Recently I have been working on embedded systems and drawing schematics. Finally, in order to ensure that the schematic is accurate, it takes me nearly two weeks to check the schematic. Here, I will summarize my experience in checking the schematic diagram for your reference. Anyone who is wrong is welcome to point out.
Often after we have drawn the circuit schematic, we also know that we need to check and check, but where do we start to check it? What should I pay attention to when checking the schematic? Let me listen to my experience.
1. Check all chip package diagram pins for errors.Of course, I am referring to the chip package I painted. In the project, I once reversed the two pins of a chip, which led to the jumper after the final plate making, which made it difficult to read.
Therefore, before the inspection and the schematic diagram must start from the package of the chip, resolutely kill the wrong package in the cradle!
2. Use protel's Tools->ERC electrical rules check to troubleshoot the files generated by it.This refers to the ERC electrical rules check of protel99, DXP ​​should also have a corresponding menu to complete such an inspection. Very useful, it can help you find a lot of errors, according to the error file it generates, check your schematic against the error file, you should be amazed: "I draw so carefully, there will be so many mistakes. ?"
3. Check if all network nodes net are connected correctly (emphasis)Common errors that are common are:
(1) Originally two nets should be connected, but they are inadvertently marked as inconsistent. For example, I used to put the DDR clock pin of the main chip as DDR_CLK, and the clock pin corresponding to the DDR chip as DDRCLK, because the names are inconsistent. In fact, these two feet are not connected together.
(2) Some nets only indicate one, and the other end of the net is forgotten to mark it.
(3) The same net label has multiple places to reuse, causing them all to be connected together.
4. Check if the function pins of each chip are connected correctly, and check if all the chips have missing pins.The function pins of the chip must not be connected incorrectly. For example, the audio processing chip I use has three clock pins, LCLK, BCLK, and MCLK. It must have one-to-one correspondence with the three audio clock pins of the main chip. Already working.
Whether there are missing pins is very easy to check, carefully observe each chip to see if there are any missing pins that are not connected, check the datasheet, see what function of the pin, if not needed in the system, use X to Pin X is off.
5. Check whether all external capacitors, inductors, and resistors have a valid value, rather than random values.In fact, when a newbie draws a schematic, it is often unclear how some peripheral resistors and capacitors are worth. At this time, do not take values ​​at random. Often, the values ​​of these peripheral circuit resistors and capacitors are described on the chip's datasheet. Some datasheets also give a typical reference circuit, or some calculation formula of resistors and capacitors, as long as you are careful enough, most of the values ​​of resistors and capacitors can be found. Occasionally, there is no basis for it. You can search other people's design cases or typical connections on the Internet. In short, don't set these values ​​at will.
6. Check if all chip power supply terminals are capacitively filtered.The importance of capacitive filtering at the power supply side does not need to be said more. In fact, anyone who has done hardware should know. Under normal circumstances, the input of the circuit power supply will introduce some ripple. In order to prevent these ripples from affecting the logic of the chip too much, it is necessary to add some capacitors such as 0.1uf to the power supply end of the chip to play some filtering. The effect, when checking the circuit schematic, you can carefully observe whether the filter circuit is added to the power supply end of the chip.
7. Detect all interface circuits of the systemThe interface circuit generally includes the input and output of the system, and it is necessary to check whether the input has the proper protection, etc., and whether the output has sufficient driving capability, etc.
Input protection generally includes: kickback current protection, optocoupler isolation, overvoltage protection, and so on.
The need for insufficient output drive capability plus some pull-up resistors improves drive capability.
8. Check whether each chip has power-on and reset sequence requirements. If required, you need to design the corresponding delay circuit.For example, the DM6467 chip used in my project has a sequential order for power supply voltage. It must first supply power to the 1.2V power supply terminal, then supply power to the 1.8V power supply terminal, and finally supply power to the 3.3V power supply terminal. Therefore, we pass the three voltages generated by the power chip through a delay chip (in fact, a triode can also be used, using the clamp voltage), and then sequentially delivered to the main chip.
9. Check the ground of each chip, the analog ground connected to the analog ground, the digital ground connected to the digital ground, whether the digital ground is separated from the analog ground.Generally, chips for processing analog signals include: sensor chip, analog signal acquisition chip, AD conversion chip, power amplifier chip, filter chip, carrier chip, DA conversion chip, analog signal output chip, etc., often only when these processing analog signals exist in the system. Analog and digital grounds are involved in the chip or circuit.
The grounding pin of the general chip is connected to the analog or digital ground in the chip manual. It can be connected according to the datasheet.
10. Observe whether each module has a better solution (optional)In fact, when I first designed the first draft of the schematic, I often didn't think so much. When the whole system was formed, you often found that many places can be improved and optimized. The power modules in our project have been revised 4 times before and after. Every time I find a better solution, the power supply scheme is simple and practical, and the effect is much higher. I think this is continuous improvement and continuous optimization. The benefits!
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