Digital Outputs (Data And Clocks); Digital Inputs; Xtal And Load Cap Value Selection; Figure 151: Crystal Circuit - Analog Devices ADV7850 Hardware Manual

16.4

DIGITAL OUTPUTS (DATA AND CLOCKS)

The trace length that the digital outputs have to drive should be minimized. Longer traces have higher capacitance, which requires more
current, which can cause more internal digital noise. Shorter traces reduce the possibility of reflections.
Adding a series resistor of a value between 50 to 200 ohms can suppress reflections, reduce EMI, and reduce the current spikes inside the
ADV7850. If series resistors are used, they should be placed as close as possible to the ADV7850 pins and the trace impedance for these
signals should match that of the termination resistors selected.
If possible, the capacitance that each of the digital outputs drives should be limited to is less than 15 pF. This can be accomplished easily by
keeping traces short and by connecting the outputs to only one device. Loading the outputs with excessive capacitance increases the
current transients inside the ADV7850, creating more digital noise on its power supplies.
16.5

DIGITAL INPUTS

The following digital inputs on the ADV7850 are 3.3 V inputs that are 5.0 V tolerant:
• HS_IN1/TRI5
• VS_IN1/TRI6
• HS_IN2/TRI7
• VS_IN2/TRI8
• DDCA_SCL
• DDCA_SDL
• DDCB_SCL
• DDCB_SDA
• DDCC_SCL
•
DDCC_SDA
• DDCD_SCL
•
DDCD_SDA
• VGA_SCL
•
VGA_SDA
Any noise that gets onto the HSync and VS inputs trace will add jitter to the system. Therefore, the trace length should be minimized, and
digital or other high frequency traces should not be run near it.
16.6

XTAL AND LOAD CAP VALUE SELECTION

The ADV7850 uses a 27 MHz crystal.
taken when using a crystal circuit to generate the reference clock for the ADV7850. Small variations in reference clock frequency can
cause auto detection issues and impair the ADV7850 performance.
These guidelines are followed to ensure correct operation:
• Use the correct frequency crystal, which is 27 MHz. Tolerance should be 50 ppm or better.
•
Know the C for the crystal part number selected. The value of capacitors C1 and C2 must be matched to the C
load
Rev. A May 2012
Figure 151 shows an example of a reference clock circuit for the ADV7850. Special care must be
27 MHz
C1
47pF

Figure 151: Crystal Circuit

XT AL
C2
47pF
431
ADV7850
for the
load