1. Which aspects should the circuit board DEBUG start from?

As far as digital circuits are concerned, first determine three things in order:

1) Confirm that all power values ​​meet the design requirements. Some systems with multiple power supplies may require certain specifications for the order and speed of the power supplies.

2) Confirm that all clock signal frequencies are working properly and there are no non-monotonic problems on the signal edges.

3) Confirm whether the reset signal meets the specification requirements.

If these are normal, the chip should send out the first cycle (cycle) signal. Next, debug according to the operating principle of the system and the bus protocol.

2. In the case of a fixed circuit board size, if more functions need to be accommodated in the design, it is often necessary to increase the PCB trace density, but this may increase the mutual interference of the traces, and at the same time, the traces are too thin and the impedance Can’t be reduced, please introduce the skills in high-speed (>100MHz) high-density PCB design?

When designing high-speed and high-density PCBs, crosstalk interference (crosstalk interference) really needs special attention, because it has a great impact on timing and signal integrity. Here are a few points to note:

1) Control the continuity and matching of the characteristic impedance of the wiring.

The size of the trace spacing. It is generally seen that the spacing is twice the line width. It is possible to know the influence of trace spacing on timing and signal integrity through simulation, and find the minimum tolerable spacing. The result of different chip signals may be different.

2) Choose the appropriate termination method.

Avoid two adjacent layers with the same wiring direction, even if there are wirings that overlap each other, because this kind of crosstalk is greater than that of adjacent wiring on the same layer.

Use blind/buried vias to increase the trace area. But the production cost of the PCB board will increase. It is indeed difficult to achieve complete parallelism and equal length in actual implementation, but it is still necessary to do so.

In addition, differential termination and common mode termination can be reserved to alleviate the impact on timing and signal integrity.

3. The filtering at the analog power supply often uses an LC circuit. But why is the filtering effect of LC worse than RC sometimes?

The comparison of LC and RC filtering effects must consider whether the frequency band to be filtered and the choice of inductance are appropriate. Because the inductance of an inductor (reactance) is related to the inductance value and frequency. If the noise frequency of the power supply is low, and the inductance value is not large enough, the filtering effect may not be as good as RC.

However, the cost of using RC filtering is that the resistor itself consumes energy and has poor efficiency, and pay attention to the power that the selected resistor can withstand.