Intel SA-1100 manual Pixel Clock Divider PCD, AC Bias Pin Frequency ACB

Peripheral Control Module

11.7.6LCD Controller Control Register 3

LCD controller control register 3 (LCCR3) contains seven different bit fields that are used to control various functions within the LCD controller.

11.7.6.1Pixel Clock Divider (PCD)

The 8-bit pixel clock divider (PCD) field is used to select the frequency of the pixel clock. PCD can be any value from 1 to 225 (0 is illegal) and is used to generate a range of pixel clock frequencies from CCLK/6 to CCLK/514 (where CCLK is the programmed frequency of the CPU clock). The pixel clock frequency should be adjusted to meet the required screen refresh rate. The refresh rate depends on: the number of pixels for the target display; whether single- or dual-panel mode is selected; whether monochrome or color mode is selected; the number of pixel clock waitstates 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; and the width of the frame clock or HSYNC signal. All of these factors alter the time duration from one frame transmission to the next. Different display manufacturers require different frame refresh rates depending on the physical characteristics of the display. PCD is used to alter the pixel clock frequency in order to meet these requirements. The frequency of the pixel clock for a set PCD value or the required PCD value to yield a target pixel clock frequency can be calculated using the two following equations. Note that programming PCD = 8’h00 is illegal.:

CCLK

PixelClock = -----------------------------

2(PCD + 2)

CCLK

PCD = ------------------------------------- – 2 2(PixelClock )

11.7.6.2AC 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 (L_BIAS). In passive mode, 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 L_BIAS is reversed, and the whole procedure starts again. The number of line clocks between each ac bias pin transition ranges from 1 to 256. The user should program ACB with the desired number of line clocks minus one.

This pin is used by the LCD display to periodically reverse the polarity of the power supplied to the screen to eliminate dc offset. If the LCD display being controlled has its own internal means of switching its power supply, ACB should be set to its maximum value to reduce power consumption (8’hFF). Note that the ACB bit field has no effect on L_BIAS in active mode. Because the pixel clock transitions continuously in active mode, the ac bias pin is used as an output enable signal. It is asserted automatically by the LCD controller in active mode whenever pixel data is driven out to the data pins to signal the display when it may latch pixels using the pixel clock.

SA-1100 Developer’s Manual 11-39