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 (LcdAC). 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 LcdAC is reversed, and the whole procedure starts again. The number of line clocks between each 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.
The ACB bit field has no effect on LcdAC in active mode. This is due to the fact that 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 controller in active mode; this occurs whenever pixel data is driven out to the data pins to signal to the display when it may latch pixels using the pixel clock.