5V comes from the TTL level, 5 is True, 0 is False, and then the PN section with lower voltage drop is used to derive the 3.3 level.
12V and 24V come from car batteries. In the early years, passenger cars had two systems, 12V and 24V. Nowadays, small cars are 12V and commercial vehicles are 24V. The origin should be lead-acid batteries.
Therefore, 3v and 5v generally appear in signal circuits or vcc power supplies such as single-chip computers, while 12v/24v generally appear in low-voltage power supplies, such as motherboards, graphics cards, axial fans, and monitors.
The hardware determines the system foundation. If the lithium battery is used early, it is estimated that there will be a 3.7/7.4 system.
Because most chips are 5V TTL level, it is necessary to level compatibility, level matching, and avoid level conversion operations, so the working voltage of many MCU microcontrollers is 5V.
The voltage drop of the early (196x) transistor circuit (TTL) single transistor is 0.7v. There are often multiple transistors in series in a circuit, such as 4 transistors in series, and the power supply must be at least 0.7x4=2.8v to ensure the normal operation of the circuit.
So there were 3v, 5v and other standards at the earliest. Later, after the LM7805 (197x) power IC came out, 5V became the de facto standard.
TTL refers to TTL level, between 0~5V, less than 0.2V output low level, higher than 3.4V output high level, the full name is Transistor-Transistor Logic, namely BJT-BJT logic gate circuit, which is a digital electronic technology A commonly used logic gate circuit has been applied earlier and the technology is relatively mature.
TTL is mainly composed of BJT (Bipolar Junction Transistor, that is, bipolar junction transistor, transistor) and resistance, which has the characteristics of fast speed.
The earliest TTL gate circuit was the 74 series. Later, 74H series, 74L series, 74LS, 74AS, 74ALS and other series appeared, but due to shortcomings such as large power consumption of TTL, it is gradually being replaced by CMOS circuits.
Why is it that many are 5V, and there are a large number of power chips that support 5V, with a voltage fluctuation of 5%, and the voltage standard, used in A/D, should be 5.12V.
Because 512 is 2 to the Nth power, so every word of A/D is an integer. It is easier to calculate as unsigned, but I haven’t seen any product that uses this voltage, most of which are 5V. Why? No need?
Because the cost of making a standard voltage of 5.12 will increase exponentially, the accuracy difference between 5V and 5.12V is a hundred times, 0.12V after the decimal point, it is basically difficult to achieve a high-precision standard voltage, the market general voltage is 5V, which rises by a certain percentage.
In the STC12 series MCU data manual released in November 2008, the voltage range of STC12C series MCU is 3.3 to 5.5V; the voltage range of STC12L series MCU is 2.2 3.6V.
If you choose the STC12C series of single-chip microcomputers, as long as the operating frequency of the single-chip microcomputer is not too high, there is no concern about using 3.7V power supply. The official claim that the anti-interference ability of the MCU single-chip microcomputer can reach 4000V, but the actual application is different.
Most single-chip microcomputers are TTL level, and their high and low levels are defined differently;
When the power supply voltage is 5V: 51, the avr microcontroller is 5V;
when the power supply voltage is 3.3V: 51, the high level of the avr microcontroller is 3.3v;
arm such as lpc2138, the power supply voltage can only be 3.3v, io output high level It is 3.3V, but the io port can withstand 5V voltage.
There are currently two main operating voltages for single-chip computers: one at 3.3V and the other at 5V.