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    What are the main operating frequencies of RFID systems?

    Low frequency, 125~l34kHz;
    High frequency, 13.56MHz;
    Very high frequency, operating frequency is 860 ~ 960MHz;
    Microwave frequency band, 2.45 ~ 5.8GHz, etc.

    What is the difference between the Thumb instruction set and the ARM instruction set?

    • The Thumb instruction set has no coprocessor instructions, semaphore instructions, and instructions to access the CPSR or SPSR, and there are no multiply-add instructions and 64-bit multiply instructions. And the second operand of the instruction is restricted. Except for the conditional execution function of branch instruction B, all other instructions are executed unconditionally. Most Thumb data processing instructions are in a 2-address format. The difference between Thumb instruction set and ARM instruction set generally has the following 4 points:
      1. Branch instruction. Compared with the jump under the ARM code, the relative program transfer, especially the conditional jump, has more restrictions on the scope, and the transfer to the subroutine is an unconditional transfer.
      2. Data processing instructions. Data processing instructions operate on general-purpose registers. In most cases, the result of the operation must be placed in one of the operand registers, not the third register. There are fewer data processing operations than ARM states. Access to registers R8 ~ Rl5 is subject to certain restrictions. Except MOV and ADD order visit register R8 ~ Rl5, other data processing orders always update the ALU status mark in CPSR. The Thumb data processing instructions that access registers R8-Rl5 cannot update the ALU status flag in the CPSR.
      3. Single register load and store instructions. In Thumb state, single register load and store instructions can only access registers R0 to R7.
      4. Multi-register load and multi-register store instructions. The LDM and STM instructions can load or store any subset of registers in the range R0 to R7. The PUSH and POP instructions use the stack instruction Rl3 as the base address to implement a full decrement stack. In addition to R0~R7, the PUSH instruction can also store the link register Rl4, and the POP instruction can load the program instruction PC.

    What are the characteristics of the VxWorks operating system?

    1. Reliability: The users of the operating system hope to work in a stable and reliable environment, so the reliability of the operating system is the first problem that users should consider. And stability, reliability has always been a prominent advantage of VxWorks. Since the ban on sales in China has been lifted, Vxworks has won more and more users in China with its good reliability.
    2. Real-time: The core meaning of real-time is determinism, not pure speed, which means that what should be done must be done within the specified time, and the ability to respond to external asynchronous events. For hard real-time performance, all work must be completed within the specified time under any circumstances (100%), so the real-time performance is measured by the length of time to complete the specified function and respond. The real-time performance of Vxworks is very good, the overhead of the system itself is very small, and the system utilities such as process scheduling, inter-process communication, and interrupt handling are concise and effective, and the delay caused by them is very short. The multitasking mechanism provided by VxWorks adopts the preemptive priority scheduling and round robin scheduling mechanism for task control, which fully guarantees reliable real-time performance, so that the same hardware configuration can meet the requirements of stronger Real-time requirements leave more room for application development.
    3. Tailorability: When users use the operating system, not every component in the operating system needs to be used. For example, graphics display, file system and some device drivers are often not used in some embedded systems. Vxworks consists of a small kernel and some system modules that can be customized as needed. The minimum size of VxWorks kernel is 8 KB. Even if other necessary modules are added, the space occupied is very small, and the system features of real-time and multi-tasking are not lost. Due to its high flexibility, users can easily customize or develop this operating system to meet actual application needs.

    Briefly describe the basic components of a transmitter?

    1. Modulator: As its name suggests, this component can modulate a signal onto a carrier, assuming that the modulated data bandwidth is between 200 kHz and 2 MHz.
    2. Carrier frequency: The transmitter needs to work in the narrow absorption window of water molecules – the blue-green light range of 460nm ~ 480nm.
    3. Laser: Laser can generate carrier wave. We can assume that the output power is 0.1 W ~ 10 W.
    4. Projection optical system: an indispensable device in an optical transmitter to focus the beam.
    5. Beam control: a system used to control the beam pointing to the receiver.

    What are the two types of reset?

    There are two types of reset: cold reset and warm reset.

    Common problems in circuit design:

    Phenomenon 1: The PCB design requirements of this board are not high, so use a thinner wire.
    Comments: Automatic routing will inevitably occupy a larger PCB area, and at the same time generate many times more vias than manual routing. In products with large batches, the factors considered by PCB manufacturers to reduce prices, in addition to business factors, are the line width and the number of vias. They affect the yield of the PCB and the consumption of the drill bit respectively, which saves the cost of the supplier and finds a reason for the price reduction.
    Phenomenon 2: These bus signals are all pulled with resistors, which makes them feel more at ease.
    Comments: There are many reasons why signals need to be pulled up and down, but not all of them have to be pulled. The pull-up and pull-down resistors pull a simple input signal, and the current is less than tens of microamps. But pulling a driven signal, its current will reach the milliamp level. The current system often has 32 bits of address and data, and there may be 244/245 isolated buses and other signals. Several watts of power consumption are consumed by these resistors.
    Phenomenon 3: How to deal with these unused I/O ports of CPU and FPGA? Leave it empty for now, and talk about it later.
    Comments: If the unused I/O port is left floating, it may become an input signal that oscillates repeatedly due to a little interference from the outside world, and the power consumption of the MOS device basically depends on the number of flips of the gate circuit. If you pull it up, each pin will also have a microampere 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 use it to your heart’s content.
    Comments: The power consumption of an FPGA is proportional to the number of flip-flops used and the number of flips, so the power consumption of the same type of FPGA at different times in different circuits may vary by 100 times. Minimizing the number of flip-flops that are high-speed overruns is a fundamental way to reduce FPGA power consumption.
    Phenomenon 5: The power consumption of these small chips is very low and need not be considered.
    Comments: It is difficult to determine the power consumption of the less complex chip inside, it is mainly determined by the current on the pin.
    Phenomenon 6: Since it is a digital signal, the steeper the edge, the better.
    Comments: The steeper the edge, the wider its spectral range and the greater the energy in the high frequency part. Signals with higher frequencies are easier to radiate (for example, microwave radios can be made into mobile phones, while long-wave radios cannot be made in many countries), and the easier it is to interfere with other signals. However, the transmission quality on the wire itself becomes worse, so those who can use low-speed chips try to use low-speed chips.

    What are the characteristics of the grating sensor?

    The grating sensor has the characteristics of high precision, large measurement range, and easy to realize measurement automation and digitization.

    What are the power management features in some DSP devices?

    • Including the following aspects:
      • Reduced operating voltage: Many manufacturers offer low-voltage DSP versions (3.3V, 2.5V, or 1.8V). Under the same clock, this processor consumes far less power than similar products powered by 5V.
      • “Sleep” or “Idle” mode: Most processors have the ability to shut down part of the processor’s clock to reduce power consumption. In some cases, a non-maskable interrupt signal can resume the processor from “sleep” mode. In other cases, only a few external interrupts can be set to wake up the processor. Some processors can provide multiple “sleep” modes with different power saving features and latency.
      • Programmable clock divider: Some DSPs allow the processor clock to be changed under software control in order to use the lowest clock frequency for a particular task to reduce power consumption.
      • Peripheral Control: Some DSP devices allow the program to stop the work of peripheral circuits not used by the system.

    What are the crystal membrane electrodes that can be classified according to different sensitive membranes?

    Crystal membrane electrodes can be divided into homogeneous membrane electrodes (single crystal membrane electrodes, polycrystalline membrane electrodes) and heterogeneous membrane electrodes (precipitated membrane electrodes) according to different sensitive membranes.

    What are the advantages of VLAN?

    1. Restricting the broadcast domain: The broadcast domain is limited to one VLAN, which saves bandwidth and improves network processing capability.
    2. Enhance the security of the local area network: packets in different VLANs are isolated from each other during transmission, that is, users in one VLAN cannot communicate directly with users in other VLANs.
    3. Improve the robustness of the network: the fault is limited to one VLAN, and the fault in this VLAN will not affect the normal work of other VLANs.
    4. Flexible construction of virtual work groups: VLANs can be used to divide different users into different work groups, and users in the same work group do not have to be limited to a fixed physical range, making network construction and maintenance more convenient and flexible.


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