HDCP
To establish an encrypted link, both the source and receiver have a set of 40 unique 56-bit keys that make up each device’s private keys. Each of these keys is associated with the device’s unique key selection vector (KSV). Each HDCP-compatible device has a unique 40-bit KSV from all other HDCP devices. The source initiates HDCP authentication by exchanging KSV values with the receiver over AUX to validate that they both support HDCP and have valid keys. If the set of keys is found to be invalid or corrupt, the HD image will not be transmitted. This exchange of keys affirms to the source that the receiver is authorized to receive HDCP content. The HDCP system allows for up to seven level repeaters to be transmitted in series between a source and a receiver. This key exchange and validation process only takes a fraction of a second.
Both the source and the receiver then generate a shared secret key value that cannot be determined by eavesdropping on the data exchange. The secret key is used to encrypt HDCP content by the source and decrypt by the receiver (see Figure 4).
Figure 4. Key Selection Vector
Direct Drive Monitor Controls
The buttons on the front of a monitor control the power on/off as well as configure the panel. The monitor will typically have green and orange LEDs to provide status and feedback to the user. The receiver has several pins to link to these front panel switches and LEDs.
An LCD panel is often backlit with a Cold Cathode Fluorescent Lamp (CCFL), which is powered by a lamp inverter. The enable signal for controlling the map is called the Pulse Width Modulation (PWM) signal, and it typically has a frequency range of 100Hz to 4KHz. If the duty cycle is 100 percent on, then the light will have full brightness. If the duty cycle is 50 percent on, then the brightness will be at 50 percent, and if the duty cycle is 15 percent, then the brightness will be off.
When a user pushed the brightness button on the monitor, the receiver will:
- Adjust the backlight PWM register setting, which will change the on/off duty cycle of the backlight
- Save the new value in a readable register
- Set a bit to notify the source of an update
- Generate an HPD interrupt to notify the source
In response to the HPD interrupt, the source will:
- Clear the bit requesting service
- Display a brightness control bar as a visual indication to the user. The bar will be removed after a few seconds if there is not additional button activity.
Figure 5. Sample Brightness Display
If a user pushes a button to increase the panel brightness, the current luminance value stored in a register on a chip is updated with a higher value, and an interrupt is generated to the source. The source will, in turn, show a display brightness adjustment bar on the screen for five seconds (see Figure 5).
NEXT: Power Modes
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