1) MCU reset pin circuit design, general use of RC mode, please calculate the basis of RC selection value;
2) The current circuit board design for 5V to 3.3V mode, Chip asm117-3.3, trouble to give the current circuit board power supply formula advantages and disadvantages, power efficiency, system minimum operating voltage (with 5V power drop as an example), relative to the use of Ldo power supply and other advantages and disadvantages;
3) 3.3V System simulation Reference, digital reference layout, 0R resistor used methods and scenarios;
4) Protection circuit design, anti-reverse charging design, PCB layout, battery capacity and service life evaluation of the battery part;
5) 5V power supply over-voltage, overcurrent protection circuit design, PCB layout, power estimation;
6) A variety of crystal oscillator resonant circuit design, PCB layout;
7) Common key design schematic design and PCB layout, such as ordinary IO query detection; Using analog ad port long key detection
1.RC's selection, q=c*? U=it=? RIC, get t=? RC. which? U refers to the amount of change in voltage, generally taking greater than or equal to 0.8. The value of T is found in the data sheet. )
2. Moderator Select the chip is asm1117-3.3 it? Power efficiency is roughly 3.3/5=66%. The linear pressure drop of this chip? U=1V, so the minimum operating voltage is 4V. This is the Ldo, then here is relative to DC to say: This ldo pressure drop, even in small pressure differential environment efficiency is very low, belonging to the early products. The advantage of LDOs is that the circuit is simple and the ripple is small. Used in low-power load, especially some ldos have a small quiescent current, it is suitable for those long-time sleep equipment. It is efficient to use in high current situations.
3.3.3V of analog and digital ground available 0R resistor or bead connection. In order to reduce the interference-mainly digital to the analog signal, according to the frequency of the digital system to add some filter capacitance and the corresponding frequency of the bead. 0R resistance is a lot of, do connect wire, jumper, fuse, filter, there is a single point of grounding.
4. Battery part of the protection circuit, mainly short-circuit protection, overshoot and over-put, for short-circuit is a string fuse. Overshoot-the general charging chip has voltage detection, which controls the upper limit of the charging voltage. Over-put, can be added in the system voltage detection circuit and standby low-power design, so that the system when the battery voltage is low, automatically go to sleep state. Alternatively, a hardware lock can be designed to cut off the power when the voltage is lowered. Anti-Recoil, string low-voltage drop diode. The layout of the PCB board in fact, as in general, DC Circuit, mainly divided into power circuit and control circuit (the essence is to control the opening and closing of a MOS tube), power circuit input and output lead strong toughness, haha, input foot filtering is very important, to close to the chip, capacitance parameters according to the chip data sheet recommended. Ground to cover a large area of copper to reduce the chip heat, lower RDC, improve efficiency. The most important of the control circuit is the voltage feedback circuit, voltage sampling point voltage to stabilize, feedback loop as far as possible with the ground parcel or for reference. The capacity of the battery is generally calculated at 60% of the normal capacity.
5. Power String PTC Self-recovery fuse to prevent overcurrent. 5V system overvoltage, generally choose TVs tube, depending on the power, choose SMA,SMB or SMC package.
6. Crystal oscillator in the circuit is very important, divided into active crystal and passive crystal oscillator.
Active crystal is relatively simple, generally used in high-frequency circuits, PCB layout, close to the main chip, not too near the edge of the board, power lines, ground and clock lines to short and thick. Passive crystal, the inside of the chip is actually an inverter, and the external circuit together constitute a oscillation circuit. Therefore, the whole loop should be small, to avoid the crystal vibration through the loop to produce radiation. In addition the load capacitance to the crystal oscillator and chip selection is appropriate, otherwise it may not be vibration. In particular, like STM32, there are other chips that have a small crystal-driven current.
Hardware circuit Design Chapter