The E-Paper display from Pervasive Displays with a interface board is available from Embedded Artists : http://www.embeddedartists.com/products/displays/lcd_27_epaper.php The 2.7 inch display have 264*176 pixel, monochrome.

Technology¶

You can look at the webside from Pervasive to see how the display works. http://www.pervasivedisplays.com/technology/home

This type of display have ultra low power consumption - due to its bi-stable nature. It requires only very little power to update the display and needs no power to maintain an image. You can disconnect the display - the image is still there. The viewing angle is like real paper - near 180°.

There are also some disadvantages of this technology. To change the image, we have to rewrite the full display in 4 steps. Invert, clear, invert new, new image. This process is visible and take a while 2s at room temperature. If it gets colder the display reacts slower and the interface timing has to be slow down. To compensate this, there is a LM75 temp sensor on the interface board. We also need ram to double buffer the display. 264 * 176 / 8 = 5808 Byte. To double buffer we need 11616 byte of ram. This is no problem for most mbed devices, but it will not run on the LPC11U24 or LPC800-MAX.

Interface¶

The graphic data is transferred to the display via spi. The maximum speed is 12Mhz. There are also some control signal and the I2C for the temperature sensor. Together we need 12 signals.

Software¶

Fonts¶

How to get nice looking fonts ?

To print characters to a graphic screen we need a font. To code a font by paper is ok for a small lcd, but for a 320*240 pixel display we need bigger fonts. A 12*12 pixel font is readable, but a lot of work to construct.

Fonts can be made with the GLCD Font Creator also from http://www.mikroe.com .

With this program you can load a window font and convert it into a c-array. To use this Font with my lib you have to add 4 parameter at the beginning of the font array. - the number of byte / char - the vertial size in pixel - the horizontal size in pixel - the number of byte per vertical line (it is vertical size / 8 ) You also have to change the array to char[]. After that you can switch between different fonts with set_font(unsigned char* font); The horizontal size of each character is also stored in the font. It look better if you use bigger fonts or italic. The letter M is wider than a l.

Here are some Fonts from me : http://mbed.org/users/dreschpe/code/TFT_fonts/

The small made for the mbed lab board : http://mbed.org/users/dreschpe/code/LCD_fonts/

And from Peter Holzleitner : http://mbed.org/users/pholzleitner/code/SourceCodePro31-SB/

Text commands :

You can use the claim() function to redirect the output to stdout or stderr to the TFT. After claim(stdout) you can simply use the printf function to print to the TFT.

• locate(x,y); function is used to setup the cursor position. x,y are the pixel position.
• printf(...); print text to the buffer with format options.

Graphics¶

Graphic commands :

• cls(); Fill the screen with background color
• pixel(x,y,color); set a single pixel at x,y with 1 : black or 0 : white
• line(x0,y0,x1,y1,color); draw a line from x0,y0 to x1,y1 with color
• rect(x0,y0,x1,y1,color); draw a rectangle x0,y0 to x1,y1 with color
• fillrect(x0,y0,x1,y1,color); draw a filled rectangle
• circle( x0,y0,radius ,color); draw a circle around x0,y0 with radius
• fillcircle(x0,y0,radius ,color); draw a filled circle around x0,y0 with radius

All this commands are writing to the frame buffer only ! To change the active display we have to call

• write_disp(); This refresh the display.