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Table of Contents
DAC
The LCD breakout has 6 bit x RGB output, which we can turn into a 6-bit DAC → RGB analog 0.7Vpp into 75 ohms use 278 & 556 ohm resistors for the R2R ladder, this will give you ~0.7v signal when plugged into a 75 ohm termination such as a monitor.
VSync
We want to get the Vsync to about 60hz
By default, the 'internal' LCD sync rate is 117 Hz
chumby-:/ # cat /proc/driver/chumbyfwfb/fb_stats PXP frequency: 59 Hz LCDIF frequency: 117 Hz VSYNC Edge IRQs: 49077 Cur Frame Done IRQs: 49224 Missed PXP firings: 5
This is set by the HW_CLKCTRL_PIX register
chumby-:/ # regutil -r HW_CLKCTRL_PIX Value at 0x80040060: 0x0000001d
Lets divide the frequency by 2 which means multiplying the divider by two
chumby-:/ # regutil -w HW_CLKCTRL_PIX=0x38 Setting 0x80040060: 0x0000000e -> 0x00000038 ok
chumby-:/ # cat /proc/driver/chumbyfwfb/fb_stats PXP frequency: 32 Hz LCDIF frequency: 62 Hz VSYNC Edge IRQs: 69919 Cur Frame Done IRQs: 70118 Missed PXP firings: 5
Actually, to get the closest to 60 Hz. I went with HW_CLKCTRL_PIX=0x37 (so says the tek 'scope)
How the Hsync is 17.08 KHz (we want 2x that 31.46 KHz)
THIS IS ALL WRONG. START OVER!
Horizontal Sync HW_LCDIF_VDCTRL2
For the LCD, this is by default = 0x45000190
Per horizontal line: 640 pixels Front porch: 16 pixels Sync pulse width: 96 pixels Back porch: 48 pixels
SO!
HW_LCDIF_VDCTRL2 = (96) « 24 | (640 + 16 + 48) = 0x600002C0
Vsync HW_LCDIF_VDCTRL1
For the LCD, this is by default = 0x0000011a (dec. 282)
Per vertical line: 480 horizontal lines Front porch: 11 horizontal lines Sync pulse: 2 horizontal lines back porch: 31 horizontal lines
so HW_LCDIF_VDCTRL1 = 542 (0x21e)
HW_LCDIF_VDCTRL0
VSYNC_OEB = 0 (generate vsync) ENABLE_PRESENT = 0 (no enable necessary) 0x0 VSYNC_POL = 0 (negative sync!) HSYNC_POL = 0 (negative sync!) DOTCLK_POL = D/C ENABLE_POL = D/C 0x0 VSYNC_PERIOD_UNIT = 1 (count in lines) VSYNC_PULSE_WIDTH_UNIT = 1 (count in lines) 0x3 HALF_LINE = 0 HALF_LINE_MODE = 1 (no half line for vga, right?) 0x4 VSYNC_PULSE_WIDTH = 2 0x2
→ 0x00340002
HW_LCDIF_TRANSFER_COUNT
By default, (32x0x240 LCD) = 0x00f00140
For 640x480 VGA it should be = 0x01E00280
HW_CLKCTRL_PIX
CLKGATE = 0 DIV = 0x11
Results?
HSync = 31.4Khz (should be 31.46)
Vsync = 58 Hz (should be 60.04)
Vsync negative pulse = 63.7 uS (should be 63.57) (it is low from the negative edge of one HSync to another negative Hsync edge)
Hsync negative pulse = 4.34 uS (should be 3.81)
