Quad HDMI Input, Fast Switching Multiplexer Using the ADV7612 Receiver with Extended Temperature Range
This circuit shows the expandability of the ADV7612 in applications requiring four multiplexed HDMI inputs of up to 225 MHz TMDS (1080p60, 12 bits per channel; 148.5 MHz LLC pixel clock) or UXGA (1600 × 1200, 10 bits per channel; 162 MHz LLC pixel clock). The circuit offers a cost effective solution to this application and operates over the extended industrial temperature range of −40°C to +85°C.
In order for multiple ADV7612 devices to share the same bus,
we need to consider the output state of the devices, capabilities
of tri-stating buses, and the electrical parameters of the load on
the bus. Additionally, the devices must be controlled from an
I2C bus in a non-conflicting manner. The board layout of this
circuit is critical and should follow a straight line using
controlled impedances to reduce risk of reflections and crosscoupling.
Complete PCB layouts are contained in the design
support package downloadable at www.analog.com/CN0224-DesignSupport. An ADV7511 HDMI transmitter was used as a backend device.
Bus Output States
After resetting, the ADV7612 tri-states pins P0-P35, HS,
VS/FIELD/ALSB, DE, LLC, AP0…AP5, SCLK/INT2, and
MCLK/INT2. These pins can be set to the active state using
registers TRI_PIX, TRI_SYNCS, TRI_LLC, TRI_AUDIO as
described in the UG-216 Hardware User Guide, available at
Video and Audio Bus Loading
Only one ADV7612 can access the AV buses at a time; the
second must remain tri-stated. Assumming an output driver
resistance (P0…P35) of 10 Ω to 20 Ω (highest drive strength)
and a trace characteristic impedance of 75 Ω, a series resistor of
55 Ω to 65 Ω is required to match the characteristic impedance
of the trace. The maximum capacitance of a tri-stated output
bus driver on the ADV7612 is 20 pF (refer to Electrical
Specifications in the ADV7612 data sheet).
Layout and Termination Considerations
For this design, it is important to make sure the transmission
line is properly terminated and has controlled impedance.
Otherwise, reflections (which may occur on longer lines) can
have a negative impact on transmitted data.
For pixel lines (P0…P35), video synchronization (VS/FIELD/ALSB, HS, DE), and audio lines (AP0, AP1/I2S_TDM, AP2...AP5, MCLK/INT2, SCLK/INT2)—other than LLC—it is suggested to use series termination resistors of 51 Ω at the ADV7612 driver side, and tracks having a characteristic impedance of 75 Ω.
The line locked clock (LLC) line has the same characteristic impedance of 75 Ω and should have no series resistor, but should be terminated at the far end with a symmetrical termination (150 Ω to +3.3 V and to 150 Ω to GND), as shown in Figure 2.
Even though theory suggests a best termination value between 50 Ω and 60 Ω, it was observed during tests that a symmetrical 75 Ω (2 ×150 Ω) termination increases the swing and centers the signal around mid-supply (1.65 V), which is desireable.The ADV7511 HDMI transmitter is included on the board and is used to transmit the multiplexed output of the two ADV7612’s.
Figure 2. Terminations for P0…P35 Data Lines and LLC Traces
Figure 3 to Figure 6 show waveforms for various terminations. In each case, a symmetrical LLC termination was placed at the far end (close to the ADV7511), and series termination resistors as close as possible to the two ADV7612 devices, as shown in Figure 2.
Measurments were taken on the ADV7511 pins with Tektronix P6243 FET probes (1 MΩ resistance, 1 GHz bandwidth, less than 1 pF capacitance) and a Tektronix TDS5104B scope.
From the waveforms we can see that using 2 × 150 Ω terminations on the LLC line ensures a maximum swing of 3.3 V.
Using 75 Ω on the data lines slows the edges too much. 33 Ω and 15 Ω on data lines caused undershoots on falling edges (Figure 5 and Figure 6) and overshoots on rising edges (not illustrated). Therefore, 2 × 150 Ω was chosen for LLC, and 51 Ω was used on data lines, which is illustrated in the eye diagrams shown in Figure 9 and Figure 10.
Figure 3. Termination: Symmetrical 2 × 150 Ω on LLC line, 75 Ω on Data Lines (HS). Vertical Scale: 1 V/div, Horizontal Scale: 2 ns/div
Figure 4. Termination: Symmetrical 2 × 100Ω on LLC line, 5 1Ω on Data Lines (HS). Vertical Scale: 1 V/div, Horizontal Scale: 2 ns/div
Figure 5. Termination: Symmetrical 2 × 68 Ω on LLC, Series 33 Ω Termination on Data Lines (HS). Note 0.5 V Undershoot. Vertical Scale: 1 V/div, Horizontal Scale: 2 ns/div
Figure 6. Termination: Symmetrical 2 × 33 Ω on LLC, Series 15 Ω Termination on Data Lines (HS). Note 1 V Undershoot. Vertical Scale: 1 V/div, Horizontal Scale: 2 ns/div
When the CS line is pulled high, I2C communication is disabled.
A simple inverter reduces the resources required on the microcontroller side, as shown in Figure 7.
Figure 7. I2C Access
CEC implementation is not mandatory on the board, and it is up to end user to implement it. If CEC is not desired, CEC pins should be left floating (as described in UG-216, Appedix B: Recommended Unused Pin Configuration). This user guide is included at http://ez.analog.com/docs/DOC-1751.
In the other case, separate engines should be used to handle
There are two ways of driving the ADV7612 clock. Both parts
may have separate crystals connected to XTAL_N and XTAL_P
pins, or they can share same signal clock. In the circuit, the 1.8 V
signal clock from the oscillator is provided to pins XTAL_P of
both parts. In this configuration, XTAL_N must be left floating.
It is critical to ensure proper layout routing and grounding to
eliminate coupling between sensitive lines. The length of each
trace of the bus should be kept equal.
Interrupts from both devices must be considered. The ADV7612 has two possible interrupts : INT1 (INT1 pin) and INT2 (available through SCLK/INT2, MCLK/INT2 or HPA_A/INT2).
It is advised not to use INT2 via pins MCLK/INT2 or SCLK/INT2, as tri-stating the audio bus with the TRI_AUDIO register will also tri-state these pins.
Figure 8. Dual ADV7612 Board Solution with ADV7511
Figure 9. Screenshot from Scope. Signal Driven from ADV7612-U43. LLC Line (162 MHz) Measured at ADV7511’s Input and Pixel Line P35. Red Rectangle Shows Eye Mask for ADV7511. 2 × 150 Ω Symmetrical Termination on LLC and 51 Ω Series Resistors on Data Lines.
Figure 10. Screenshot from Scope. Signal Driven from ADV7612-U1. LLC Line (162 MHz) Measured at ADV7511’s Input and Pixel Line P35 Red rectangle Shows Eye Mask for ADV7511. 2 × 150Ω Symmetrical Termination on LLC and 51Ω Series Resistors on Data Lines
Layout should consist of very short traces. In the ideal case,
traces connecting two pins of the same function between the
two ADV7612’s should be as short as possible and should share
a common series termination resistor placed as close as possible
to both devices and then connected to the bus. In practice, this
is not possible due to layout constraints; therefore, each device
requires its own series termination resistor (see Figure 2). The
video traces should be kept as close to the same length as
possible for delay matching.
Evaluation and Test
The circuit was evaluated using two video generators (Quantum Data 882) to generate UXGA 1600 × 1200 pixels, 30-bits, and 1080p60, 36 bits (Samsung2 and MoirèX patterns). As an HDMI sink (output from ADV7511), an Astro VA-1831 video analyzer was used. Additionally, video signals from the ADV7612 (LLC and P35) were observed on the ADV7511 pins with a P6243 (1 pF, 1 MΩ, 1 GHz) probe attached to a Tektronix TDS5104B oscilloscope. The resulting waveforms are presented as eye diagrams with ADV7511 eye masks and are shown in Figure 9 and Figure 10 for UXGA (162 MHz, 30 bits) with the Samsung2 pattern. The MoirèX pattern used during tests also showed a similar safety margin.
Figure 11. Test Setup
Video timing measured by the Astro VA-1831 showed no anomalies. Even and odd vertical lines of MoirèX were analyzed (1080p60 36-bit and UXGA 30-bit), and it showed that all bits toggle properly at the same time without any leakage between lines (MoirèX pattern). The Astro VA-183 also showed stable HDMI synchronization signals and packets having correct CRC checksum. This indicates properly received clock and synchronization information by the backend ADV7511.
- Prepare test configuration as shown in Figure 11.
- Power up board and measuring equipment; start DVP Evaluation Software.
- In DVP Evaluation Eoftware, load ADV7612 board.
- Run INIT_PARTS_AND_SET_PORT_A_ver4.py script in DVP Evaluation Software (refer to design resources).
- Press Init button. Once board is initialized, click one of the buttons: Port A, Port B, Port C, or Port D to select desired input (see Figure 12).
Figure 12. Software Used for Selecting HDMI Input