Embedded design is a huge project, today to talk about several aspects of the design considerations hardware circuit, first of all, let's understand the embedded hardware architecture.
We know, CPU is the soul of this system, all the peripheral configurations are associated with, it also highlights a feature of embedded hardware design can be cut. In doing embedded hardware design, the following points need attention.
First, power is determined
Power for the embedded system can be seen as the role of action of the air on the human body, and even more important: human breathing air have like oxygen, carbon dioxide and nitrogen content, but stable, which is equivalent to the power supply system in a variety of clutter, we hope to get clean and stable power supply to meet the requirements, but due to various factors, but our dreams. This should focus on two aspects:
a, voltage
Embedded system requires a variety of power such as the common order of 5v, 3.3v, 1.8v, etc., to minimize power supply ripple, LDO devices used in embedded systems. If DCDC not only bigger in size, its ripple also a very troublesome problem.
b, current
The normal operation of embedded systems requires not only stable enough power, but also have enough current to power devices in the choice of when to consider its load, generally 30% of the margin left my design.
If multilayer, dividing the power part of the power required at the time of layout, then divided note path, as far as possible the amount of power are placed together. If a double-sided, then the trace width must be noted that, as far as possible in the case of widening the board permitted. Suitable decoupling capacitors as close to the power pin.
Second, determine the crystal
Crystal is at the heart of embedded systems, its stable or not directly related to its operational status and communication performance. Common vibration passive crystal, the active crystal, first to determine its oscillation frequency, and secondly to determine the crystal type.
a, passive crystal
Which selects the matching capacitors and matching resistors, typically based on this part of the reference manual. In the single-chip design, often using plug-in crystal oscillator with ceramic capacitors. In ARM, in order to reduce space and ease of wiring, often with quadrangular passive crystal chip capacitors. Although we are more familiar with fixed crystal matching circuit, but in order to achieve foolproof, is to look at the Reference Manual to determine the size of the capacitor, resistor and other details need to match.
b, active crystal
Better and more accurate clock signal, but in contrast, the price is higher than the crystal missed, so this is at the cost of the hardware circuit design concern.
In doing board design Note crystal alignment chip as close as possible, away from the key clock signal traces. Increasing the guard ring in the case of ground conditions permit. If it is plywood, but also talk about the key signal traces away from the crystal.
Third, set aside the test IO port embedded commissioning phase, rich in resources pins, I usually reserved for an IO port to connect an LED or horn, laying the groundwork for the preparation of the next software. During operation of the embedded system appropriately controlling the IO interface, to determine whether the system is running.
Fourth, extended memory device
If there is an embedded system power supply, crystal oscillator and CPU, so this is what we are familiar with the minimum system. If the embedded system requires an operating system to run big point, then the CPU will not only need to have a MMU, CPU but also the need for external SDRAM and NANDFLASH. If the cpu has NANDFLASH and SDRAM controller, so do not use too much consideration on the address line of hardware design. If there are no associated controller, you need to pay attention to use the address line.
LAYOUT when this part is a key reason is to make the correlation signal lines of equal length to ensure equal delay signal, a clock signal and differential DQS line traces. Various wiring techniques need to be used when integrated wiring, such as symmetrical and cpu, daisy chain wiring, T-wiring, which need to be selected according to the number of how much memory, in general, the larger the number, the more complex wiring but we know that the key point, everything solved.
Fifth, the function interface of an embedded system is the most important thing is to control the peripheral modules through a variety of interfaces, designers achieve preset goals. Common interface with a serial port (used to connect Bluetooth, WIFI module and 3G), USB interface, a network interface, the JTAG port, audio and video interface, HDMI interface and the like. Because these interface with external module is connected, good EMC design is important work. In addition, note the use of differential lines at LAYOUT time.
Sixth, the screen
This feature is the reason listed separately, because of its dispensable. If an embedded system only as a peripheral device connected to the connector module is connected via interfaces to a host computer or directly linked to the network, the screen is not required. But if you do it is a consumer product, with frequent user interaction, which had a few nagging.
The preferred screen is a capacitive screen embedded in the circuit design should be noted that the display layout of the touchscreen cable and cable. In the process of alignment in the shortest possible next to the host cpu, taking care to take the differential signal line pair, RGB control signal routing length. Various signal trace spacing 3W follow the rules to avoid mutual interference. In the design of the screen, make sure the power and interference avoidance, to prevent the splash screen and screen Huaping phenomenon.
The same also to share some information with reference to hardware design
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