Error one:
These pull-up/pull-down resistors have little relationship with the resistance, so choose an integer 5K!
Comments: There is no 5K resistance value on the market, the closest is 4.99K (accuracy 1%), followed by 5.1K (accuracy 5%), and its cost is 4 times and 2 times higher than the 4.7K with 20% accuracy. . The resistance of 20% precision is only 1, 1.5, 2.2, 3.3, 4.7, 6.8 (including integer multiples of 10); similarly, 20% precision capacitors only have the above values, if other ones are selected Values ​​must use higher precision, and the cost can be multiplied several times without any benefit.
Error two:
This part of the circuit is designed to be free of problems as long as it requires software.
Comments: Many electrical features on the hardware are directly controlled by the software, but the software is often accidental. After the program runs away, it is impossible to predict what will happen. The designer should ensure that no permanent damage is caused in a short period of time, regardless of the hardware in which the software is being operated.
Error three:
This logic can also be used with the 74XX gate circuit, but it is too earthy, still use CPLD, it looks more upscale.
Comments: The door of the 74XX is only a few cents, and the CPLD has at least dozens of pieces. The cost has been increased by N times, and the work of production, documentation, etc. has been added several times.
Error four:
The PCB design requirements of this board are not high, just use a thin line and automatically cloth it!
Comments: Automated wiring must occupy a larger PCB area, and at the same time produce more than a lot of vias than manual wiring. In large batches of products, the factors considered by PCB manufacturers to cut prices are line width and The number of holes, which affect the yield of the PCB and the consumption of the drill bit, respectively, saves the cost of the supplier, and finds a reason for the price reduction.
Error five:
Our system is powered by 220V, so we don't have to worry about power consumption.
Comments: Low-power design is not just to save power. More benefits are reduced cost of power modules and cooling systems, and reduced electromagnetic radiation and thermal noise due to reduced current. As the temperature of the device decreases, the lifetime of the device increases accordingly (for every 10 degrees increase in the operating temperature of the semiconductor device, the lifetime is reduced by half).
Error six:
These bus signals are pulled with a resistor and feel relieved.
Comments: There are many reasons why the signal needs to be pulled up, but not all of them have to be pulled. The pull-down resistor pulls a simple input signal, and the current is tens of microamps or less. However, if a signal is driven, the current will reach milliamperes. The current system is usually 32 bits of address data, and there may be After the 244/245 isolated bus and other signals are pulled up, the power consumption of a few watts is consumed by these resistors (do not use the concept of 8 cents per kilowatt to treat the power consumption of these watts).
Error seven:
How to deal with these unused I/O ports of CPU and FPGA? Let it be empty first, and then talk about it later.
Comments: If the unused I/O port is left floating, it may become an input signal of repeated oscillations due to a little interference from the outside world, and the power consumption of the MOS device basically depends on the number of times the gate circuit is flipped. If you pull it up, each pin will also have a micro-ampere current, so the best way is to set it as an output (of course, you can't connect other driven signals outside).
Error eight:
There are so many doors left in this FPGA that you can use it!
Comments: The power consumption of FGPA is proportional to the number of flip-flops used and the number of flips, so the power consumption of the same model FPGA at different times of different circuits may differ by a factor of 100. Minimizing the number of flip-flops at high speeds is the fundamental way to reduce FPGA power consumption.
Error nine:
The power consumption of these small chips is very low, no need to consider.
Comments: It is difficult to determine the power consumption of the chip that is not too complicated inside. It is mainly determined by the current on the pin. An ABT16244 consumes less than 1 mA without load, but its indicator is each foot. Can drive 60 mA load (such as matching tens of ohms of resistance), that is, the maximum power consumption of up to 60 * 16 = 960mA, of course, only the power supply current is so large, the heat is falling on the load.
Error ten:
There are so many control signals in the memory. I only need to use the OE and WE signals on this board. The chip selection is grounded, so the data is much faster when I read the operation.
Comments: Most of the memory power consumption is more than 100 times larger than when the chip select is valid (regardless of OE and WE), so CS should be used to control the chip as much as possible, and if other requirements are met, It is possible to shorten the width of the chip select pulse.
Error 11:
How do 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, CML), there are overshoots. As long as they are not very large, they do not necessarily need to match, even if the match is not the best match. The output impedance of TTL is less than 50 ohms, and some even 20 ohms. If such a large matching resistor is used, the current is very large, the power consumption is unacceptable, and the signal amplitude will be too small to be used. In addition, the output impedance of the general signal at the output high level and the output low level is not the same, and the method is completely matched. Therefore, the matching of signals such as TTL, LVDS, and 422 can be accepted as long as the overshoot is acceptable.
Error 12:
Reducing power consumption is a matter for hardware personnel, and it has nothing to do with software.
Comments: The hardware is just a stage. The software is the software. The access of almost every chip on the bus and the flipping of each signal are almost controlled by software. If the software can reduce the number of external accesses (use more register variables, More use of internal CACHE, etc., timely response to interrupts (interrupts are often active low with pull-up resistors) and other specific measures for specific boards will greatly contribute to reducing power consumption.
Error 13:
This CPU with a frequency of 100M can only handle 70%, and it is fine to change the 200M frequency.
Comments: The processing power of the system involves a variety of factors. In the communication business, the bottleneck is generally in the memory, the CPU is fast, and the external access is not fast.
Error 14:
CPU with a larger CACHE, it should be faster.
Comments: The increase in CACHE does not necessarily lead to an increase in system performance. In some cases, closing CACHE is faster than using CACHE. The reason is that the data moved to CACHE must be reused multiple times to improve system efficiency. Therefore, in the communication system, only the instruction CACHE is generally opened, and even if the data CACHE is opened, it is limited to a part of the storage space, such as the stack portion.
At the same time, the program design also needs to take into account CACHE's capacity and block size. This involves the length and jump range of the key code loop body. If a loop is just a little bigger than CACHE, and it is repeated again, it will be miserable.
Error fifteen:
A CPU can't handle it, and with two distributed processing, the processing power can be doubled.
Comments: For moving bricks, two people should be twice as efficient as one person; for painting, one more person can only help. Use several CPUs to have more knowledge of the business before you can determine, minimize the cost of coordination between the two CPUs, so that 1+1 is as close as possible to 2, and not less than 1.
Error sixteen:
This CPU comes with a DMA module, and it is certainly faster to use it to move data.
Comments: The real DMA is to start the bus at the same time after the hardware grabs the bus, and read it in one cycle, there are some. However, many DMAs embedded in the CPU are just analog. Before starting each DMA, you need to do a lot of preparatory work (set the start address and length, etc.). In the transmission, it is often read into the chip for temporary storage, and then write it out. That is, it takes two clock cycles to move data once, which is faster than software to move (no need to take instructions, no extra work such as loop jump), but if you only move a few bytes at a time, you still have to do a bunch of preparations. It is also generally involved in function calls, which is not efficient. So this kind of DMA is only applicable to big data blocks.
Error 17:
In order to ensure a clean power supply, the decoupling capacitor is much better.
Comments: In general, the more power capacitors are, the more stable the power supply will be, but there are too many disadvantages: waste of cost, difficulty in wiring, and too much power-on current. The key to the design of the decoupling capacitor is to select the pair of capacitors and place them in the right place. The general chip manuals have a design reference for the decoupling capacitors. It is best to follow the manual.
Error 18:
You can't blame me for a user error.
Comments: It is right to ask the user to operate strictly according to the manual. However, when the user is a human being, there is a mistake when it is impossible to say that the wrong button is broken and the wrong plug is inserted. Therefore, all kinds of mistakes that users may make must be protected.
Error 19:
The reason for this board's bad is that there is a problem with the board at the opposite end, and it is not my responsibility.
Comments: For a variety of external hardware interfaces should have sufficient compatibility, you can not because the other party's signal is not normal, you will rest. It should not only affect the part of the function that is related to it, but other functions should work properly, should not be completely striked, or even permanently damaged, and once the interface is restored, you should immediately return to normal.
Error twenty:
Our system requirements are so high, including MEM, CPU, FPGA and other chips must be the fastest.
Comment: Not all parts of a high-speed system work at high speeds, and for every level of device speed, the price is almost doubled, and it also has a significant negative impact on signal integrity issues.
Twinkle System Technology Co Ltd , https://www.pickingbylight.com