Here are some of the learning experiences, ideas, and knowledge I gathered while revisiting analog and digital circuits. Feel free to share your thoughts and learn together!
1. HC is a CMOS level, whereas HCT is a TTL level.
2. LS inputs are open to a high level by default, but HC inputs must not be left floating. HC typically requires an external pull-down resistor to ensure a defined state when inactive. LS does not have this requirement.
3. The LS output has a strong pull-down but weak pull-up, while HC has balanced pull-up and pull-down capabilities.
4. Working voltage: LS operates only at 5V, while HC can operate between 2V and 6V.
5. CMOS can drive TTL circuits, but the reverse is not possible. When driving CMOS with TTL, a pull-up resistor must be added to raise the voltage to between 2.4V and 3.6V for proper CMOS input recognition.
6. Driving capability differs: LS can drive up to 5mA at high level and 20mA at low level, while CMOS can drive about 5mA at both levels.
7. RS232 logic levels are +12V for low and -12V for high.
8. 74 series is for commercial use, while 54 series is for military applications.
9. TTL high level is >2.4V, low level is <0.4V, with a noise margin of 0.4V.
10. OC (open collector) and OD (open drain) gates require external pull-up resistors to function properly. They can only sink current and need an external power supply to provide a high level. These are often used as driver circuits for high-voltage or high-current loads.
11. If the input current of a CMOS device exceeds 1mA, it may get damaged.
12. For long signal transmission lines, a matching resistor should be connected at the CMOS circuit end to prevent signal reflections.
13. Adding a 10kΩ resistor in series with a gate’s input can cause a low-level input to appear as a high level due to insufficient current.
14. When using a 3.3V CMOS circuit to drive a 5V CMOS circuit, such as a 3.3V microcontroller driving a 74HC chip, several solutions exist. One simple option is to replace the 74HC with a 74HCT chip, which is compatible with 3.3V signals. Alternatively, you can add a level-shifting IC or configure the MCU's I/O as open-drain with a 5V pull-up resistor.
15. When a logic gate outputs a high level, it sources current; when it outputs a low level, it sinks current.
16. Open-drain circuits require an external pull-up resistor to set the high level. The pull-up resistor's voltage determines the output level. This allows for level shifting between different voltage domains. However, the rising edge may be slow, so it's better to rely on the falling edge if possible.
17. Several level conversion methods:
(1) Transistor + pull-up resistor method – uses a BJT or MOSFET with a pull-up resistor to generate a higher output voltage.
(2) OC/OD device + pull-up resistor – similar to the first method, suitable when the device already has an open-drain output.
(3) 74xHCT series chips – these are TTL-compatible CMOS devices that can convert 3.3V to 5V.
(4) Over-limit input step-down method – many modern CMOS devices allow inputs above their supply voltage, enabling simple level reduction using resistive dividers.
(5) Dedicated level shifter chips like the 74LVC1G125 or 164245 – ideal for bidirectional communication but more expensive.
(6) Resistor divider method – a simple way to reduce a 5V signal to 3.3V using two resistors.
(7) Current-limiting resistor – useful in specific situations where current needs to be controlled.
18. Non-polar capacitors (e.g., 0805, 0603) are used for AC coupling, while polar capacitors (like aluminum electrolytic or tantalum) are used for DC filtering.
19. Common IC package types include PQFP, BGA, PGA, PLCC, SOP, TOSP, SOIC, etc.
20. QFP packages have gull-wing leads on all four sides. BGA uses ball grid arrays. PLCC has internal hook legs. SOJ and SOIC have gull-wing leads on both sides.
21. Shielded cables suppress static electricity, while twisted pairs reduce electromagnetic interference.
22. Analog signal sampling anti-jamming techniques include differential amplifiers, shielded twisted pair cables, converting voltage to current, and using RC filters.
23. Unused IC pins should not be left floating to avoid interference. Unused op-amps should be connected as buffers, and unused microcontroller I/Os should be set to output. Multiple power and ground pins on microcontrollers must all be connected.
24. Resistor color codes: 4 bands for standard resistors, 5 bands for precision ones.
25. Resistors serve functions like current limiting, voltage division, biasing, filtering, and impedance matching.
26. Capacitors are used for DC blocking, bypassing, coupling, filtering, compensation, charging/discharging, and energy storage.
27. Capacitance values are usually in pF for small capacitors and uF for electrolytic capacitors.
28. Key capacitor specifications include capacitance, voltage rating, and maximum operating temperature.
29. Inductors are used for filtering, oscillation, magnetic energy storage, and notch filtering.
30. Inductors can be air-core or core-based (including iron-core or copper-core).
31. Semiconductor diodes are classified by material (silicon, germanium) and purpose (rectifier, Zener, LED, photodiode, varactor).
32. FETs are voltage-controlled, while BJTs are current-controlled. Choose FETs when signal source current is limited, and BJTs when signal voltage is low.
33. A socket is a rectangular plug-in connector, while a slot is a rectangular groove.
34. To test a crystal oscillator, use a multimeter on RX10K. A good one should show infinite resistance; otherwise, it may be damaged.
35. When an IO port outputs a high level, it sources current; when it outputs a low level, it sinks current.
36. When driving inductive loads, a current-limiting resistor or a diode should be used.
37. The 9013 transistor provides up to 300mA of drive current.
38. Output data should be latched if peripheral speed is too slow, and input data should use three-state buffers to avoid bus conflicts.
39. 8-bit parallel output ports require latches (e.g., 74LS377, 74LS273), while parallel input ports need three-state gates (e.g., 74LS373, 74LS244).
40. Serial-to-parallel expansion: 74LS165 for input, 74LS164 for output.
41. Keyboard interface modes include software scan, timer scan, and interrupt mode. Dual-function keys can also be designed.
42. For TTL loads, focus on DC characteristics due to high current and low capacitance. For MOS loads, focus on AC characteristics due to low current and high capacitance.
43. Pay attention to bus load balancing to avoid overloading any single pin.
44. Pull-up resistors improve signal integrity, reduce EMI, suppress static, and minimize reflection in long-distance transmission.
45. Two-stage voltage regulators are better for stable output.
46. Transmission line impedance matching includes terminal parallel, series, DC blocking, and clamp diode methods.
47. Grounding types include housing grounding (true ground) and working grounding (floating ground).
48. MCU grounding includes digital, analog, power, signal, AC, and shielded grounds.
49. One-point grounding is used for low-frequency circuits (<1MHz), while multi-point grounding is used for high-frequency circuits (>10MHz).
50. Digital and analog grounds should be separated and connected at a single point to reduce noise.
51. High-frequency bypass capacitors are small (e.g., 0.1uF), while decoupling capacitors are large (e.g., 10uF or more).
52. Diode application circuits include limiting, clamping, switching, rectification, and low-voltage regulation.
53. Bypass capacitors are small (0.1uF, 0.01uF), while decoupling capacitors are larger (10uF+).
54. Pull-up resistor usage:
- Improve TTL-to-CMOS compatibility.
- Required for OC/OD gates.
- Enhance drive capability on microcontroller pins.
- Prevent static damage on CMOS inputs.
- Improve noise margin and bus immunity.
- Reduce signal reflection on long lines.
55. From a power-saving perspective, large resistors are preferred to limit current; from a drive capability perspective, smaller resistors are better for higher current.
56. Pull-up resistors stabilize undefined signals at a high level and limit current flow.
57. Bypass capacitors eliminate unwanted AC energy, while decoupling capacitors isolate local power and reduce noise.
58. Active buzzers have built-in oscillators and activate with power, while passive buzzers require a square wave signal (2K–5KHz) to sound.
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