The next candidate to look at for integration is the DDC bus, which also handles HDCP, the content protection of the HDMI standard. Since the price for a quality HDMI cable can easily exceed the price for a DVD player, an end customer might be enticed to buy a cheap cable over the Internet. An investigation of some of these cheap cables found them to violate the HDMI specification, some of them quite severely. This can degrade the DDC signal enough to disturb the content protection. Depending on the equipment, the source can show for some seconds a message like “display connected - non HDCP compliant” before the screen turns dark. The likelihood is high the TV set will be sent back as faulty by the end customer. Sometimes, this issue is solved by reducing the value of the DDC pull up resistors.
However, this approach is violating the HDMI specification, which specifies them to be 47kOhm. To avoid this situation, we include a DDC buffer plus slew rate accelerator. Not only does this relax the situation on the “cheap cable” side, it also enables a simple way of multiplexing and is providing a capacitive decoupling between input and output. Finally, it serves as a bi-directional level shifter, since DDC operating voltage is 5V according to the HDMI standard whilst modern system chips operate at lower voltage levels.
Figure 5: DDC buffer and slew rate accelerator for DDC data and DDC clock
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Figure 6: Feeding a TV set over a cable violating the HDMI specification (above) and DDC signal lines (below). Left: The cheap HDMI cable has deteriorated the HDPC signal, the DVD player is blocking out the TV after giving an error message for some seconds. Right: Same setup as left but having an IP4778CZ38 with DDC buffer and slew rate accelerator in the loop. The signal is restored, the DVD player is accepting the TV set.
Another application to be included is the hot plug application, where the HDMI sink is signalling to the HDMI source readiness to transmit the screen specific parameters like resolution, color depth and so on over the DDC bus. We include this application and protect it against ESD pulses as well.
Figure 7: Block diagram of the hot plug application for a HDMI source (left) and sink (right)
The coffee tables of end users are typically covered by several remote controls. To address this issue, several AV equipment manufacturers are employing the CEC bus for communication between their applications. Since the CEC bus is operating in the audible 1kHz range, the HDMI specification recommends using a slew rate limiter to avoid crosstalk of the CEC bus to the audio parts. Hence NXP offers devices, having an integrated CEC slew rate limiter, as well.
Figure 8: Block diagram of the CEC slew rate limiter
Summary
NXP Semiconductors offers a wide portfolio of devices for the HDMI interface. having different levels of integration. For example, IP4777CZ38 (for HDMI sources) and IP4778CZ38 (for HDMI sinks) include IEC level ESD protection, DDC buffering and slew rate accelerating, the Hot Plug Detect application and a CEC Slew Rate Limiter. Further information on these devices are available at:
http://www.nxp.com/pip/IP4778CZ38_1.html
and
http://www.nxp.com/pip/IP4777CZ38_1.html
Further ‘clean interface’ devices from NXP Semiconductors can be expected on short notice.
There are several benefits of integrating the various circuits required to operate an HDMI port to a single device.
- Besides direct cost savings, integration is also saving a lot of indirect costs.
- Pick-and-place operations will sum up to impressive amounts and can easily exceed the value of the assembled component. Less devices also reduces purchasing, stock-keeping and quality costs.
- Further benefits are in shortening the time to market and giving easy access to design know-how without the need to develop a proven RF design again..
- Integration increases the performance of circuits, especially when symmetry is required. The only feasible way to realize some applications, for example TMDS equalizers, is through an integration of the circuit into one chip.
Taking all these benefits into account, the future of HDMI interface turns out to be ‘clean’.
About the author:
Dr. Stefan Seider is International Product Manager for NXP Semiconductors Integrated Discretes. He can be reached at stefan.seider@nxp.com
