Fullscreen HTML5Med Res (??MB)HTML4

We have recently started using e-paper displays in our projects. We have looked at some modules incorporating them before, but this is the first time we’ve integrated a bare e-paper panel into a design. It was more involved than expected; this article explains what we did to make it work. By Tim Blythman F or our Human Comfort Indicator project, we decided to use an e-paper display to achieve low power consumption with a screen that can be read at any time. e-paper is readable in ambient light, so it does not require a backlight, which could otherwise have a substantial power requirement. These screens might also be called eInk or EPD, where EPD stands for electronic paper display or electrophoretic display. ‘Electrophoretic’ refers to the motion of charged or polarised particles in a liquid medium due to an electric field. The laboratory technique known as electrophoresis involves separating different molecules based on their size and electrical charge. The common factor is the presence of an electric field affecting charged or polarised particles. We reviewed a small e-paper module in the June 2019 issue (siliconchip. au/Article/11668) with a resolution of 200×200 pixels, measuring 1.54 inches (39mm) along its diagonal. It had an IL3820 controller and was configured to use an SPI (serial peripheral interface) bus. We created some demonstration code for the Arduino and Micromite. Display controller ICs for e-paper most operate similarly to LCD and OLED controllers. They have numerous internal registers to configure the device and some RAM (random access memory) that holds a representation of what needs to be displayed. The big difference with e-paper displays is that they are not constantly refreshed, like other display types. Instead, an explicit command is required to perform the refresh. It can take a second or longer, so it needs to be done under the control of the software. Since e-paper needs no power to maintain its display, the proportion of time it spends updating will dictate the total and average power consumption. The downside is that it is difficult to tell the difference between an e-paper device that is working and one that has not updated. Nearly all the e-paper modules we have seen use an SPI interface. Like many LCD and OLED controllers, many can also support I2C or parallel interfaces, although these may not be available due to the circuitry used on the module; they are usually designed for just one interface type. With low power consumption being an important aspect of the Human Comfort Indicator, we found that many modules included circuitry that made this target difficult to achieve, since they often contained unnecessary circuitry that would waste power. Thus we had to base our design on a bare display panel and provide the support Subcapsule addressing enables high-resolution capability Transparent Top Electrode Positively Charged White Pigment Negatively Charged Black Pigment Clear Fluid Bottom Electrode Fig.1: the operation of an e-paper display with particles having different colours, sizes and charges in a clear medium. Source: https://w.wiki/7qVD 66 Silicon Chip Australia's electronics magazine siliconchip.com.au Photo 1: the panel we selected includes a controller IC and a display panel integrated into a COG assembly. It connects via the FFC on the right. Source: www.buydisplay.com/serial-2-9-inch-e-paper-screen-128x296-for-electronicshelf-label-lcd circuitry needed to operate the controller. Fig.1 shows the operation of an e-paper display. With an electrode on each side of the panel, it’s easy to set a pixel by simply setting the electrode polarity. In practice, the electric field is pulsed and reversed several times to ensure that all the particles (which are smaller than the pixels) are not stuck. The type of display shown in Fig.1 uses particles with different colours, sizes and charges moving through a clear medium; some displays may use a coloured medium to further expand the range of colours that can be displayed. There are also variants that use multi-coloured, polarised particles. Since the panel uses electric fields to control the display, the support circuitry involves generating voltages above normal logic levels; the panel we are using spans +20V to -20V. Fortunately, the movement of the tiny particles in the display does not require much current. Controller operation Typically, once the circuit is powered up, you need to initialise the display controller with some configuration commands, then fill its RAM with data indicating what to display. Performing a refresh then sends the data to the panel. For low-power designs, the controller can be put to sleep between refreshes to ensure it uses the least amount of power. This requires it to be initialised again before the next refresh. The refresh process is controlled by a look-up table (LUT). This is effectively a simple program that describes how the panel electrodes are strobed, for how long, and with what voltages. siliconchip.com.au The values in the table depend on the characteristics of the panel. Some controllers can work with two-colour and three-colour panels; they will need different LUTs than monochrome panels. For the module that we reviewed in 2019, the LUT was stored by the microcontroller and sent to the controller over the SPI bus, as is fairly common. Some panels also have OTP (one-time programmable) memory that contain one or more LUT programs, which can be used directly by the controller or automatically loaded. Sometimes the controller is attached to the panel glass using a COG (chipon-glass) process. Connecting traces are made with transparent, conductive ITO (indium tin oxide) material. In this case, the OTP LUT parameters are typically programmed by the panel manufacturer, with the LUT optimised for the specific panel the controller is connected to. Many controllers that we investigated had multiple LUTs, each optimised for a specific temperature range. An onboard temperature sensor allows the controller to automatically pick the correct LUT. This is important because the temperature of the medium in the panel affects its viscosity and thus the rate at which the coloured elements move. Some (but not all) controllers also include LUTs for full and fast refresh. As the name suggests, a fast refresh is quicker than a full refresh, but it may show remnants of the previous display (a phenomenon known as ‘ghosting’). One trick that we saw in some epaper software libraries is to override the internal temperature sensor to make it use a faster LUT. In practice, we found this made a negligible difference to the speed we could Australia's electronics magazine achieve. Suffice to say that there are many factors involved in refreshing e-paper displays. The website www.buydisplay.com sells a wide range of bare display panels of various types, including LCD, OLED and e-paper, plus breakout modules. They also sell many of their items on eBay. Notably, they provide detailed data sheets and sample code for all their modules and panels, so we found it quite easy to get things working. We ordered several panels, breakout boards and modules from them to suit Arduino and Raspberry Pi boards. Many of the e-paper panels are fitted with a 24-pin flat flexible cable (FFC) connector, so they can be interchanged with other panels. With this option, we were able to try several panels easily to see what might work best in our application. Best-laid plans Several three-colour and four-colour e-paper displays are now available, and we thought they might be handy to show different states or conditions. However, these displays can take up to 15 seconds to perform a refresh, which we decided was too long. A multi-colour display consists of particles with different sizes or charges that move at different rates through the liquid layer. Thus, the controller must set each colour in turn and make sure that the other colours are not affected, significantly slowing the refresh process. A full refresh of a multi-colour panel typically involves the display flashing rapidly for a long period before settling on its final output, which would be quite distracting for a device that should sit unobtrusively in a home. Such a long refresh would also tend to use more power than a simpler panel. In the end, we decided to keep things simple and use a monochrome (black and white) display. The ER-EPD029-2B is a 2.9-inch (74mm) e-paper display using the SSD1680 controller IC. The panel has a resolution of 296 × 128 pixels, ample for the information we want to display. This controller has support for two sets of LUTs, allowing either a full or fast refresh without having to tweak any parameters. A 2.9in/74mm display is large enough to be clearly visible, with a 67 × 29mm display area (see Photo 1), and June 2026  67 the panel is quite well priced relative to its size. This controller can support multi-colour displays, but we are using the monochrome variant. There is also the ER-EPD029-2R version that supports a black, white and red display, which looks otherwise identical. e-paper circuitry The glass e-paper panel consists of the controller IC connected to the display matrix. There are 24 lines brought out via a 0.5mm pitch FFC. This is connected to a ZIF (zero insertion force) socket on both the commercial modules and our PCB. As well as providing the SPI interface for communication, there are some extra lines that need to be provided to the controller. Nine of these lines connect to 1μF capacitors and bypass various voltages for the controller. There are a few other components, too. The main reason they are connected this way is that it would not be easy to provide that amount of capacitance on the glass substrate. Fig.2 shows the circuit that is used on the breakout modules that we tested. This is about the minimum needed to support the display panel; the modules also include level-­ conversion circuitry that is not shown, so that the module can interface with Scope 1: the blue trace is the PREVGL line, the red trace is the PREVGH line, the green trace is the VGH line and the yellow trace is the VSL line (all shown in Fig.2) during a 1.5-second refresh period. microcontrollers running at different voltages. The circuit around the Mosfet and inductor is used to generate dual rails with voltages of up to ±20V. Scope 1 shows the measurements on some of these lines during a full refresh cycle on the panel. This circuitry is managed by the controller IC, including the drive to the Mosfet. The GDR line signal (not shown in Scope 1) starts out at around 300kHz with a duty cycle lower than Fig.2: the SSD1680 e-paper controller requires some support circuitry that can’t easily be mounted on the panel. That includes capacitors, a Mosfet and an inductor. 68 Silicon Chip Australia's electronics magazine 10%. Once the voltages have stabilised, the driver starts skipping pulses, presumably to limit and regulate the voltages. The PREVGH line is arranged in the standard boost configuration, with a 0.47W resistor allowing the controller to sense the rising current in the inductor to limit it safely. The PREVGL line is created by coupling the switching node to a point between the two diodes, acting as a charge pump, which brings the PREVGL line to -20V. Several internal regulators supply lines such as VSL (the yellow trace in Scope 1). You can see the various regulated voltages remaining steady despite PREVGL sagging under load. The data sheet for the SSD1680 controller indicates that the voltages are as expected and the internal regulator currents are less than 1mA. We noted that the total 3.3V supply current jumped up to about 5mA, which seems reasonable. With several internal regulators needing input and output bypassing, it’s easy to see why so many capacitors are needed. Fig.2 also shows the six control lines needed. As well as the three pins needed for a unidirectional SPI bus (SCK, MOSI and CS), there is a reset line and a data/command (D/C) selector. This is used to differentiate between commands and display data when addressing the controller. These are all common to other display controllers, such as those used for graphics LCDs. The sixth line is a BUSY signal that the controller drives when it is busy siliconchip.com.au Photo 2: nowadays there are even colour e-paper displays available. Some, like the Waveshare display shown here, only provide a select few colours, while others have access to the full spectrum. Source: www.waveshare.com/product/ raspberry-pi/displays/e-paper/3.97inch-e-paper-hat-plus-g. htm Photo 3: Amazon’s Kindle is one of the most well-known e-paper devices. e-paper devices like the Kindle have very low refresh rates, typically at 10Hz or less. Source: www.amazon. com.au/dp/B0CFPL6CFY refreshing the display and should not be interrupted. Once the refresh is completed, the controller can be shut down by taking its reset line (RES) low or sending a sleep command. While it might appear that the controller supports an I2C interface via the TSDA and TSCL lines, this is not used by a host to talk to the controller. Instead, these pins allow an external I2C temperature sensor to be read by the controller IC. The BS1 line can be used to select a 9-bit SPI mode that removes the need for a separate D/C pin, since the D/C bit is sent as the ninth bit. We have no shortage of pins, and the 8-bit SPI mode is much easier to implement, so that’s what we used. Summary Our experience with the Arduino display module gave us the knowledge and experience we needed to design the hardware and software to work directly with e-paper display panels. While it would have been nice to use a multi-colour display, we did not think they were suitable for our application; thus, we chose the monochrome panel seen in Photo 1. In another situation, such as a price tag in a shop, a slower/flickering refresh would not really be a problem as it would be updated so infrequently. A splash of colour would be nice there (eg, to separate the price from the product name), so colour e-paper displays clearly suit some applications. By designing our own hardware, we have been able to achieve our target of very low quiescent power consumption. You can see the result of this in the Human Comfort Indicator project, SC which starts on page 43. Dual-Channel Breadboard Power Supply Our Dual-Channel Breadboard PSU features two independent channels each delivering 0-14V <at> 0-1A. It runs from 7-15V DC or USB 5V DC, and plugs straight into the power rails of a breadboard, making it ideal for prototyping. Photo shows both the Breadboard PSU and optional Display Adaptor (with 20x4 LCD) assembled. Both articles in the December 2022 issue – siliconchip.au/Series/401 SC6571 ($40 + post): Breadboard PSU Complete Kit SC6572 ($50 + post): Breadboard PSU Display Adaptor Kit siliconchip.com.au Australia's electronics magazine June 2026  69