Casio Tips Its Hat to Renesas for Its Battery-Less, Solar-Powered SmartwatchMarch 19, 2020 by Gary Elinoff
The secret sauce? Renesas' silicon-on-thin-buried-oxide (SOTB) process technology.
Renesas has announced that Casio Computer Co. has chosen Renesas' RE01 controller as the primary controller of Casio's new shock-resistant smartwatch. The GBD-H1000, a new member of Casio's G-SHOCK line of smartwatches, is said to function on harvested energy. It also features a heart rate monitor and GPS functionality.
The GBD-H1000 measures its wearer’s heart rate by means of an optical sensor. Other sensors measure temperature, compass bearing, and barometric pressure. The watch also contains technology that measures distance traveled.
Casio's G-SHOCK watch and Renesas’ RE family controller. Image used courtesy of Renesas
Even with all these capabilities, Casio claims that the new watch needs no battery since it can be completely powered via solar power. The secret to this power-efficiency feat? Renesas says it's their SOTB processing technology.
What Is SOTB Process Technology?
Renesas’ RE01 Group products, the “brains” animating Casio’s new watch, are extremely low-power devices based on silicon-on-thin-buried-oxide (SOTB) process technology.
In semiconductors, large geometries serve to reduce leakage current; less power is wasted when the device is on standby. But, larger geometries mean that more power is consumed when the device is active. The trade-off is illustrated below.
Low active current vs. low standby current—the usual trade-off. Image used courtesy of Renesas
Renesas' SOTB process technology lets designers “have their cake and eat it too,” benefiting from the low leakage currents of larger geometry node size while having the low active current typical of smaller silicon geometries.
Silicon-on-thin-buried-oxide (SOTB) process technology. Image used courtesy of Renesas
As illustrated above, the “secret sauce” is a thin insulating oxide film ("BOX," or buried oxide) deposited on top of the base silicon substrate. Then, an extremely thin silicon layer with no impurities is deposited on top of the BOX layer forming a dopantless channel transistor under the gate, enabling very low operating voltage.
The small geometries, as expected, also tend to enable SOTB-based devices to operate at high frequencies.
IC designers will also be happy to learn that they can combine SOTB on the same substrate as standard silicon processes.
Renesas’ RE01 Product Family
In addition to watches, Renesas’ RE01 product family is applicable anywhere where high performance and low power consumption are mandatory. These applications may include healthcare, security, home appliances, metering, building automation, IoT, and of course, wearables.
Because the glory of these devices is the ability to operate sans battery. Instead, they are equipped with an on-board energy harvesting control circuit.
The RE01 Group devices can operate at 64 Mhz with a 1.62 V power source and require as little as 4 µA. All are based on the ubiquitous Arm Cortex M0+ core. They include a parallel MIP-LCD interface, 2D graphics, 1.5 MB flash, and 256 KB SRAM. Units can handle data logging and firmware updates with security functions including a trusted secure IP.
Block diagram of RE01. Image used courtesy of Renesas
Three family members are now available with three more under development.
The three available at present are the R7F0E017D2DBN, the R7F0E015D2CFB, and the R7F0E011D2CFP. The R7F0E017D2DBN measures 4.47 mm by 4.27 mm while the others are significantly larger. It must be assumed, then, that the R7F0E017D2DBN is the one used in the Casio watch.
Renesas offers a marvelously detailed datasheet. There are some naming inconsistencies in the document, but they should be of no consequence to designers.
With or Without Batteries
Renesas offers a guide as to how its RE01 devices can form the basis of a smartwatch or wearable device.
Wearable and or hybrid watch example using RE01. Image used courtesy of Renesas
As can be seen, a provision is made for an optional battery.
The RE01 1500KB evaluation kit. Image used courtesy of Renesas
Included in this kit are a mainboard, solar panel, USB cable, and expansion board.
Wearables Powered by On-Board Solar Power—Really?
The promise has been made by others before, and it has sometimes fallen flat. Engineers, do you think SOTB technology will make this technological feat a reality this time?
Do you have experience with solar-powered wearables? If you're able to share your experience, let us know about the ins and outs of designing such a device in the comments below.