
DDC MODULE
S3C8639/C863A/P863A/C8647/F8647
17-12
THE DDC INTERFACE
DDC2BI MODE
Overview
DDC2B capable graphic hosts have limited and mono-directional communications with the display devices. At the
contrary, DDC2Bi mode is an extension of the DDC2B level in order to offer a bi-direction communication
between the computer graphic host and the display device. DDC2Bi brings DDC2B+ functionality to DDC2B
graphic hosts using a simple S/W driver. So DDC2Bi display device is made by simple S/W upgrade to DDC2B+
capable displays. DDC2Bi protocol relies on the DDC2B H/W definition and the Access bus messages protocol.
The Graphic host behaves as an IIC single master host, and the display device behaves as an IIC slave device.
The DDC2Bi is a modification of the Access bus multi-master protocol to fit single master communication.
DDC2Bi Host and Display Device
DDC2Bi host is considered as an IIC single master capable device. The virtual IIC slave address of the host is
50/51H. But DDC2Bi display device is considered as a fixed address display device (6E/6F), and uses only IIC
slave mode to communicate with the host.
A display dependent devices are geographically located around the display and follow the same DDC2Bi data
protocol than the display device. And fixed address IIC slave devices group all the existing stand-alone and
brain-less IIC slave device. These devices can coexist and be connected to the DDC/IIC-bus.
DDC2Bi S/W Implementation
In order to describe the display that the received message is of DDC2Bi type, the source address byte bit 0 is set.
And when the host expects an answer from the display, the host reads the answer message at the display device
slave address 6FH. The checksum is still computed by using the 50H, virtual host address.
A null message can be defined as an Access bus message without any data byte. The null message is used in
the following cases:
— To detect that the display is DDC2Bi capable by reading it at 6FH, IIC slave address.
— To describe the host that the display does not have any answer to give to the host
— The enable application report has not been sent prior application messages exchange with the host
DDC2Bi Communication
In the DDC2Bi communication, it is capable to retrials when a communication fails (bus error or bad checksum).
So the host is responsible for resending its message and trying to get an answer from the display again. When
the communication fail is occurred, the DDC2Bi devices must answer by the retry of host.
The DDC2Bi capable device must properly send and receive all its supported messages. This determines the
maximum internal data communication buffer required size for proper display operation. If the device receive a
message which size is lager than the maximum supported by the device, the message be accepted entirely by
the device, but does not need to be supported internally, and then be discarded. Therefore the DDC2Bi capable
device must acknowledge all received data bytes from the host.