Here are some of the learning experiences, ideas, and knowledge when you revisit the analog and digital circuits. Share them and learn together!
1, HC is COMS level, HCT is TTL level
2. The LS input is open to a high level, and the HC input is not allowed to open. HC generally requires an upper pull-down resistor to determine the level at which the input is inactive. LS does not have this requirement
3, LS output pull-down strong pull-up weak, HC pull-up pull down the same
4, working voltage: LS can only use 5V, and HC is generally 2V to 6V
5, CMOS can drive TTL, but the reverse is not acceptable. When the TTL circuit drives the COMS circuit, it needs to add a pull-up resistor to pull up the voltage between 2.4V and 3.6V, so that the CMOS detects the high-level input.
6, the driving ability is different, LS generally high level drive capability is 5mA, low level is 20mA; and CMOS high and low level are 5mA
7, RS232 level is +12V for logic negative, -12 for logic positive
8, 74 series for commercial use, 54 for military use
9, TTL high level > 2.4V, TTL low level <0.4V, noise tolerance 0.4V
10, OC gate, that is, open collector circuit (why there will be OC gate? Because to achieve "line and" logic), OD gate, that is, open-drain gate circuit, must be external pull-up resistor and power supply to switch the level Used as a high and low level. Otherwise, it is generally only used as a switch large voltage and a large current load, so it is also called a drive gate circuit. And can only absorb current, the external pull-up resistor and power supply must be able to output current
11, COMS input current exceeds 1mA, it is possible to burn COMS
12. When connecting the long signal transmission line, connect the matching resistor at the COMS circuit.
13. After inputting a 10K resistor in series with the input of the gate circuit, input a low level, and the input terminal presents a high level instead of a low level.
14. If a 3.3V COMS circuit is used in the circuit to drive a 5V CMOS circuit, such as a 3.3V microcontroller to drive the 74HC, there are several ways to solve this situation. The simplest is to directly replace the 74HC with the 74HCT chip. Because 3.3VCMOS can directly drive the 5V TTL circuit; or add the voltage conversion chip; there is to set the I/O port of the MCU to open-drain, and then add the pull-resistance to 5V. In this case, adjust the resistance according to the actual situation. The size is guaranteed to guarantee the rising edge time of the signal.
15. The load current when the logic gate output is high (the current is pulled), and the load current when the logic gate output is low (for sinking current)
16. Because the drain is open, the post-stage circuit must be connected to a pull-up resistor. The supply voltage of the pull-up resistor determines the output level. This open-drain connection allows devices of different levels to be connected for level shifting. One point to note: the load is charged by an external pull-up passive resistor on the rising edge, so the rising edge may not be fast enough. Try to use the falling edge.
17, several level conversion methods:
(1) Transistor + pull-up resistor method
It is a bipolar transistor or MOSFET. The C/D is connected to a positive pull-up resistor. The input level is very flexible and the output level is roughly the positive power level.
(2) OC/OD device + pull-up resistor method
Similar to (1). Applicable when the device output is just OC/OD.
(3) 74xHCT series chip boost (3.3V→5V)
Any 5V CMOS device with input 5V TTL level compatibility can be used as a 3.3V → 5V level shifter. This is because the level of 3.3V CMOS is just compatible with the 5VTTL level (coincidence), and the output level of CMOS is always close to the power supply level. Cheap options such as the 74xHCT (HCT/AHCT/VHCT/AHCT1G/VHCT1G/...) series (that letter T means TTL compatible).
(4) Over-limit input step-down method (5V → 3.3V, 3.3V → 1.8V, ...)
Any logic device that allows input levels to exceed the power supply can be used to reduce the level. The "overrun" here refers to exceeding the power supply, and many older devices do not allow the input voltage to exceed the power supply, but more and more new devices have eliminated this limitation (changing the input stage protection circuit). For example, in the 74AHC/VHC series chips, the datasheets clearly indicate "the input voltage range is 0~5.5V". If 3.3V power supply is used, 5V→3.3V level conversion can be realized.
(5) Dedicated level conversion chip
The most famous is the 164245, which can be used not only as a boost/buck but also to prevent the two sides from being out of sync. This is the most common level shifting solution, but it is also very expensive (not long ago, it was ¥45/piece, although it was retail and expensive), so if it is not necessary, it is best to use the first two options.
(6) Resistance division method
The easiest way to lower the level. The 5V level is divided by 1.6k+3.3k resistor, which is 3.3V.
(7) Current limiting resistor method
18, non-polar capacitors and polar capacitors: the former package is basically 0805, 0603. The latter uses up to Aluminum Electrolytic Capacitors, a good tantalum capacitor
19, PQFP (Plastic Quad Flat Package), BGA (Ball Grid Array Package), PGA (Pin Grid Array Package), PLCC (Plastic Leaded Chip Carrier) Lead chip carrier), SOP (Small Outline Package), TOSP (Thin Small Outline Package), SOIC (Small Outline Integrated Circuit Package)
20, common packaging form of integrated circuits
QFP (quad flat package) has gull-wing feet on all sides (package).
BGA (ball grid array) ball grid array (package)
PLCC (plastic leaded chip carrier) has internal hook legs (package) on all sides. See Figure 3.
SOJ (small outline junction) has internal hook feet (package) on both sides.
SOIC (small outline integrated circuit) has gull-wing feet on both sides (package).
Figure 1 
Figure II 
Figure III 
Figure 4 
Figure 5 
21, the shielded wire has a strong inhibitory effect on static electricity, and the twisted pair has a certain suppression effect on electromagnetic induction.
22, analog signal sampling anti-jamming technology: can use the differential input measuring amplifier, using shielded double glue line to transmit measurement signals, or change the voltage signal into a current signal, and use RC filter technology
23. Do not hang unused IC pins to avoid interference. The unused op amp is connected to the input terminal and the negative input is connected to the output. The I/O port that is not used by the microcontroller is defined as an output. There is more than one power supply and grounding terminal on the single-chip microcomputer, each must be connected, do not hang
24, resistance resistance color ring representation: ordinary color ring resistor is represented by 4 rings, precision resistor is represented by 5 rings
25, the role of the resistor is shunt, current limit, voltage divider, bias, filtering (combined with capacitors) and impedance matching
26, the role of the capacitor: DC, bypass, coupling, filtering, compensation, charge and discharge, energy storage, etc.
27, the general representation of the capacitance of the capacitor is pF, the Electrolytic capacitor is generally uF
28, the main performance indicators of the capacitor: the capacity of the capacitor (that is, the capacity to store the charge), the withstand voltage value (refers to the maximum DC voltage or the maximum value of the maximum AC voltage that the capacitor can work reliably for a long time in the rated temperature range) (indicating the maximum operating temperature that the capacitor can withstand)
29, the role of the inductor: filtering, notching, oscillation, storage of magnetic energy, etc.
30. Classification of inductors: air core inductors and core inductors. Core inductors can also be called core inductors and copper core inductors.
31. Classification of semiconductor diodes
a) divided by material: silicon diode and germanium diode;
b) According to the purpose: rectifier diode, detector diode, Zener diode, LED, photodiode, varactor diode.
32. The FET is a voltage control component and the transistor is a current control component. In the case where only a small current is allowed from the signal source, the FET should be selected; and when the signal voltage is low and the current is taken from the signal source, the transistor should be selected.
33. Socket is a socket package form, which is a rectangular socket; Slot is a slot package form, which is a rectangular slot.
34. Measurement method of crystal oscillator: Use the multimeter RX10K file to measure the positive and negative resistance values ​​of the quartz crystal oscillator. Normally, it should be infinite. If the quartz crystal oscillator has a certain resistance or zero, the quartz is indicated. Crystal oscillator has been leaking or breakdown damaged
35. When the IO port outputs a high level, the driving capability is the lowest, and the external display is the push current; when the IO port outputs the low level, the driving capability is the largest, and the external display is the pulling current.
36. Peripheral integrated digital drive circuit If driving an inductive load, a current limiting resistor or a diode must be applied.
37, 9013 provides a drive current of 300mA
38, the output data should be latched (the peripheral speed can not keep up, so it needs to be latched), the input data should have a three-state buffer (adding a high-impedance state, it will not affect the internal data bus)
39, 8-bit parallel output port (must have a latch function): 74LS377, 74LS273.8 bit parallel input port (must be a three-state gate): 74LS373, 74LS244
40, serial port expansion parallel port, parallel input port: 74LS165. Parallel output port: 74LS164
41. There are three ways to work with the keyboard:
l Programming scan mode
l Timing scanning method
l Interrupt mode. It is also possible to design an IO port for the design of the dual function keys (upper and down keys)
42. For TTL loads, the DC load characteristics should be considered mainly because the TTL current is large and the distributed capacitance is small. For MOS type load, the AC load characteristics should be considered mainly because the input current of the MOS type load is small, and the distributed capacitance is mainly considered.
43. Pay special attention to the concept of bus load balancing!
44, the benefits of pull-up resistors:
l increase the signal level
l Improve the anti-electromagnetic interference capability of the bus (electromagnetic signals enter the CPU through the DB)
l Suppress static interference (CMOS chip)
l Reflected wave interference (long-distance transmission)
45, when the voltage is regulated, the two-stage integrated voltage regulator chip is better.
46. ​​Impedance matching of the transmission line:
l Terminal parallel impedance matching (high level drop)
l Start series matching (low level up)
l Terminal parallel DC blocking (RC series grounding)
l Terminal clamp diode
47. There are two kinds of grounding: housing grounding (true grounding) and working grounding (floating ground)
48. Types in the MCU: digital ground, analog ground, power ground (large current, thick ground), signal ground, AC ground, shielded ground
49. One point grounding: low frequency circuit (less than 1MHZ). Multi-point grounding: high frequency circuit (above 10MHZ)
50. The ground and the signal ground cannot be shared. The digital ground and the analog ground are best separated, and then connected at one point.
51, æ© vibration circuit: can choose mica, high frequency Ceramic Capacitor, DC: can choose paper, polyester, mica, electrolytic, ceramic and Other Capacitors , filter: can choose electrolytic capacitor, bypass: polyester, paper, Ceramic, electrolytic and other capacitors
52, diode application circuit
(1) Limiting circuit---It is composed of the unidirectional conductivity of the diode and the constant voltage at both ends after the conduction, and the signal is limited to a certain range, and is divided into a single limiting and double limiting circuit. Mostly used in signal processing circuits.
(2) Clamp circuit --- clamp the output voltage to a certain value.
(3) Switching circuit---Using diode unidirectional conductivity to turn the circuit on and off, widely used in digital circuits.
(4) Rectifier circuit---Using the unidirectional conductivity of the diode to convert the AC signal into a DC signal, which is widely used in DC stabilized power supplies.
(5) Low-voltage voltage regulator circuit---Using the characteristics of the diodes after the diode is turned on, the voltage at both ends is basically unchanged, and several diodes are connected in series to obtain an output voltage of 3V or less.
53. The high-frequency bypass capacitor is generally small. According to the resonant frequency, it is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally large, which is 10uF or more.
54. Pull-up resistor summary:
l When the TTL circuit drives the COMS circuit, if the high level of the TTL circuit output is lower than the lowest level of the COMS circuit (typically 3.5V), then the pull-up resistor needs to be connected to the output of the TTL to improve the output. The value of the high level.
l The OC gate circuit must be equipped with a pull-up resistor before it can be used.
l To increase the drive capability of the output pins, pull-up resistors are often used on some microcontroller pins.
l On the COMS chip, in order to prevent damage caused by static electricity, the unused pins cannot be suspended. Generally, the pull-up resistor is connected to reduce the input impedance and provide a discharge path.
l The pin of the chip is added with a pull-up resistor to increase the output level, thereby improving the noise margin of the chip input signal and enhancing the anti-interference ability.
l Improve the anti-electromagnetic interference capability of the bus. When the pins are suspended, it is easier to accept external electromagnetic interference.
l The resistance mismatch in long-line transmission is easy to cause reflected wave interference, and the pull-down resistor is resistance matching, which effectively suppresses reflected wave interference.
55, from the power saving and chip sinking capacity considerations should be large enough; large resistance, small current.
From the perspective of ensuring sufficient drive current, it should be small enough; the resistance is small and the current is large.
56, pull-up is to indeterminate signal through a resistor clamped at the high level! Resistance at the same time from the current limit! Drop down the same!
57. Bypass Capacitor: Generates an AC shunt that eliminates unwanted energy entering the susceptible zone. Decoupling Capacitor: Provides a local DC power supply to the active device to reduce the propagation of switching noise on the board and direct the noise to ground (his value is approximately 1/100 to 1/1000 of the bypass capacitor)
58. Active buzzer: There is an internal oscillating circuit, which can be activated when powered. Passive buzzer: There is no internal oscillation circuit, and it needs to add 2K-5K square wave signal to ring.
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