NEC MultiSync V520 Service Manual page 55

DDC Interface
The DDC interface is a slave mode I²C interface with DDC1 function. It is fully compatible with VESA
DDC1/2B standard. The functional block diagram is shown in the below.
VSYNC
MUX
After power on or reset the DDC interface, it is in DDC1 state. The shift register shilfts out data to SDA pin
on the rising edge of VSYNC clock. Data format is an 8-bit byte followed by a null bit. Most significant bit
(MSB) is transmitted first. Every time when the ninth bit has been transmitted, the shift register will load a
data byte from data buffer (REG#18H). After loading data to the shift register, the data buffer becomes
empty and generates and INT0 interrupt. So the program must write one data byte into REG#18 every nine
VSYNC clocks.
Since the default values of data buffer (REG#22) and shift register are FFH, the SDA pin outputs high level
if no data had been written into data buffer after power on reset. When program finished initialization and set
the IEN_D bit to "1", the INT0 will occur because the data buffer is empty. The INT0 service routine should
check the DDC2B bit is "0" and then writes the first EDID data byte into data buffer. When the second INT0
occurs, the INT0 service routine writes the second EDID data byte into data buffer and so on.
SDA
1 2
VSYNC
Load data to
shift register
INT0
IEN_D
Internal Data Bus
Data Buffer
Shift Register
Address Compare
MSB
1 0 1 0 0 0 0
Address Register
Bit7
3 9
10
ENACK
MUX
R/W
ADDR
START/STOP Detect
Handshake Control
Bit6 Bit5 Bit4 Bit3
52
START
STOP
DDC2B
Bit2 Bit1 Bit0
18
SDA
SCL
Bit7
19