Electrophoretic Display Controller
The relatively new Electrophoretic display technology has advantages over other types of display in many applications and enables entirely new products, such as the e-book, to meet customer expectations. This display has high contrast and needs no backlight, therefore being legible in a wide range of lighting conditions. Furthermore, it consumes no power when it is not being updated. Industry is presently discovering new applications for this technology. ECROS Technology helped evaluate several electrophoretic displays from SiPix to see whether they would meet a customer's needs.
The customer required a single controller that would connect to a SiPix display of a particular type. Two displays were to be custom build by SiPix, one being an eight-character, seven-segment alphanumeric display and the other having a small number of graphical icons. The controller was to provide power from a USB port, a single-cell, low-voltage battery or some other higher voltage battery, whichever happened to be connected. Operating parameters were to be adjustable by simple commands using a serial interface running over a USB port. What was displayed on the alphanumeric display was also to be set over the serial interface, but the other display was to run an animation sequence of the icons.
Updating an Electrophoretic Display
Each pixel or segment of an electrophoretic display consists of an isolated zone of fluid in which particles are suspended. By applying an electric field between electrodes, the particles can be made to migrate to the front, where you can see their color, or to the back, where you see the color of the fluid. In principle, updating such a display involves simply applying the appropriate voltage between each segment electrode and the common electrode until the migration is complete and then removing the voltage.
What complicates the update is the need to get all white segments equally white and all black segments equally black, no matter what their state before the update. The simplistic approach either takes a very long time, by holding the voltage until no further migration is possible, or results in small contrast differences between segments that have changed from the other state in the update and those that have received multiple updates to the same state. To avoid this contrast or ghosting problem, display manufacturers specify complex waveforms that are different for segments changing from black to white, changing from white to black, staying white and staying black.
To generate the tens of volts required and to apply the complex waveforms, a driver chip is typically included with the display. In the case of the SiPix displays, this chip was manufactured by Solomon Systech. The chip is mounted directly to the flexible printed circuit leading from the display (not in a package). The fine-pitch traces from the segments of the display (up to 96) connect to the chip and connections to the host system lead away to a flat-flex cable connector with a 0.5 mm pitch. The host system controls the driver chip via a SPI interface.
The Major Challenge - Understanding the Data Sheets
This should have been a very straightforward project, with no particular technical challenges. However, getting information about the display and the driver chip was like getting blood from a stone. Technical support from both SiPix and Solomon Systech simply ignored direct queries, possibly because they did not recognize ECROS Technology as a customer with plans to buy product. Therefore, it was necessary to pass questions up the chain to the system integrator and the end user (who did not really want to be bothered with low-level questions). Replies were a long time in coming back and were often unconvincing. The data sheet for the driver chip was a Product Preview nearly three years old. An application note purporting to describe the drive waveforms referenced a different display series than the ones used on this project.
Eventually, it came down to guesswork. To mitigate the risk of guessing incorrectly, a hasty prototype was thrown together. Conveniently, ECROS Technology has a product, called the Dragon Rider, that couples to the Atmel AVR Dragon and turns it into a flexible development and prototyping system. A simple adapter PCB was designed to carry the flat-flex connector and the capacitors for the driver chip's charge pump. This was contrived so that by populating the appropriate capacitors and running a few wire links, all the high voltage options of the chip could be explored.
Using the prototype, firmware was written to update sample displays provided by SiPix. The update took seconds, which seemed a long time, but there were no contrast or ghosting problems. The waveforms generated by the driver chip could not be verified as the pitch of the traces on the flexible printed circuit were too fine to probe. In addition, many configuration registers in the Solomon Systech driver chip did not read back sensible values and it was not possible to get confirmation from the manufacturer that this behavior was normal. The system integrator decided to move ahead with things as they were.
Host Interface and Triple Power Supplies
The host system interface was implemented with the ubiquitous and easy-to-use FT232R USB-to-serial interface chip from FTDI. This device can supply up to 50 mA at 3.3 volts to an external circuit, so this was routed via a blocking diode to the logic and charge pump supply of the Solomon Systech driver chip. For single-cell battery operation, the Texas Instruments TPS60310 charge pump was selected. This outputs a regulated 3.3 volts from an input in the range 0.9 to 1.8 volts. Again, a blocking diode prevents conflict with the other power sources. Finally, for other batteries, the Texas Instruments TPS71530 low-power, low-dropout, linear regulator provides 3.0 volts from inputs in the range 3.1 to 24 volts. No blocking diode was used as the regulator will shut down if another supply is active. Thus, the system will use whichever of the three power sources is connected, per the requirement.
Microcontroller and Firmware Development
The Atmel ATmega328P microcontroller was chosen for ready availability and familiarity, although it is overkill for this application. In production, the ATmega88P or an even cheaper device would be preferable.
The firmware for the microcontroller was written in Atmel's AVR Studio using the GCC compiler in the WinAVR package. It can be loaded over the In-System Programming (ISP) port using any suitable interface, such as the AVRISP MkII. As the ISP port shares pins with the SPI port, the display must be disconnected so as not to conflict with the programming signals. Because of the requirement for small size, the usual 0.1 inch pitch, three-by-two connector was not used. Instead, a 0.05 inch pitch, six-by-one footprint saved a great deal of space, although requiring a simple adapter cable.
During development, firmware was loaded from AVR Studio via the debugger using the AVR Dragon in debugWIRE mode. The debugWIRE port consists only of ground, power and the reset line, so by making second adapter cable with the SPI signals not connected, it was possible to work with the display plugged in without conflict.
Although not in the original requirements, ECROS Technology suggested the inclusion of a thermistor to measure ambient temperature. (The microcontroller does include a temperature sensor, but it isn't very accurate.) This allows the drive waveform to be adjusted to give faster updates when possible (at higher temperature) but without ghosting when more time is needed (at lower temperature). The system integrator accepted this suggestion.
The project work products were delivered to the customer on time and the customer reports complete satisfaction. The custom displays arrived from SiPix with the driver chip on the other side of the flexible printed circuit with respect to the samples provided for development. As a result, the connector was backwards and the controller had to be turned upside-down to work. Fortunately, this was acceptable, but this kind of problem shows how careful you have to be with details. ECROS Technology warned the system integrator that the relationship with SiPix needed to be managed more carefully, but was not or could not be done.
Requirements are now being gathered for a cost-reduction that would eliminate the Solomon Systech driver chip and include a form-factor change for a specific application.