AN-1331: HDMI Transmitter Phase-Locked Loop Operation

Introduction

Analog Devices, Inc., High-Definition Multimedia Interface (HDMI^®) transmitters are required to operate over a wide range of output frequencies: 25 MHz to 165 MHz, 25 MHz to 225 MHz, and 25 MHz to 300 MHz. To generate a stable transition minimized differential signaling (TMDS) output clock, the HDMI transmitter employs a phase-locked loop voltage controlled oscillator (PLL VCO). Supporting such a wide range of output frequencies with a PLL VCO is not realistic because it consumes a significant amount of power and requires a significant amount of silicon area. To overcome this challenge, the PLL VCO is used in conjunction with a gearing system. The gearing system allows the PLL VCO to operate across a limited range while multiplying the output frequency (see Figure 1).

Figure 1. Example of How Multiple Ranges Enable the PLL VCO to Cover
25 MHz to 225 MHz. — Figure 1. Example of How Multiple Ranges Enable the PLL VCO to Cover 25 MHz to 225 MHz.

PLL VCO Operational Overview

The gear system is influenced by environmental variables such as temperature and voltage (see the Gear System Behavior section). In considering these variables, two conditions are required to lock the PLL VCO. The PLL VCO lock detect flag must be set, and the PLL VCO must be centered within the selected gear. Having the PLL VCO centered within the selected gear guarantees an operating safety margin to account for changes in environmental variables. A set of high trip points and low trip points are employed to establish that the PLL VCO is centered within the selected gear.

When attempting to lock to a new input frequency, the PLL VCO selects Gear 0 and checks that the two locking criteria are met. If the two locking criteria are not met, the PLL VCO selects Gear 1 and checks that the two locking criteria are met. If the two locking criteria are still not met, the search continues through the rest of the available gears.

The number of gears that the PLL VCO searches is a function of the specific transmitter in question. Later HDMI transmitters, such as the ADV7511, features a larger number of gears than earlier designs, such as the AD9388A and the AD9889B.

When the PLL VCO identifies a gear in which it can lock to the input frequency, it stops searching and indicates that it has locked. This indication comes from an I²C readback such as the PLL lock status, ADV7511 Main Map 0x9E[4]. See the ADV7511 Programming Guide for more information.

Gear System Behavior

The PLL VCO and gear system employed in the HDMI transmitter are analog circuits; therefore, they are influenced by environmental variables such as temperature and voltage. As temperature levels or voltage levels rise or fall, the positions of the gears can vary slightly. For this reason, the preset range above and below the operating point must be available before the PLL VCO locks at any point. The preset range provides a safety buffer to allow such variations.

To illustrate the variation, consider a scenario where the ambient temperature around the HDMI transmitter increases due to some system level event such as an FPGA switching into a high power processing mode. Figure 2 shows an example of this scenario. The net effect of this temperature change is a slight downward shift in the gear curves.

Figure 2. Illustration of Gear Variation with Temperature.

The gear system on HDMI transmitters such as the ADV7511 is designed and evaluated to ensure that there is sufficient gear coverage for the entire frequency range and overlap between gears to allow any temperature change or voltage change within the specified limits.

In an unlikely scenario that the HDMI transmitter needs to change from the selected gear during normal operation (for example, it moves from Point A to Point B due to temperature change), the PLL VCO changes gear so that the HDMI transmitter recovers from the event with minimal disturbance.

Design Considerations

For HDMI transmitters such as ADV7511, a few design considerations must be utilized to minimize the possibility of inducing gear related issues: power supply considerations, layout considerations, and I²C configuration.

It is important to ensure that the power supply levels are within the specified limits, and that the voltage drops caused by system activity do not cause the power supply levels to decrease outside the specified limits. The user must also filter the power supplies to ensure that the power supply noise levels are the lowest possible.
It is important to ensure that the layout keeps the power supplies isolated from each other, and it keeps them on dedicated power supply planes. External swing resistors must be carefully located close to the HDMI transmitter. Heat transfer into the board must also be optimized where possible. Following any available Analog Devices reference design is strongly recommended.
It is essential that the HDMI transmitter is configured using the Analog Devices recommended settings, which are defined during the evaluation phase of the project. These settings are optimized across the variations of the process, temperature levels, and voltage levels.