present to handle the smart watch's "Ok Google" speech recognition capabilities. To its left is a mysterious IC labelled "WL18G/31/46C1VRI $N," which as it turns out is a Texas Instruments-sourced wireless transceiver module, handling Wi-Fi, Bluetooth, and Bluetooth Low Energy protocols.

Let's now look at the two large ICs below the TMS320C5545-plus-WL18G combo, and extending down the centre of the board. The upper, labelled as 2SB28 D9QRM, is actually a Micron Technology MT46H128M32L2KQ-5 IT 4 Gbit mobile LPDDR SDRAM. The lower is a Toshiba THGBMAG5A1JBAIT 32 Gbit NAND flash memory, with an e-MMC interface, implementing the smart watch's 4 GB of resident O/S, application, and data storage.

Next, let's peruse the ICs along the right side of the PCB. Next to the Micron SDRAM is Solomon Systech SSD2848K1 (PDF) display controller, implementing the MIPI interface protocol in driving the LCD. Below it is Atmel's MXT112S capacitive touchscreen controller, supporting the LCD's touch interface facilities. And below it (and directly next to the Toshiba flash memory chip) is Texas Instruments' AFE4490, which acts as an analogue front-end companion to the previously mentioned pulse oximeter sensor.

Finally, let's peruse the PCB left-side ICs. In the upper corner of the Micron SDRAM is a Texas Instruments 1211A1 USB 2.0 PHY transceiver. Its presence is admittedly a bit baffling to me, since the Moto 360 offers no to-outside-world USB connectivity, but perhaps one or more of the smart watch's internal sub-systems requires a USB interface. Below it and directly to the side of the MT46H128M32L2KQ-5 IT is Texas Instruments' TPS659120 (PDF) power management unit. And to the side of the Toshiba flash memory is Texas Instruments BQ51051B, which interfaces with the Qi wireless power receiver for battery-charge management purposes. If you've been keeping a running tally, you've undoubtedly already noticed what a bag o'chips design win the Moto 360 represents for TI!

Over on the far left of the PCB are two other ICs, whose function begs for more explanation. The shiny one at top is Wolfson Microelectronics' (now Cirrus Logic's) WM7132 MEMS microphone, with a bottom-side sound input port location (whose ambient-air access scheme will become clear shortly). Its companion is the WM7121, with a topside port that you can see if you look closely. The two microphones work in a tandem "array" arrangement (that I've been writing about for years), which both allows the DSP connected to them to "localize" a desired sound source and, in the process, suppress spurious ambient noise.

Their shared access to the outside world consists of a small hole in the left side of the Moto 360 (opposite the watch's sole right-side button), shown above and which I admittedly didn't even notice in my unit until after I'd learned where the microphones inside could be found. I'm guessing that a membrane behind the hole allows environmental sound vibrations, but not moisture or dust, to pass through. But speaking of membranes, note that the Moto 360 is silent; there's no speaker inside, only a tiny motor for vibration, which along with the display constitutes the sole means of user communications.

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