By Leland Teschler
Executive Editor
We recently finished up a tear-down video on the widely noted wearable technology called Google Glass. You can watch the video here. And spelled out below is a run-down on what we found inside.
Google Glass is a type of wearable technology that includes an optical head-mounted display. Google glass displays information in a smartphone-like hands-free format and lets its wearers communicate with it via voice commands. It’s also got a touch pad so users can interact with it in a way analogous to swiping the screen of a smartphone. A built-in camera records video and images. Finally, a liquid-crystal-on-silicon display generates images that users see by means of beam splitters and reflecting mirrors.
Google glass is not designed to be disassembled but its plastic case can be separated at the seams – the result is a fair amount of cosmetic damage. A single Torx screw attaches the plastic cases holding the electronics to the titanium frame. With the plastic cases are off the frame, the disassembly process involves first prying the case off the prism assembly, then working back toward the housings for the main circuit board and battery.
Battery module
It might be easiest to understand Google Glass by starting with the simplest component, the behind-the-ear module. This is where the battery resides. It is a 3.7-V single-cell lithium polymer battery with a capacity of 570 mAh. It is not designed to be replaceable. It can’t be accessed without cutting into the plastic case it sits in.
The battery connects to the rest of the circuitry through a flexible circuit. Behind the flex circuit connection and the battery is a bone conduction speaker – basically a coil vibrating a metal plate – which also seems to work as a pushbutton switch.
Main circuit board
The flex circuit from the battery connects to the main circuit board residing in the next plastic module. One side of the main board contains the power on/off button. The object next to it is the connector port for the recharging cable.
The main component of note on this side of the board are the wifi transceiver made by Universal Scientific Industrial Corp. in Taiwan. It’s basically a wifi/Bluetooth transceiver on a chip. It’s also based on a Broadcom 4329 chipset.
The next item of interest here is an audio codec, this one from Texas Instruments. An audio codec basically converts analog audio signals into digital signals for transmission. This one also decodes digital signals back to audio. It has five audio input channels and, also of interest for Google Glass, it has drivers for running vibration coils as is used for the bone conduction speaker.
One other component we noticed on the main board was a 435 Series fast-acting surface mount thin-film fuse. This one is from Littelfuse. There’s a marking code on its top surface that seems to indicate this one is rated to handle 2 amps at 35 V in normal operation. These fuses are obviously tiny which is why they are often found in applications such as hand-held portable electronic devices. They are designed to open
in less than 5 seconds when they see a 200% overload. A 200% overload for this device would be something like 4 amps, and if you look at the time-to-open curve on its datasheet, you see that on average, it will open up in a few tenths of a second at that level.
Of course, you hope that never happens. If this fuse opens, it stays open. In that case, the Google Glass will no longer work. We can’t discern from the circuit board what this fuse is protecting, though it sits near the USB charging port, so it may be protecting against a short during battery charging.
Finally there is a MEMS digital microphone mounted on the main board. But it apparently is not the mic Google Glass uses to input user commands or audio because there is another microphone up near the prism and camera assembly that seems to be in a better location for picking up conversations.
It is tough to tell this component is a microphone because its audio pick-up port is on its underside, out of view. So the mic just looks like a metal can. There has been speculation in the electronics community that the mic on the main board is just used for noise canceling. But the board is fully enclosed in the plastic case, and so is the microphone. They only noise this mic would be able to pick up well would be the noise of a finger tapping or swiping the capacitive touch pad. Thus it is possible the mic is there to cancel out finger noise. But of course, this is just speculation.
Turning to the major components on the other side of the main board reveals a GPS receiver, made by Cambridge Silicon Radio. It is based on an ARM7 processor, as are a lot of smartphone style products. It also uses a sensor which doesn’t reside on this board. The sensor actually sits on another flex circuit extending into the hinged portion of the device where the display, camera, and main microphone all reside.
One large chip found on this side of the board is a 16 Gb NAND flash memory chip from Toshiba. This chip also incorporates a controller that basically acts as a memory manager, doing things like error correction, managing bad blocks, garbage collection, and translating logical addresses into physical addresses. Again, this is a flash memory chip so it is non volatile. And because it is a NAND memory, it can be written and read in blocks or pages.
Next to it is flash memory chip is a synchronous DDR2 DRAM memory chip holding 512 megabytes. (As a quick review, DDR stands for double data rate, which comes from the fact that the chip transfers data on the rising and falling edges of the bus clock signal.) This chip is what’s called the mobile version, meaning it consumes less power than conventional DDR2 chips. One reason is that it works from a 1.8-V supply rather than 2.5 V. Also, it refreshes less frequently than ordinary DDR2 under a couple of scenarios, while sitting in a chip package that’s smaller.
The synchronous DRAM is apparently integrated with the main processor in Google Glass which is an OMAP 4430 from Texas Instruments. It is a dual-core processor that uses the ARM Cortex-A9 architecture. Among the features that of particular interest for the Google Glass application are a built-in multimedia processor that comes in handy for handling video, and an image signal processor for handling screen graphics.
The small chip near the processor is an FPGA. It is from Lattice Semiconductor. It is a 1-kb mobile device and, notably, is also used in the Samsung Galaxy 4S smartphone.
Also on this side of the board is the power-management IC for the OMAP processor. It is another Texas Instruments device. It’s specifically designed for applications powered by a rechargeable battery. It’s got seven step-down converters that provide up to 2.0 A to power the memory, the processor core, I/O, and so forth. It includes a real-time clock that can provide second, minute, hour, day, month, year information, and an alarm wake up. This is also the device that handles battery charging by virtue of a built-in switched-mode charger.
Finally, there is another mechanical switch on the edge of this board with an unknown function. It does not seem to be mentioned in any of the tutorials Google has produced so far.
Touch pad
Moving to the circuit board that controls the touch pad, the main component is the capacitive touchpad controller. It is from Synaptics. The capacitive touchpad itself is laid out on the back of this circuit board. The touchpad contains electrode layers that are basically traces of conductive material. Atop the conductive traces sits an insulator layer which is usually called lens material in the parlance of capacitive touchpads. Each conductive trace has a baseline capacitance. When a finger touches the insulator layer, it changes the capacitance of the conductive traces in its vicinity. The Synaptics chip measures the change in capacitance to note the position of the finger and determine whether you are tapping, swiping, and so forth.
Video camera, display, and sensor module
A complex flex circuit connects the main logic board to an assembly mounted on the other side of the pivot hinge that lets the user move the display into a position that’s comfortable. The assembly contains the display and prism, a camera, a microphone, the gyroscope chip, and an ambient light sensor. The microphone is the same kind of MEMS digital device found on the main circuit board. The digital nature of it comes because it incorporates an analog to digital converter so the output of the microphone is a serial digital bit stream, not an analog audio signal. As in the case of the mic on the main circuit board, it is difficult to tell this is a microphone because the microphone port is on the underside. From the top it just looks like a metal can of some kind.
The flex circuit also contains a nine-axis InvenSense inertial sensor. The inertial sensor is a combo 3-axis gyroscope, 3-axis accelerometer, 3-axis digital compass, and an onboard digital motion processor. The flex circuit wraps around and through the frame to make a connection with the combination prism, display chip, and camera. The other components visible on the flex circuit are super-high-density connectors..
The glass display, the prism, and the camera are all in the pivoting section. The display chip is very small. The display chip is glued into the prism housing so that the backside of the display chip is visible initially.
Careful removal of the glue lets the display chip unseat from the housing. Google says the display chip has a native resolution of 640×360 pixels. And the pixels are roughly one-eighth the physical width of those on the iPhone 5 retina display. The Glass camera seems to be the kind common in typical smartphones. A final point to note is that the camera and display are on separate connectors. So they appear to talk to the main circuit board independently, with no direct connection between them.
All in all, as in any teardown, we’ve had to make some educated guesses about the functions of some of the components and about the rationale behind some of the design decisions. Also, we found some differences between what other Google Glass analyses found and what sat on the circuit boards of our unit.
For example, the GPS receiver chip on our unit was mounted on the second side of the main circuit board. Other teardown results posted on the web have found this chip attached to the opposite side of the board and made by a different vendor.
Thus though it is a relatively young product, there have already been revisions in Google Glass’ circuit boards. It’s probably likely that the circuitry will continue to evolve. Teardowns conducted in the future may find configurations and parts that differ from what we discerned in our analysis.
Design World