VGA Interface Testing with the VM5000
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Application Note VGA Interface Testing with the VM5000 Many devices depend on high performance color displays, including personal computers, video gaming appliances, and home theaters. Even with digital display interfaces such as DVI, analog RGB persists due to the large number of analog-only displays and the flexibility and reliability of the analog interconnect. Frequently a 15-pin RGBHV analog interface is used. Developed originally for VGA PC graphics cards, this physical interface is capable of very high performance. But impairments can and do occur because of failures or poor design. Verifying proper RGBHV interface operation can help ensure that displays work predictably and deliver top-quality results. VGA Interface Background 8 colors, including black and white, or 16 with the I-bit. VGA, or Video Graphics Array, is a graphics controller By moving to analog representations of the RGB signals, standard introduced by IBM in 1987 with the PS/2 many more intermediate signal values could be conveyed personal computer. Prior to VGA, typical computer over a 3-wire interface. Standard VGA uses 6 bit displays used simple binary combinations of R, G, and B, representations, allowing 64 independent voltage levels sometimes with an added intensity, or I bit. A drawback on each of the RGB channels, making 262,144 color of the 3-wire binary interface was the limited number of combinations possible. colors and shades of gray that could be reproduced: VGA Interface Testing with the VM5000 Application Note VGA Interface Performance The power of an analog RGB interface is its ability to convey virtually any desired video value. In practice, this is limited by noise and distortion. That is, if 6 bit representa- tions are to be carried over the interface, then the noise and distortion level of the RGB signals must be low enough to allow reliable recognition of 64 (26) discrete and linearly spaced levels over the full range. If the interface performance does not support this, due to noise, non-linearity, gain errors, or signal transient behavior such as overshoot and ringing, then the potential benefit of higher bit Figure 1. VGA connector pin assignments. representations is lost. A more serious consequence of casually extending the VGA The VGA physical interface is a RGBHV 5-wire signal, interface to higher rates is proper display synchronization. using 700mV analog RGB video levels and TTL-level digital With a larger number of displayed pixels per frame comes HV synchronization signals. A 15-pin high density D-type a higher horizontal sync frequency, and the need for shorter connector (HD-15) carries these signals. Figure 1 shows a sync rise times and smaller levels of jitter. Improper detection VGA pin-out. The RGB signal lines are sourced by Digital of the horizontal and vertical sync events can reduce the to Analog Converters (DACs) coupled to video amplifiers spatial definition in the image, and at worst result in an capable of driving the 75Ω load assumed by the interface. unlocked and unrecognizable display. TTL-compatible logic buffers typically source the HV sync signals. This physical interface ended up being widely deployed, so it was preserved by subsequent graphics controller designs. Although commonly referred to as “the VGA port,” today’s 15-pin RGBHV output usually carries display formats with resolutions and color bit depth well beyond that of the 640 x 480 VGA standard. 2 www.tektronix.com/video VGA Interface Testing with the VM5000 Application Note Parameter Value Sync Logic “1” 2.4V – 5.5V @ 8mA source Sync Logic “0” 0.0V – 0.5V @ 8mA sink Sync rise/fall time (max) 80% of pixel clock period Sync monotonic rise/fall voltage range 0.5 – 2.4V Sync overshoot/undershoot (max) 30% Hsync jitter p-p (max) 30% of pixel clock period Video Max Luminance (input=FFh) 700mV + 70mV/-35mV Video Min Luminance (input=00h) 0.000V Video rise/fall time (max) 25% of pixel clock period Video settling time (max) 30% of pixel clock period Video overshoot/undershoot (max) 12% Video Integral and Differential Linearity error ±1 LSB Video channel voltage mismatch (max) 6% over full voltage range Video channel time skew (max) 50% of pixel clock period Video Noise Injection Ratio (max) ±2.5% of Max Video Luminance Table 1. VSIS specification summary. VESA Video Signal Standard compatibility between two devices connected with the The Video Electronics Standards Association (VESA) VGA interface. Without proper synchronization, little else recognized that performance issues had become more critical matters. The second area, Analog Video Outputs, contains as the VGA interface was being applied at higher and higher specifications for RGB video levels, noise, and transient signal rates. In 1999 VESA released a voluntary standard behavior. These are the key specifications for ensuring that whose goal was to more tightly define the parameters carried the analog interface will in fact convey the desired color over the RGBHV interface. Called the Video Signal Standard, resolution, whether it is a 6, 8, or 10 bit representation. or VSIS, it provides recommended values for the most Finally, the Test Circuits section prescribes the loading important aspects of the RGBHV signal. The VSIS at both ends of the interface. Improper loading can document was subsequently revised in 2000 and 2002. substantially alter the transient response for both the sync and RGB video signals. VSIS makes recommendations in three parts. The first, Synchronization Signal Specification, covers parameters The VSIS specs are summarized in Table 1. Complete such as sync logic levels, sync jitter, and sync transition specifications can be found in the VESA Video Signal behavior. These specs are at the top of the list for ensuring Standard document, available through VESA (www.vesa.org). www.tektronix.com/video 3 VGA Interface Testing with the VM5000 Application Note VGA Test Procedure Making Connections In 2002 VESA supplemented VSIS with a companion test Frequently overlooked is the method of connecting the procedure. Simply titled “Test Procedure—Evaluation of scope to the signal. We want to observe the signal without Analog Display Graphics Subsystems,” the document affecting its characteristics; that is, we want insignificant describes recommended methods for measuring VSIS loading from the scope connection. The 1 MΩ input resist- parameters. Twelve tests are detailed, each listing required ance of most scopes makes this easy at low frequencies. equipment, test patterns, test procedure, and necessary At high frequencies, however, the scope input capacitance analysis. If you plan to do VSIS testing, this is an important begins to dominate. For example, a 20pF input presents document. It helps clarify some of the VSIS specifications an 80Ω reactive load at 100 MHz. This will affect the and their intent. performance of a 75 Ohm system operating in this The VSIS document and companion Test Procedure frequency range. are both available through VESA. Taken together, they VSIS provides clear guidance on test connections and provide an effective approach for assessing VGA loads. The RGB analog video source is expected to be interface performance. 75Ω. The test load is specified to be 75Ω, with a tolerance Implementing the VESA Tests of ±1.5Ω at DC, rising to ±15Ω at 60% of the pixel clock frequency. If the load is predominately a parallel RC, the The VSIS and Test Procedure are written around an maximum load capacitance permitted is: oscilloscope. This is a good fit, given the time-domain nature of many of the specifications. Plus, oscilloscopes are Cmax = 1/(377*PCF), commonly available, and most engineers and technicians where PCF is the pixel clock frequency in Hz know how to operate them. Despite this familiarity, some A second consideration is the effect this load capacitance care must be exercised in selecting an appropriate scope. has on the system risetime. The -3dB bandwidth of the First, the scope needs to have an adequate number of interconnect, assuming a 75Ω source and load, is: inputs. At least two vertical inputs with a separate trigger -3dB BW(Hz) = 1/(236*C) input are needed. This is to support those measurements Since VESA recommends that the measurement system that compare two signals; for example, the timing between -3dB point be at least 5X PCF, the maximum C from a the R and G video channels. A five channel device, with bandwidth perspective is: one input for each of the RGBHV signal lines, would be CmaxBW = 1/(1178*PCF), with PCF in Hz ideal. This would allow all measurements to take place with one connection. For example, XGA (1024x768) at 75 Hz uses a 78.75 MHz PCF, so Cmax is 1/(377*78.75 MHz) = 34 pF. CmaxBW is Second, each signal input needs to provide the appropriate 1/(1178*78.75 MHz) = 11pF. A direct connection to a 20pF vertical accuracy, sensitivity, and bandwidth for the scope will meet the VSIS load spec, but will not provide measurements. The Test Procedure recommends vertical the 5X PCF bandwidth regardless of the scope vertical accuracy of ±2% over the 200mV to 1.5V range. It also bandwidth. At higher PCFs the allowed load C is quite recommends a minimum -3dB bandwidth of five times small. Display format 2048x1536 at 85 Hz has a nominal the inverse of one pixel time, or five times the Pixel Clock 388 MHz PCF, so Cmax = 7pF and CmaxBW = 2pF. Frequency (PCF). This is to prevent degrading transient response measurements like rise time. With formats that use very short pixel times, this can push the bandwidth requirement to beyond 2 GHz. 4 www.tektronix.com/video VGA Interface Testing with the VM5000 Application Note Figure 2. Test connections for RGB measurements. Figure 3. VM5000 option VGA system with MIU. There are two common solutions to this problem.