Graphical LCDs based on KS0108
Most monochromatic graphical LCDs use a controller compatible with KS0108. Each one has an internal 512-byte memory, so it allows resolutions up to 64x64 pixels. Bigger displays tend to use multiple controllers, one per fraction of the screen. For instance, a 128x64 LCD has two controllers, a 196x64 has 3, and a 128x128 has 4 of them.
Each controller is independent so they don’t transmit any information between them. For activating one of the controllers, two control lines are used: CS1 and CS2 (chip select). They act as a 2-bit address, so they could be used to select up to four different ICs. They don’t have an internal font generator, so in order to write texts the device that drives this LCDs has to store the pixels of each wanted glyph, for example in the Flash memory of a microcontroller or an external memory.
The LCDs have these pins:
- VSS: 0V, ground reference
- VDD: 5V, supply voltage for LCD
- V0: Contrast adjustment
- D/I: selects between Data (1) or Instruction (0)
- R/W: selects between reading (1) or writing (0) to the controller
- E: When a pulse is applied, the data transfer happens
- DB0..7: Data bus (8-bit bus)
- CS1..2: Lines to select the controller
- RES: Reset signal, if 0 all controllers get reseted
- Vee: Negative voltage OUTPUT
- K y A: Backlight, typically LEDs
Most LCDs feature an internal negative voltage generator, which is needed by the controller to turn on the LCD pixels/segments. To control the contrast, a 20 kiloohm variable potentiometer can be used, connected between
Vss (connected to the side pins) and the middle pin (wiper) can be connected to
K pins are typically connected to LEDs that act as a backlight, and they should be connected to 5V and 0V, with a series resistance of 100-200 ohm to limit the current.
How to send a command or data to the LCD::
- Activate the desired controller (combination of CS1 and CS2)
- Set D/I line in the wanted state: 1 if a command needs to be sent, 0 if data will be sent
- Place the data or the command in the data bus
- Make a pulse in the E line
- Wait a small time (~500nS). Alternatively, the LCD status can be read to avoid hardcoding delays in the code, but this requires an extra signal line.
The display itself is split into in 8 horizontal pages (each with 8 pixels of height) and 64 vertical lines/columns.
Turn on/off display
D/I = 0, DB = 0x3F (on) or 0x3E (off)
Move pointer - X axis
D/I = 0, DB = 0x40 + line (0 to 63)
Move pointer - Y axis
D/I = 0, DB = 0x9C + page (0 to 7)
Write data to screen
D/I = 1, DB = DataToWrite
When data is written, the address is defined by the pointer. After writing a value, the X position is incremented. This can be used to copy lots of data quickly, only requiring to change the
page after sending 64 bytes.