This article mainly introduces the knowledge points about TTL level, CMOS level, RS232 level, and hopes to help you learn. TTL level signals are utilized most because the usual data representation is binary, +5V is equivalent to logic "1", 0V is equivalent to logic "0", which is called TTL (Transistor-Transistor Logic transistor-transistor logic Level) Signaling system, which is the standard technique for communication between various parts of a device controlled by a computer processor. The TTL level signal is ideal for data transmission inside a computer controlled by a computer processor. First, the data transmission inside the device controlled by the computer processor is not required for the power supply and the heat loss is low, and the TTL level signal is directly It is connected to the integrated circuit without the need for expensive line drivers and receiver circuits. Furthermore, the data transfer inside the device controlled by the computer processor is performed at high speed, and the operation of the TTL interface can meet this requirement. In most cases, TTL communication uses parallel data transmission, while parallel data transmission is not suitable for distances over 10 feet. This is due to both reliability and cost. Because of the problems of bias and asymmetry in the parallel interface, these problems have an impact on reliability. TTL output high level "2.4V, output low level = 2.0V, input low level 2.4V. Enter L: 2.0V The TTL device outputs a low level of less than 0.8V and a high level of greater than 2.4V. Input, below 1.2V is considered to be 0, above 2.0 is considered to be 1. Output L: 0.9*Vcc. Enter L: 0.7*Vcc. Since the CMOS power supply uses 12V, the input is lower than 3.6V, the noise margin is 1.8V, higher than 3.5V, and the noise margin is 1.8V. Higher noise margin than TTL. The level of logic 1 is -3 to -15V, and the level of logic 0 is +3 to +15V. Note that the definition of the level is inverted once. What is the difference between TTL and CMOS levels? 1. The upper and lower limits of the level are defined differently, and the CMOS has a larger area of ​​noise immunity. The same is 5 volt power supply, ttl is generally 1.7V and 3.5V look, CMOS is generally 2.2V, 2.9V look, inaccurate, for reference only. 2. The current drive capability is different. ttl generally provides 25 mA drive capability, while CMOS is typically around 10 mA. 3. The required current input size is also different. Generally, ttl needs about 2.5 mA, and CMOS requires almost no current input. 4. Many devices are compatible with TTL and CMOS, and the datasheet will explain. General devices are interchangeable if speed and performance are not considered. However, it should be noted that sometimes the load effect may cause the circuit to work abnormally, because some ttl circuits require the input impedance of the next stage as the load to work properly. 1. Logic level of TTL circuit and CMOS circuit VOH: Output voltage of logic level 1 VOL: Output voltage of logic level 0 VIH : Input voltage of logic level 1 VIH : Input voltage of logic level 0 TTL circuit threshold: VOHmin = 2.4V VOLmax = 0.4V VIHmin = 2.0V VILmax = 0.8V CMOS circuit threshold (power supply voltage is +5V) VOHmin = 4.99V VOLmax = 0.01V VIHmin = 3.5V VILmax = 1.5V 2. TTL and CMOS logic level shifting CMOS level can drive TTL level The TTL level does not drive the CMOS level and requires a pull-up resistor. 74LS Series: TTL Input: TTL Output: TTL 74HC Series: CMOS Input: CMOS Output: CMOS 74HCT Series: CMOS Input: TTL Output: CMOS CD4000 Series: CMOS Input: CMOS Output: CMOS
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