Issue 145 August 2002
Driving the NKK Smartswitch
Part 2: Graphics and Text

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Start • Graphic Generation • Switch Input • Sources & PDF

In Part 1, I described the basic interface to drive the Smartswitch. In this article, I will add the bells and whistles to allow both text and messages to be placed anywhere on the screen.

GENERATING AN IMAGE

The software I described last month merely converts the bits stored in the memory array cMatrix[][] to pixels on the LCD. Now, I will show you how to generate an image on the display that enables a level of abstraction.

I’ve tried to create an interface that enables the mixing and matching of graphics and text. As I mentioned before, the LCD output is maintained in RAM, so writing an object to RAM affects only specific pixels. Everything else is left untouched.

Interface to these functions is through up to 12 bytes of RAM (cCMDs[12]). The application software that uses this interface writes these desired bytes and then waits for a completed signal. Bit 0 of cCMDs[0], when set by the application software, indicates that there is a valid set of command and data bytes in this array.

The Display Interface Handler processes this array and when it’s complete it clears bit 0 of cCMDs[0]. The application software polls the RAM location cCMDs[0] and waits for bit 0 to go to zero. After it does, the software updates registers cCMDs[1] to cCMDs[11] as needed and then sets bit 0 of cCMDs[0] to one.

The command set implements all the functions necessary to control the display. The commands include clearing and turning on a pixel block, writing text, placing graphics, reverse video, and changing the backlight. You can download the command set from the Circuit Cellar web site.

CHARACTER GENERATION

In order to represent a character in dot matrix format, the symbol must be reduced to a series of pixel patterns. These patterns are held in a look-up table where the character code (normally ASCII) and row number access the specific pattern. This is called a character generator.

Five columns by seven rows is a common format for small displays, but I felt a second, slightly larger format, 7 × 9, also would be beneficial because the Smartswitch is a small device. A 5 × 7 format accommodates a maximum of six characters per line, while a 7 × 9 only allows for four characters per line. I implemented only two character sets in order to economize on program memory space.

Creating a character set can be tedious, so I was delighted when I discovered that I could use Excel to expedite the process. This brilliant idea from Alberto Bitti was published as a design idea in another magazine in September 2001. [1] I was motivated enough to create some macros to speed up the generation process. This implementation was published in April 2002. [2] You may download the associated Excel file from the Circuit Cellar web site.

In summary, you have to enter the desired matrix pattern for each character and the Excel worksheet generates the bit pattern as a number for every row. This is then saved to a file, which can be edited and read into the application (see the cCharSet1 and cCharSet2 arrays in the C module LCDmem.c).

In order to ensure that previous entities are cleared, and to allow spacing between characters, I have added a column of blank pixels to the right of each character and a row of blank pixels underneath each character. If the most significant bit of a character in RAM is set, that character is underlined in the row underneath the character. A standard ASCII character set goes from 0x20 to 0x7F. I have allowed for an additional 32 characters, from 0 to 0x1F, to permit customized characters.