Registers
1071
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Liquid Crystal Display Controller (LCDC)
Table 23-22. LCD Raster Timing Register 2 (RASTER_TIMING_2) Field Descriptions (continued)
Bit
Field
Value
Description
15-8
ACB
0-FFh
AC Bias Pin Frequency. This value defines the number of Line Clock (LCD_HSYNC) cycles to
count before transitioning signal LCD_AC_ENB_CS. This output may be used to periodically invert
the polarity of the power supply in order to prevent a display DC charge build-up on the LCD panel.
AC Bias Time Period = [2 × ((Line Clock) × (ABC))]
7-0
Reserved
0
Reserved
23.3.11.1 AC-Bias Pin Frequency (ACB)
NOTE:
The 8-bit ac-bias frequency (ACB) field has no effect 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 can
latch pixels using the pixel clock.
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. 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 ac-bias pin is reversed, and the whole procedure starts again. The
number of line clocks between each ac-bias pin transition ranges from 1–256 (program to value required
minus 1). This line is used by the LCD display to periodically reverse the polarity of the power supplied to
the screen to eliminate DC offset.
23.3.11.2
AC-Bias Line Transitions Per Interrupt (ACB_I)
The 4-bit ac-bias line transitions per interrupt (ACB_I) field is used to specify the number of line transitions
to count before setting the ac-bias count status (ABC) bit in the LCD controller status register that signals
an interrupt request. After the LCD controller is enabled, the value in ACB_I is loaded to a 4-bit down
counter, and the counter decrements each time the ac-bias line state is inverted. When the counter
reaches zero it stops, the ac-bias count (ABC) bit is set in the status register. Once ABC is set, the 4-bit
down counter is reloaded with the value in ACB_I and is disabled until ABC is cleared. Once ABC is
cleared by the CPU, the down counter is enabled, and again decrements each time the ac-bias line is
flipped. The number of ac-bias line transitions between each interrupt request ranges from 0 to 15.
Programming ACB_I = 0000 disables the ac-bias line transitions per interrupt function.
23.3.11.3
Invert VSYNC (IVS)
The invert VSYNC(IVS) bit is used to invert the polarity of the frame clock (VSYNC).
•
When IVS = 1, the frame clock (VSYNC) is active low.
•
When IVS = 0, it is active high.
23.3.11.4 Invert HSYNC (IHS)
The invert HSYNC(IHS) bit is used to invert the polarity of the line clock (HSYNC).
•
When IHS = 1, the line clock (HSYNC) is active low.
•
When IHS = 0, it is active high.
23.3.11.5 Invert Pixel Clock (IPC)
The invert pixel clock (IPC) bit is used to select the edge of the pixel clock that drives pixel data out onto
the LCD data lines.
•
When IPC = 1, data is driven onto the LCD data lines on the falling edge of the pixel clock.
•
When IPC = 0, data is driven onto the LCD data lines on the rising edge of the pixel clock.