Have you find the proper test point?
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Skilled testers are very familiar with the importance of the test point position which determines test results and accuracy, and affects signal judgments accordingly. So can every tester find the proper test point and understand its importance? The answer is no.For that, at least, some junior testers are easy to make basic mistakes when they are testing.
There is a funny story about my junior tester friend. When he found the DDR3 system was behaving incorrectly, he tried to test the clock signal of DDR3 by oscilloscope and he got the result like figure 1. Since the waveform in figure 1 showed clock signal was non-monotonic, so my friend maintained that it was a serious problem about signal quality and design, and it should be solved immediately.
But is that really so? When I got this non-monotonic waveform, I asked my friend the test position first and he told me he tested at the front of the resistances. Then I guess I already know the problem and I tried to check the topology of clock signal. (See figure 2)
My friend tested the TP1 in figure 2 and he got a non-monotonic waveform like figure 1. While the most valuable waveforms always come from the receiver but not the driver, because the waveform of the driver is often non-monotonic due to reflections. So if my friend tested TP2 at the first place, it would be another story. We can verify it through simulation. (See figure 3)
The red graph and grey graph in figure 3 refer to the simulation waveform of TPI (non-monotonic) and the one of TP2 separately. My friend was surprised that little test points change so many things. After he tested TP2, he got the results like figure 4.
The waveform in figure 4 is definitely fine. This, in turn, shows that my friend got a wrong decision at the first place. The DDR3 system’s problem had nothing with signal quality and PCB design. After checking again, we found the main problem is about the boot software. So we just updated the boot software, the system works well now.
Is it amazing? Have you find the proper test point? A little test point does change a lot of things. What we should do is just care about it and never ignores it.