​Eight Mistakes in PCB Circuit Design and Principles of PCB Layout Review

The PCB manufacturer explains the eight mistakes in PCB circuit design and the principle of PCB layout review

Phenomenon 1: The PCB design requirements of this board are not high, so use thinner lines to automatically cloth

Comments: Automatic wiring will inevitably occupy a larger PCB area and generate many times more vias than manual wiring. Among products in large batches, the factors considered by PCB manufacturers for price reduction, apart from business factors, are the line width and the number of vias. They respectively affect the yield of PCB and the consumption of drill bits, saving the cost of suppliers, and thus finding reasons for price reduction.

Phenomenon 2: These bus signals are pulled with resistance, which makes me feel relieved.

Comment: There are many reasons why signals need to be pulled up and down, but not all of them need to be pulled up and down. Pull up and down the resistance to pull a simple input signal, and the current will be less than tens of microamps. But pull a driven signal, and the current will reach milliampere level. Today's systems often use 32 bits of address data, and possibly 244/245 isolated buses and other signals. If you pull up, several watts of power consumption will be consumed by these resistors.

Phenomenon 3: How to deal with these unused I/O ports of CPU and FPGA? Let it be empty first, and let it be later.

Comment: If the unused I/O port is suspended, a little interference from the outside may become an input signal that oscillates repeatedly. The power consumption of MOS devices basically depends on the number of flips of the gate circuit. If you pull it up, each pin will also have microamp current, so the best way is to set it as an output (of course, no other driving signals can be connected outside)

Phenomenon 4: There are so many gates left in this FPGA, so you can play it to your heart's content

Comment: The power consumption of FGPA is proportional to the number of flip-flop used and the number of flips, so the power consumption of the same type of FPGA in different circuits at different times may vary by 100 times. Minimizing the number of flip flops is the fundamental way to reduce FPGA power consumption.

Phenomenon 5: The power consumption of these small chips is very low, so it is unnecessary to consider

Comments: It is difficult to determine the power consumption of a chip that is not too complex internally. It is mainly determined by the current on the pin. An ABT16244 consumes less than 1 mA without a load, but its indicator is that each pin can drive a load of 60 mA (such as matching a resistance of tens of ohms). That is, the maximum power consumption of a full load can reach 60 * 16=960mA. Of course, the power supply current is so large that the heat falls on the load.

Phenomenon 6: There are so many control signals in the memory. I only need to use OE and WE signals on this board. Just ground the chip selection, so that the data will come out much faster during the read operation.

Comment: The power consumption of most memories will be more than 100 times greater when the chip selection is valid (regardless of OE and WE) than when the chip selection is invalid. Therefore, CS should be used to control the chip as much as possible, and the width of the chip selection pulse should be shortened as much as possible when other requirements are met.

Phenomenon 7: Why are these signals overshoot? As long as the match is good, it can be eliminated

Comments: Except for a few specific signals (such as 100BASE-T and CML), there are overshoots. As long as they are not very large, they do not need to be matched, even if the matching is not the best. For example, the output impedance of TTL is less than 50 ohms, and some even 20 ohms. If such a large matching resistance is also used, the current will be very large, the power consumption will be unacceptable, and the signal amplitude will be too small to use. In addition, the output impedance of general signals at high output level and low output level is not the same, and there is no way to achieve complete matching. Therefore, the matching of TTL, LVDS, 422 and other signals is acceptable as long as overshoot is achieved.

Phenomenon 8: Reducing power consumption is a matter for hardware personnel, not software.

Comment: The hardware is just a stage, but the software is the singer. The access to almost every chip on the bus and the overturning of every signal are almost controlled by the software. If the software can reduce the number of accesses to external memory (use more register variables, use more internal CACHE, etc.) Timely response to interrupts (interrupts are usually low level and have pull-up resistors) and other specific measures for specific boards will make a great contribution to reducing power consumption.

Principle of PCB layout review

1. Whether the system layout can ensure the reasonable or optimal wiring, whether it can ensure the reliable wiring, and whether it can ensure the reliability of circuit work. In the layout, it is necessary to have an overall understanding and planning of the signal direction and the power and ground network.

2. Whether the size of the printed circuit board is consistent with the size of the processing drawing, whether it meets the requirements of the PCB manufacturing process, and whether there are behavior marks. This requires special attention. The circuit layout and wiring of many PCB boards are designed beautifully and reasonably, but the precise positioning of the positioning connector is neglected, which makes the designed circuit unable to connect with other circuits.

3. Whether the components have conflicts in 2D and 3D space. Pay attention to the actual size of the device, especially the height of the device. The height of components without layout in welding shall not exceed 3mm generally.

4. Whether the layout of components is dense, orderly and orderly, and whether they are completely distributed. In the layout of components, it is not only necessary to consider the direction and type of signals, places needing attention or protection, but also the overall density of component layout to achieve uniform density. The circuit board manufacturer explains the eight major mistakes in circuit design of circuit boards and the principle of reviewing PCB drawings during PCB layout.