Mr. High Speed.
Author |Zhou Wei.
We often encounter many systems connected through high-speed connectors, and the signal is routed according to the protocol of PCIe3 or PCIe4, often the impedance of many connectors is usually 100ohm standard, and PCIe3 or PCIe4 is 85ohm standard according to the protocol or chip requirements, so at this time our line impedance is better according to 85 or 100 ohms
The following are some chip requirements or protocol requirements for line impedance.
To sum it up, it's PCIe 30 below 100ohm, PCIe3 or above is the standard of 85ohm, the higher the rate, the stricter the impedance tolerance requirements.
Now one of our customers is experiencing a practical problem above. The customer's system is composed of a CPU board and an FPGA board, in which the CPU board has 4 high-speed connectors on the board and one connector on the FPGA board, and the signal in the middle uses PCIe30 protocol, may be upgraded to support PCIe 4 later0 protocol, while the connector in the middle is a regular backplane connector, as shown in the figure below:
Then the problems reported by the customer are as follows:
At the same time, the eye diagram of the test was also relatively poor, and the eyes were almost open, and it looked like they hadn't woken up.
The position of the corresponding CPU board is shown in the following figure:
On the same board, two of the connectors are easy to have problems, and the other two have not found any problems, in this case, our treatment is to look at the PCB design first, and then look at the next step according to the design.
Open the PCB design file and look at the traces of TX0 on the Card3 connector and RX1 on the Card4 connector, as shown below:
The card3 connector on the tx0 is at the bottom layer, the connector has also been hollowed out, the BGA is not optimized, and the other signals go over together, the optimization method is similar, there is no reason for the signal alone to be poor;
RX1 on the Card4 connector is at the ART12 layer, except for the BGA, there is no optimization, but this signal is at the edge of a set of traces. From the design point of view, there are not too many problems, it is more conventional.
From the design point of view, there is really no obvious problem, there must be a demon when things go wrong, and the children's shoes who understand our high-speed Mr. routine must know our next operation, that is, a mule is a horse, pull it out and yo-yo - the physical board to test the impedance to see what the difference is in the impedance.
The system impedance of card3 is connected to the backplane.
From the perspective of the system impedance of card3, first of all, the impedance of the connector with the biggest problem is high, up to 107ohm, and secondly, the impedance of the tx0 signal with the problem is the lowest on card3, which is about 83ohm, and the impedance of the bottom plate is about 90 91ohm
The system impedance of the card4 base plate.
From the point of view of the system impedance of card4, the biggest problem is still the high impedance of the connector, the highest to about 107ohm, the highest impedance is exactly the rx1 signal that has the problem, the overall impedance of card4 is about 88ohm, and the impedance of the bottom plate is about 94ohm, at this time, from the perspective of reflection, the impedance on the rx1 line and the impedance deviation of the connector is the largest;
Judging from the test results of the impedance of the two board subsystems, there are several common characteristics: firstly, the impedance of the connector is high, and secondly, the impedance of the bottom plate is controlled at about 92 94ohm, and whether the subcard is card3 or card4, the impedance should be controlled according to the impedance requirements of 85ohm, and the overall impedance is below 90ohm.
Seeing this, here comes our question
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Questions in this issue. From the perspective of problem solving, how can the daughter board where the card3 and card4 connectors are located be designed to solve the current problem?Everyone is welcome to speak up.
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