Detailed explanation of the specific differences between 74HC04 and 74HC14
It also has an inverter function, do you know the specific difference between 74HC04 and 74HC14?
74HC04
It is well understood for 74HC04, input low level, output high level; input high level, output low level.
Recommended operating conditions:
The figure below is the input characteristic table of TI 's 74HC04. For example,
When the chip power supply voltage Vcc=4.5V, the input terminal must input at least 3.15V voltage to be recognized as a high level by 74HC04 ;
In the same way, only when the input voltage is not greater than 1.35V can it be recognized as a low level .
In TE ST CONDITIONS (test conditions), select IOH=-4mA and IOL=4mA respectively. For another example, as above, when the chip power supply voltage Vcc=4.5V, and the input terminal is recognized as low level , the output Terminal VOH is high level , and the minimum value is 3.84V ;
Similarly, when the chip power supply voltage Vcc=4.5V, and the input terminal is recognized as high level , the output terminal VOL is low level , and the maximum value is 0.33V .
From the above analysis, it can be seen that after the inversion of 74HC04, the output high level or low level of several volts is mainly related to the power supply voltage of the chip !
The above analysis for 74HC04 is also applicable to 74HC14! Regarding the above data, they are exactly the same in the manual ! That is, when it is used simply as an inverter, it can be used universally . But in addition to the basic function of the inverter, the 74HC14 is also a Schmitt trigger .
74HC14 pin diagram:
For 74HC14,
When the input VI is greater than the threshold voltage VT+, the output VO changes from high level to low level;
When the input VI is less than the threshold voltage VT-, the output VO changes from low level to high level;
Through the above analysis, we understand that the Schmitt trigger is based on the inverter and the basic concept of the threshold voltage.
VOH and VOL have been introduced above, let’s take a look at VT+ and VT-:
When the chip power supply voltage Vcc=4.5V, the typical value of the positive threshold is VT+=2.5V, and the typical value of the negative threshold is VT-=1.6V. That is to say, when the input is greater than 2.5V, the output is low; when the input is less than 1.6V, the output is high. This range is wider than the range of 74HC04 "when the input is greater than 3.15V, the output is low; when the input is less than 1.35V, the output is high (see the first figure)", thus avoiding level interference.
How does the Schmitt trigger 74HC14 overcome level interference than the inverter 74HC04?
Set the chip power supply voltage Vcc=4.5V, when the input voltage is greater than 3.15V before time T, no matter it is 74HC04 or 74HC14, the output is low level; but when the input voltage fluctuates at time T, there is a rapid dip, this When the input voltage is less than 3.15V but greater than 2.5V, then the output of 74HC04 will be uncertain, because its input is neither high level nor low level range at this time, and 74HC14 will still consider the input to be high level, so The output is low! At this point, we can see the profound skill of 74HC14, right?
Therefore, 74HC14 is often used in this way to combat interference:
Connect the two Schmidts before departure. For example, when the chip power supply voltage Vcc=4.5V, the input is 3V, and the output is at least 3.84V, generally close to 4.5V. Even if the input fluctuates, as long as it is not less than 2.5V, the output will not change. This can counteract sudden interference of the input signal .