Segmented Displays Enable a Variety of Smart Surfaces
Because they are ultra-thin, rugged, and flexible, high-contrast segmented electronic-paper displays can be placed in a variety of devices.
by Joanna Au and Shigeaki Kawano
ELECTRONIC-PAPER DISPLAYS (EPDs) made popular by applications such as electronic books and newspapers are also enabling a host of new applications. Not only will electronic paper using electrophoretic technology eventually become a good alternative to liquid-crystal displays (LCDs) for specific products or applications, they will help enable new products and applications, such as smart cards, and new concepts for wrist watches and display-enabled key fobs.
Electrophoretic displays made of plastic are ideal solutions in applications where a glass-based display is not viable. EPDs can endure physical stress while retaining display content with very low power requirements. EPDs, such as those based on E Ink's SURF (segmented, ultra-thin, rugged, and flexible) technology are characterized by their low-power requirements, high visibility, ruggedness, readability in sunlight, and low profile. A number of applications characterized by "more-with-less" aspects recently have become possible by using this segmented-display technology in conjunction with a series of microcontrollers (MCUs) and drivers from Epson.
In contrast to active-matrix displays used in electronic books, which can display any image, segmented displays are low-information-content displays with a pre-determined set of images designed for a target application. Segmented displays are driven from a single wire per segment, similar to segmented alphanumeric LCDs. Possible applications include battery-powered health and capacity meters (for products involving battery life and memory storage); electronic shelf labels; hand-held data collectors; and the aforementioned wrist watches, bi-directional key fobs, and smart cards.
Having a thickness as thin as 330 μm,1 EPDs can fit almost anywhere. The plastic construction provides a shatterproof display with conformability and flexibility for a portable ruggedness. Displays can also be non-rectangular in shape, allowing designers to create more innovative products.
Among the first such products to highlight the benefits of EPD technology were a series of watches developed by Seiko (Fig. 1). Approximately 1000 watches were sold in 2006 as limited editions. Because of their ultra-high contrast without the need for backlighting (the display is readable even in low-ambient-light conditions), innovative curved surfaces, and low-power MCUs and EPD drivers, these products are a good example of the possibilities inherent in a fusion between EPD and semiconductor technologies.
Fig. 1: EPDs, custom IC drivers, and low-power MCUs made it possible to create an innovative design for Seiko's bracelet-style wristwatch for women. The black-and-white display runs along the exterior of the band and is capable of displaying the time in "efficiency" mode with easily recognizable numerals or in high-concept "mystery mode," as shown here. In mystery mode, the numerals appear in script in somewhat mysterious patterns. For example, the time shown here is 12:58, reading from the lower left-hand side of the display upwards (the "1" is only partially visible at the left) and right to the "8". Image courtesy of Epson.
Power Considerations of Segmented EPDs
Calculating the power consumption for a segmented-display solution depends upon a number of variables including the size of the display, display-switching duty cycle, choice of electronics, and the use of available low-power and sleep modes.
One of the key features of EPDs that enables low-power applications is their ability to maintain an image when disconnected from a power source. Because the display only requires power while the image is changing, the usage model for the device will have an impact on the power consumed. An in-store point-of-purchase sign that updates every 2–3 sec to grab the attention of shoppers will have considerably different power requirements than an electronic shelf label that is updated once daily with a product price. To take even further advantage of the image stability, the controller chips and driver electronics can be shut down or kept in a low-power mode until the next display update.
Next, consider smart-card applications. The use of smart cards has been growing as institutions take aim at reducing the fraud that can occur with conventional credit cards. Demand for display functions has therefore emerged as the next level of improvement.
A standard six-digit seven-segmented EPD designed for smart-card applications1 has an active area dimension of 20.3 mm x 6.1 mm. The active area defines the maximum switchable area of the display cell. An EPD consumes about 1 μA/cm2 with a typical 240-msec pulse. Driving the entire smart-card display to white or to black with a 240-msec pulse would draw 1.24 μA at 15 V, or 18.6 μW. Using this number provides the worst-case scenario for updating the smart-card display. To derive more accurate calculations, the area for all six digits of the display is 51.0 mm2. Updating only the six numbers will consume 0.51 μA of current at 15 V, or 7.65 μW of power.
One-time-password (OTP) smart cards are becoming popular because they offer an extralayer of security. These cards contain an IC that generates a continually changing checksum. The owner activates the card, usually by pressing a button, causing the current checksum to be displayed. When the user enters this checksum into a Web page or ATM terminal, the host system can verify that the user is actually in possession of the smart card at the time of the transaction. Unlike a memorized password,the checksum is only valid for a minute or two, so a potential thief cannot gain access to the account by simply snooping on the password.
Evaluating a typical display usage model for an OTP card, the display starts off with a fully white display. When a button is pressed, a six-digit number is updated to the screen and is held there for approximately 30 sec, long enough for the user to transfer the number. After the 30 sec are completed, the numbers are erased to white and the display flashes black and then white. In this example, the whole cycle takes about 30.96 sec. Writing and erasing the six digits takes about 1.6% of the transaction cycle, and the black/white flash at the end is also 1.6% of the transaction cycle. During the remaining 96.8% of the cycle, the display draws no power. Averaged over the entire write-erase cycle, the power draw of the display alone is 0.42 μW per transaction.
Power Considerations for an EPD System
The display is only one component of the total display system. An ultra-low-power display should be matched with ultra-low-power electronics for optimal performance.
For the Seiko watches mentioned above, a dedicated EPD driver featuring low power consumption to take advantage of EPD features was developed. Drawing on accumulated LCD driver and MCU technologies, a high-voltage process, capable of withstanding approximately 20 V, was combined and optimized with a low-leakage 20-V processor. An integrated booster circuit (DC-DC converter) was added that reduced the conventional current consumption of the power-supply circuit when active (unloaded) down to around 5 μA. By utilizing the characteristic image stability of EPDs, operating voltage and power consumption for the driver power supply could be removed when not in use. The commercialization of the EPD watch, which required frequent display switching, was enabled by these low-power-consumption technologies.
MCU, EPD driver, and real-time clock (RTC) technologies are available for the smart-card market, including a conventional 4-bit MCU2 series and a recently introduced 16-bit MCU,2 both featuring low power consumption. RTCs consuming just 160 nA lead the market in low power consumption, based on a newly developed low-voltage oscillator circuit. OTP cards are an example of a product that utilizes these technologies. A typical system diagram for one of these cards is shown in Fig. 2.
Fig. 2: A one-time-password (OTP) card diagram shows the relationship between MCU, EPD driver, and EPD. Source: Epson.
From the battery, 3 V of power is supplied to the MCU, RTC, and EPD driver. The MCU uses 32 kHz and 1 MHz for the clock. The EPD driver drives the six-digit display described in detail earlier. The MCU and RTC transmit data bi-directionally. The EPD driver is controlled by the MCU, and input buttons connect directly to the MCU.
When the card is in use, the power consumption will be 310 nA, the sum of the MCU halt current of 150 nA and the RTC power consumption of 160 nA. By combining the MCU and the RTC on one chip, it is possible for both to share circuits, enabling a reduction in power consumption. When the password is displayed by pressing a switch (6-μA current for 300 msec), the MCU shifts to operating mode, and the 1-MHz clock starts operating to communicate with the other components. The power consumption of the MCU at this time is 600 μA. After the clock starts up, it obtains the time data from the RTC. At this time, the RTC uses 40 μA of current during transmission (about. 300 msec). The calculation and processing time for this data is taken to be 200 msec (this differs depending on the type of calculation). The power consumption when the clock is at 1 MHz is 600 μA.
Next, a 15-V power source is created using the EPD driver's DC/DC converter. The DC/DC converter is set at 3 V x 5 = 15 V. When starting up, charge-up consumes 30 μA for 5 msec (recommended circuit configuration for this EPD driver). The loss at start-up is 5 μA. When 15 V is applied, the display consumes 1 μA, and for the DC-DC converter power current it is five times that at 5 μA (simplified calculation). During the transmission of the 48-bit data (about 60 μsec), 65 μA of power is consumed. For one display, data is transmitted at a minimum of three times; for white display, black display, and discharge. During the display process, white and black take 250 msec, and for all drivers at 0-V output it takes 5 msec. This information above is summarized in Table 1.
Under these conditions, the expected life of a 14-mAh battery would be 5 years if used 10 times a day (erase 10 times) and 2.5 years if used 100 times a day (erase 100 times). This combination of MCU, EPD driver, RTC, and EPD is designed to meet customer needs with respect to repeated use and battery life cycle.
In order to extend battery life indefinitely, a similar display circuit can be built for electronic-shelf-label applications utilizing IC tags, an MCU integrated with a solar-charging control circuit, and the EPD driver. The solar-charging control circuit was originally designed for use in clocks and achieves a low-power-consumption level of 150 nA in Halt mode and 3.5 μA when active (power circuit on), while easily meeting the increased power requirements of the RF-controlled clock functions. Combined with a secondary battery, the electronic shelf labels can be designed to run off ambient store light, eliminating the need to change the batteries.
There are a few drawbacks to EPD technology, including its lack of maturity in the marketplace. Electronic-paper players are few, and an ecosystem and infrastructure around EPD applications has yet to develop. LCDs, for example, have more standard drivers available to them. Presumably, this drawback will fade in time as the market develops. Other drawbacks include voltage requirements: EPDs require much higher driving voltages (> 10 V), even though their current requirements are very low. And last but not least, for the time being, EPDs have slower response times than other display media. However, despite these drawbacks, many of which should be overcome in the future, EPDs offer unique solutions for a variety of novel applications. With their ultra-thin, rugged, and flexible properties, segmented EPDs provide a high-contrast display that can be used on many types of devices. By choosing low-power system electronics to match the low-power features of the display, designers can realize solutions requiring both portability and long battery life.
1Refers to E Ink SURF™ Displays.
2Refers to Epson's MCU, EPD driver, or RTC technology. •