By Charming Su, SVP, Pervasive Displays

E-paper displays are an extremely versatile, low power alternative to using LCD and LED technologies. They particularly suit applications such as smart labeling in retail outlets and supermarkets. E-paper displays are a bi-stable technology, are light and thin, making them suitable for a wide range of applications. Perhaps their most notable attribute is that they only need a power source to change the displayed information. An e-paper display does not require any power and will happily display the same image for years. Another valuable attribute of e-paper displays is that they are readable in direct sunlight. Recent applications have seen them incorporated into battery-less applications where the energy to update the display is obtained through energy harvesting techniques.

E-paper display update methods

An e-paper display suits applications where the image does not have to be updated constantly, for example, a smart utility meter. This is because the display update process involves four operations. E-paper displays use two image buffers, one for the current image and the second buffer for the new image. Changes are made on a pixel-by-pixel basis by first inverting the current image and then displaying a completely white image. The new image is then inverted, and finally, the new image is displayed. The four-step process, termed a global update, is used to prevent the ghosting effect that e-paper displays commonly encounter where the previous image is partly visible after an update. During the global update operation, every pixel is changed, which also prolongs the lifetime of the e-paper display by allowing more pigment material movement with each pixel and reducing the ghosting effect.

Viewing an e-paper display during a global update can be distracting as it goes through the stages documented above. Speeding up the update process is possible through two alternative update methods; fast and partial.

Fast and partial e-paper update methods

A fast update does not change every pixel the way a global update does. Instead, it compares the two image buffers and only changes the pixels it needs to update (see Table 1). A partial update only changes the desired portion of the screen that needs changing similarly. For example, when changing the price on an intelligent retail shelf label, the product description, product image, and product size will remain the same; only the price needs to change. For a partial e-paper display update, only the specific portion of the display is updated by bounding within the image buffer.

Fast or Partial Update		Previous Image
		Black	White
New Image	White	White	No Changes
	Black	No Changes	Black

Table 1 – The update process for fast and partial update methods

(Source: Pervasive Displays)

Table 1 applies to both fast and partial updates and invokes three possible actions: change to black, change to white, no change.

While partial and fast updates provide a convenient and more visually appealing method of quickly updating a screen using minimal power consumption, it is recommended that a global update is regularly performed to maintain display performance and prevent ghosting.

From a broader application perspective, the developer can implement a display with either an internal or external timing controller. An internal timing controller (iTC), typically implemented as an embedded chip-on-glass, requires less design-in effort. However, an external timing controller (eTC) uses a host microcontroller to drive each stage of the update process. This approach gives the designer more flexibility to control the power consumption profile and how the image data is sent.

To find out more about how fast and partial updates can enhance your e-paper display application, download the Pervasive Displays white paper here.