Misunderstanding 1: Cost saving
Common mistake 1: What color should the indicator light on the panel choose? I personally prefer blue, so choose it.
Positive solution: For the indicator lights on the market, red, green, yellow, orange, etc., regardless of size (under 5MM) and packaging, they have been mature for decades, so the price is generally less than 50 cents. The blue indicator light was invented in the past three or four years. The technology maturity and supply stability are relatively poor, so the price is four or five times more expensive. If you design the panel stack indicator color without special requirements, don’t choose blue. At present, the blue indicator light is generally only used in occasions that cannot be replaced by other colors, such as displaying video signals.
Common mistake 2: These pull-down/pull-up resistors don’t seem to matter much with their resistance values. Just choose an integer 5K.
Positive solution: In fact, there is no resistance value of 5K in the market. The closest is 4.99K (accuracy 1%), followed by 5.1K (accuracy 5%). The cost price is 4 times higher than that of 4.7K with 20% accuracy. 2 times. The resistance value of 20% precision resistance has only 1, 1.5, 2.2, 3.3, 4.7, 6.8 types (including integer multiples of 10); correspondingly, the 20% precision capacitor also has only the above several capacitance values. For resistors and capacitors, if you choose a value other than these types, you must use higher accuracy, and the cost is doubled. If the accuracy requirements are not large, this is costly waste. In addition, the quality of resistors is also very important. Sometimes a batch of inferior resistors is enough to destroy a project. It is recommended that you buy them in genuine self-operated stores such as Lichuang Mall.
Common mistake 3: 74XX gate circuit can be used for this logic, but it is too dirty, so use CPLD, it seems much more high-end.
Positive solution: 74XX gate circuit is only a few cents, and CPLD is at least dozens of dollars (GAL/PAL is only a few dollars, but it is not recommended), the cost has increased many times, not to mention, it is returned to production, documentation, etc. Add several times the work. Under the premise of not affecting performance, it is obviously more appropriate to use the 74XX with higher cost performance.
Common mistake 4: The PCB design requirements of this board are not high, just use a thinner wire and automatically arrange it.
Positive solution: Automatic wiring will inevitably take up a larger PCB area, and at the same time it will produce many times more vias than manual wiring. In a large batch of products, PCB manufacturers have important considerations in terms of line width and the number of vias in terms of pricing. , They respectively affect the yield of PCB and the number of drill bits consumed. In addition, the area of the PCB board also affects the price. Therefore, automatic wiring is bound to increase the production cost of the circuit board.
Common mistake 5: Our system requirements are so high, including MEM, CPU, FPGA and all chips must choose the fastest.
Positive solution: Not every part of a high-speed system works at high speed, and every time the device speed increases by one level, the price almost doubles, and it also has a great negative impact on signal integrity problems. Therefore, when choosing a chip, it is necessary to consider the degree of use of different parts of the device, rather than using the fastest.
Common mistake 6: As long as the program is stable, longer code and lower efficiency are not critical.
Positive solution: CPU speed and memory space are both bought with money. If you spend a few more days to improve program efficiency when writing code, then the cost savings from reducing the CPU frequency and reducing the memory capacity are definitely worthwhile. The CPLD/FPGA design is similar.