LCD Controller Registers
11-44
ac-Bias Pin Frequency (ACB)
The 8-bit ac-bias frequency (ACB) field is used to specify the number of line
clock periods to count between each toggle of the ac-bias pin (LCD, AC). After
the LCD controller is enabled, the value in ACB is loaded to an 8-bit
down-counter, and the counter begins to decrement using the line clock. When
the counter reaches zero, it stops, the state of LCD, AC is reversed, and the
whole procedure starts again. The number of line clocks between ac-bias pin
transition ranges from 0 – 255 (program to value required minus one). This line
is used by the LCD display to periodically reverse the polarity of the power
supplied to the screen to eliminate DC offset.
Note:
The ACB bit field has no effect on LCD.AC in active mode. This is because
the pixel clock transitions continuously in active mode; the ac-bias line is
used as an output enable signal. The ac bias is asserted by the LCD control-
ler in active mode; this occurs whenever pixel data is driven out to the data
pins to signal to the display when it can latch pixels using the pixel clock.
Pixel Clock Divider (PCD)
The 8-bit pixel clock divider (PCD) field is used to select the frequency of the
pixel clock (see Table 11–21). PCD can generate a range of pixel clock
frequencies from LCD_CK/2 to LCD_CK/255, where LCD_CK is the LCD
controller clock from the OMAP5910 clock management logic (see Chap-
ter 15, Clock Generation and System Reset Management). The pixel clock
frequency must be adjusted to meet the required screen refresh rate. The
refresh rate depends on:
-
The number of pixels for the target display
-
Whether monochrome or color mode is selected
-
The number of pixel clock delays programmed at the beginning and end
of each line
-
The number of line clocks inserted at the beginning and end of each frame
-
The width of the VSYNC signal in active mode or VSW line clocks inserted
in passive mode
-
The width of the frame clock or HSYNC signal