The difference between TTL level and CMOS level

     Output high level>2.4V, output low level<0.4V. At room temperature, the general output high level is 3.5V, and the output low level is 0.2V. Minimum input high level and low level: input high level>=2.0V, input low level<=0.8V, noise margin is 0.4V.
2. CMOS level:
     1 logic level voltage is close to the power supply voltage, 0 logic level is close to 0V. And it has a wide noise margin.
3. Level conversion circuit:   
     Because the high and low levels of TTL and COMS are different, level conversion is required when connecting to each other: that is, use two resistors to divide the level!
4. Comparison of TTL and COMS circuits:   
(1) TTL circuit is a current control device, while COMs circuit is a voltage control device.   
(2) TTL circuit is fast and has a short transmission delay time (5-10ns), but it consumes a lot of power.   
  COMS circuit is slow and has a long transmission delay time (25-50ns), but it consumes a lot of power.   
  The power consumption of the COMS circuit itself is related to the pulse frequency of the input signal. The higher the frequency, the hotter the chip set is. This is a normal phenomenon.   
(3) Locking effect of COMS circuit:   
  Due to the input of too much current, the internal current of the COMS circuit increases sharply. Unless the power is cut off, the current continues to increase. This effect is the locking effect. When the locking effect occurs, the internal current of the COMS can reach more than 40mA, which can easily burn the chip.   
Defensive measures:   
(1) Add clamping circuits to the input and output ends to ensure that the input and output do not exceed the specified voltage.   
(2) Add a decoupling circuit to the power input end of the chip to prevent instantaneous high voltage from appearing at the VDD end.   
(3) Add a line resistor between VDD and the external power supply to prevent it from entering even if there is a large current.   
(4) When the system is powered by several power supplies, the switches should be in the following order: when turned on, turn on the power supply of the COMS circuit first, and then turn on the power supply of the input and load; when turned off, turn off the power supply of the input signal and the load first, and then turn off the COMS   

5. Compatibility:
   The power supply voltage of CMOS integrated circuits can vary over a large range, so the requirements for power supply are not as strict as those of TTL integrated circuits. Therefore, they can be compatible with TTL level if conditions permit. It should be noted that their driving capabilities are different. CMOS has a larger driving capability. Sometimes the low level of TTL cannot trigger the CMOS circuit, and sometimes the high level of CMOS will damage the TTL circuit. Attention should be paid to compatibility.
Note:
1. CMOS is composed of field effect transistors, and TTL is composed of bipolar transistors.   
2. The logic level range of CMOS is relatively large (5~15V), and TTL can only work at 5V.   
3. The difference between the high and low levels of CMOS is relatively large and has strong anti-interference ability, while TTL has a small difference and poor anti-interference ability.   
4. CMOS power consumption is very small, and TTL power consumption is relatively large (1~5mA/gate)   
5. The operating frequency of CMOS is slightly lower than that of TTL, but the speed of high-speed CMOS is almost the same as that of TTL.

For example, which ones are TTL? Which ones are CMOS?

74 series; 74S series; 74LS series; 74AS series; 74ALS series, etc. belong to TTL type integrated circuits

CD series; 74HC series; 74HCT series; 74BCT series, etc. belong to CMOS integrated circuits


LS and HC have different definitions of high and low levels. The high level of HC is specified as 0.7 times the power supply voltage, and the low level is specified as 0.3 times the power supply voltage. LS specifies the high level as 2.0V and the low level as 0.8V. The load characteristics are different. The pull-up and pull-down capabilities of HC are the same, while the pull-up of LS is weak and the pull-down is strong. The input characteristics are different. The input resistance of HC is very high, and the level is uncertain when the input is open. The LS input has an internal pull-up, and the input is high when it is open.

74HC is a CMOS circuit, and CMOS circuits are the leading products in MOS circuits. MOS circuits use insulated gate field effect transistors as switching elements. Therefore, they are also called unipolar integrated circuits. According to the type of its conductive channel, MOS circuits can be divided into PMOST, NMOS and CMOS circuits. CMOS circuits developed rapidly along the 4000A--4000B/4500B (collectively referred to as 4000B)--74HC--74HCT series. The HCT series is also compatible with TTL levels, expanding the scope of application.