Recently, due to project requirements, it is necessary to conduct a thorough test on the USB eye diagram. The testing process is very simple, but the road to debugging is not so simple. Not being simple often means bringing a lot of gains.
The oscilloscope is connected across the output end of the receiving filter, and the scanning period of the oscilloscope is adjusted so that the horizontal scanning period of the oscilloscope is synchronized with the period of the received symbol. 111), these signals are superimposed to form a pattern similar to the human eye. At the same time, according to the requirements of the association, the bandwidth of the oscilloscope should be at least 1.5GHZ, preferably 2GH or 2.5GHZ.
It mainly uses the afterglow method to cumulatively superimpose and display the results of the bits of the serial signal collected. The superimposed graphic shape is also called an eye diagram because it looks like an eye. The eye diagram represents the bit information of the serial signal and has become an important tool to measure the signal quality. Through the eye diagram, the influence of intersymbol crosstalk and noise can be observed, so as to estimate the quality of signal integrity.
The measurement of the USB eye diagram is mainly divided into device and host.
USB host: D+ and D- each connected to a 15kΩ pull-down resistor;
USB device: high-speed and full-speed D+ connected to a 1.5kΩ resistor to pull up to 3.3V; low-speed D- connected to a 1.5kΩ resistor to pull up to 3.3V . After the high-speed chirp KJ handshake (the frequency of Chirp KJ is about 10KHz) is successful, the 1.5kΩ pull-up on D+ is automatically disconnected; when the high-speed device enters suspend, the 1.5kΩ pull-up on D+ is enabled to 3.3V, and when exiting suspend , disconnect the 1.5k pull-up on D+.
The hub's full-speed driver and the device's full-speed driver are functionally equivalent to each being a 45Ω resistor, and the combined 90Ω differential resistance produces a zero-coefficient reflection. In this way, the requirement that the characteristic impedance of the cable is 90Ω is generated.
In HS mode, the single-ended impedance of DP/DM to ground is 45Ω (a terminal resistance with impedance of 45Ω formed by the full-speed driver), and the matching resistance in series must be 0, otherwise there is a problem with the eye diagram; DP/DM of LS/FS must There is a matching resistance greater than 0. The DC-level of DP/DM in HS mode is 17.78mA x (45Ω//45Ω) = 17.78mA x 22.5Ω = 400mV, of which 17.78mA is a current source (generally requires an external reference resistor for PHY), so the differential mode voltage is 800mV. DC-level of LS/FS is 3.3V.
The signal integrity problem caused by the 10Ω resistor in series on the HS PHY DP/DM is shown in the following calculation
Udevice = 17.78mA*(45//(10+45)) = 440.1mV, Uhost = (45/(45+10))*440.1 = 360.1mV
This is why we need to control the maximum and minimum of the eye diagram at about ±400mV, and the reason why it is not easy to put resistance on D+ D-.
It doesn’t matter if you don’t understand it, you can ignore it for the time being. In the following articles, the editor will focus on introducing these. It is recommended to turn back and read this content at that time.
The measurement parameters of the eye diagram are different from the measurement parameters of the waveform. In more cases, it is a statistical measurement. The main measurement parameters are eye height, eye width, and eye jitter.
Measurement of Eye Height
Eye height reflects the degree of eye opening in the vertical direction. The measurement method is to first count the level distribution of the eye diagram at the center of the eye diagram, and obtain the positions of high level (One Level) and low level (Zero Level) according to the position with the highest probability of occurrence of the histogram distribution; and then Then according to the noise distribution on the high and low levels, 3 sigma (RMS value of the noise) are pushed inward, so as to obtain the measurement result of eye height.
Measurement of Eye Width
Eye width reflects the degree to which the eye diagram is opened in the horizontal direction. The measurement method is to make statistics on the horizontal distribution of the eye diagram at the intersection position of the eye diagram, and obtain the horizontal positions of the intersection point 1 (Cross1) and the intersection point 2 (Cross2) according to the position with the highest probability of the histogram distribution; and then According to the jitter distribution near the crossing point, 3 sigma (RMS value of jitter) are pushed inward to obtain the measurement result of eye width.
Eye Jitter Measurement
The jitter of the eye diagram reflects the time uncertainty of the signal, and if the jitter is too large, the eye width of the signal will be reduced. The jitter of the eye diagram refers to the horizontal jitter of the signal near the intersection point of the eye diagram, which can be measured by RMS or Peak-Peak value.
Regarding the eye diagram, it is far from enough for us to only know the relevant key test parameters, and it is also necessary to quickly determine whether the tested signal meets the specification requirements of the bus. When using instrumentation testing, templates are often used.
The template, that is, the range requirements of the signal high and low levels and the range of jitter are defined in advance, and then the eye diagram test file is generated on this template.
The template mainly consists of three parts:
The top area defines the maximum amplitude requirement of the signal.
The bottom area defines the minimum amplitude requirement for the signal.
The middle region defines the eye opening requirements for the signal.
When performing an eye diagram test, put the eye diagram directly on the template. If the long-term cumulative measurement does not press on the template, it means that the signal quality basically meets the requirements.
PASS
If the signal under test is pressed against the test template, it indicates that there is an obvious problem with the quality of the signal under test.
FAIL
Usb signal line design
In terms of USB high-speed signals, we often like to connect resistors, inductors or parallel TVS tubes at the D+ and D- terminals for static or surge protection. Since it is a high-speed signal (rate 480Mbps, frequency 240MHZ), the selection of TVS tube should be cautious. The TVS tube has a junction capacitance, and the capacitance of the junction capacitance should be paid special attention to. In the projects that Xiaobai has done, the capacitance value of the TVS tube junction capacitance is often selected below 1pf. If the capacitance is too large, the opening of the eye diagram will be smaller due to the influence of RC delay, which may overwhelm the middle mask. At the same time, the selection of series resistance should also be appropriate, because it affects the eye height.
Therefore, we should pay special attention to this aspect when debugging the eye pattern pressure template.
At the same time, the common failures of the eye diagram test can be briefly summarized as follows
: poor PCB design: too long traces (long traces mean large losses), mismatched trace impedance (requires 90±10% ohms), interference from other sources (Suggest three-dimensional wrapping ground processing),...At the same time, the line width is recommended to be thicker.
Bad
ESD cables, EMI components do not meet USB requirements,
bad clocks, PLL
power interference ,
ferrite beads and common mode chokes.
Do not use the latest USBET
bad test fixture
In the adjustment of the eye diagram, it is better to make the eye diagram as wide as possible. The
larger the eye diagram opening, the greater the tolerance for noise and jitter; the
larger the eye diagram opening, the better the accuracy of the receiver in judging the signal.