Talking about Communication Interface Technology

Now, do you have a very strange feeling that more and more devices use serial communication, and also say that the serial port is so simple that several lines are faster than the parallel port of a dozen lines, which is somewhat inexplicable anyway. . If you feel this way, it is recommended to continue watching. If you feel normal, please detour. Let's talk about the interface that everyone is most familiar with, that is, the UART interface, RS232 type DB9 interface, the old-fashioned computer has, can be connected to the modem, we do MCU51 MCU project, generally use this UART interface, serial port rate 115200bps, 57600bps Wait, then come out USB to serial port PL2302 and FT232, the rate can go to 921600bps, this is the highest. Because all kinds of single-chip microcomputers have UART interface, no matter MCU51 or ARM, this is the most familiar to everyone.

The synchronous serial port has SPI, IIC, etc., especially the SPI speed is relatively high, and can reach several M, tens of M. The UART and the like belong to the asynchronous serial port, because the data synchronization signal recovery needs to be considered, so the speed is not high. Everyone knows the hard disk IDE interface, printer interface, as well as the parallel port of single-chip microcomputer, ARM, etc. These speeds are much faster than the serial port, but there are many more lines. It can be understood as a parallel connection of multiple serial ports. In other words, the parallel port is the serial port to send data at the same time. But everyone will also find such a phenomenon, that is, there are two types of parallel port. For example, when you connect to SDRAM, it is more obvious. SDRAM is much faster than ordinary parallel port. SDRAM speed is very easy to 100MHz, and ordinary parallel port speed Can only be in the vicinity of tens of M, especially for ordinary mobile phones, know that the flash interface speed can not go. This is actually not caused by the interface speed, but the bus mode is different. The earliest intel80 interface, the read signal is Data, Adrr, nCS, nRD, nWR, this time you need to send the address, the chip can be decoded to obtain data, so the speed is slow, the general MCU51 is such a bus, Called intel80, in addition to many black and white LCD screen with 68K interface, nRD, nWR replaced with R / W, E.

The new interface generally uses Data, Addr, CLK, RAS, CAS, WE, etc. This is generally called the burst mode parallel port, and intel80, 68K is generally called the normal mode parallel port. The biggest advantage of burst mode is that after configuration, the address is only sent once, and then the data is read continuously through CLK, and the address is continuously read back, so that the address is no longer needed, the decoding process is selected, and the internal default is continuous. Read, write, can be pipelined, so the speed is much faster, of course, this kind of CPU usually has high-speed RAM, or it needs to read and write the whole screen, such as larger color LCD screen, 3.5inch~10inch are This type of interface, on the LCD screen, some people call this RGB interface. It is most common on ARM9 processors, such as 8 data lines of R, G, and B, but it is often not necessary to remove some low-level data to form 565 format, followed by CLK, VS (frame sync signal), HS (row sync signal) ), IIC configuration signal. One thing in common here is that the propagation of information is based on electrical signals, which are generally judged by how much V or less than V is 1 or 0. The carrier of the electrical signal is the electric field and voltage, so the above discussion is based on the conventional circuit loop, and the carrier medium of the signal is relatively simple, as long as it can be looped. As we all know, the conventional circuit is an approximation of the electromagnetic field theory at low speed, which is the steady state after the electromagnetic field propagates multiple fold reflections. When the signal frequency is higher, this folding reflection will lead to a decrease in the sharpness of the signal. That is to say, the signal integrity is reduced, and the square wave becomes a sine wave and becomes unrecognizable.

Therefore, in order to further increase the communication speed, it is necessary to change the carrier of the transmission and change from the electric field to the electromagnetic field. Therefore, the transmission medium is changed into a dedicated transmission line, such as a microstrip line, a coaxial line, etc., and the entire impedance of the transmission line is uniform. Claim. Some friends think of differential signals when they think of high-speed transmission, but they understand that they are only anti-interference ability to the outside world. It does not solve their own malformation during signal energy transmission, as long as the differential signal is further constrained to meet the electromagnetic field impedance requirements. Further solve the problem of signal malformation, so this time simply understand the difference, can only say a part, and highly reliable signal transmission, many of which use coaxial lines, microstrip lines, there is no difference in this The concept is.

Based on this theory, the current high-speed serial interface comes out. For example, USB, 1394, network port, SATA, LVDS, etc. are all developed based on electromagnetic field theory. They all have a common feature that requires impedance matching, and this is The most basic thing in electromagnetic field theory.

Because the technology based on electromagnetic field is far faster than the rate based on electric field theory, this is equivalent to a leap of 1, 2 orders of magnitude, so the current high-speed serial port is higher than the ordinary low-speed parallel port rate, this is Not surprising. Then everyone has to ask, further increase the speed, is the parallelization of the existing high-speed serial port, this is on the one hand, such as DDR2, it is already the case, but there is a trend, such as SATA 1X, 2X, 4X is not a simple parallel, but should be networked. That is to say, from 1X, 2X to 4X, from one channel to four network channels. The communication method of data has been networked like the current Internet. Because of these interfaces, there may be a bad contact, and the channel will not work, but it will not affect the data communication, but the rate is slow. Or the conventional parallel mode, it completely failed.

Post knot

I am very grateful to everyone for your discussion. Some readers have found that the mistakes have been revised. Some readers have pointed out some mistakes in the use of words. Indeed, it is difficult to write a rigorous article. It is difficult and there is no such energy. Please understand me. The intention can be. This article emphasizes, what I want to explain is why the conventional serial port develops to the parallel port and goes back to the high-speed serial port, and emphasizes that the theoretical basis of the previous serial port is different from the current serial port, and the transmission medium is also different, such as the previous one. Serial port, just one or two loop lines can be used. The current serial port requirements are based on transmission lines or microstrip lines, coaxial lines, and also match. And the current parallel port is not the conventional parallel port. It may seem to be a combination of multiple serial ports, but it is actually networked, because the previous parallel port is invalid, but now it is the serial port network. After the parallel port, removing one will not fail. Secondly, everyone will use the conventional circuit such as differential to explain, this can only be said to be the traditional electrical theory, there is nothing wrong with it, but to the high speed, especially after the G level, it belongs to the microwave level, because the wavelength is only a few centimeters. When the size of the transmission line is close to the signal wavelength, optical reflection and dispersion will occur. Because the electromagnetic field is transmitting, if the matching is not good, the energy transmission cannot be absorbed in the past, and the reflection will cause oscillation. Wave distortion occurs. This is not an explanation of interference, but a problem of its own. If you still can't understand, you can think about such a problem, that is why the fiber that is transmitted over long distances generally requires a single-mode fiber rather than a multimode fiber. Because of the multi-mode fiber, multiple types of modes appear after the light comes in. Transmission, and the characteristics of each type of rate will be different, similar to the dispersion, resulting in blurred and unrecognizable light, so a single mode is required for long-distance transmission. After the high-frequency microwave level, after the electromagnetic field characteristics come out, everyone should consider this energy, because the signal is based on energy, and if the energy is not completely absorbed, it will reflect. This is easy to understand in reality, but the classic circuit avoids it. One problem is the signal transmission rate problem and its establishment process. It thinks that the rate is infinite, but in fact, he is limited, that is, the speed of the electromagnetic field. Since the speed of the electromagnetic field exists, the problem of energy matching cannot be avoided.

Of course, the electromagnetic field is more complicated, and I can explain it clearly by simply describing a few words. In the follow-up article, I intend to write some easy-to-understand articles about electromagnetic fields, but please don't be too demanding, especially in terms of rigor.