TTL level VIH/VIL is generally 2V/0.8V, VOH/VOL is generally 2.4V/0.4V, whether it is 3.3V or 5V TTL is the same; CMOS VIH/VIL is generally 70%VCC/30% VCC, VOH/VOL are generally 80% VCC/20% VCC, so different levels cannot be pushed each other!
In addition, the speed of CMOS is relatively fast, and general high-speed devices are used!
Common logic level standards
The commonly used level standards are TTL, CMOS, LVTTL, LVCMOS, ECL, PECL, LVPECL, RS232, RS485, etc., as well as some high-speed LVDS, GTL, PGTL, CML, HSTL, SSTL, etc. The following briefly introduces their respective power supply, level standards and precautions for use.
TTL: Transistor-Transistor Logic triode structure.
Vcc: 5V; VOH>=2.4V; VOL<=0.5V; VIH>=2V; VIL<=0.8V.
Because there is still a lot of free space between 2.4V and 5V, it is not good for improving the noise margin, and it will increase the system power consumption in vain and affect the speed. So later, a part of it was "cut off". That is LVTTL behind.
LVTTL is divided into 3.3V, 2.5V and lower voltage LVTTL (Low Voltage TTL).
3.3V LVTTL:
Vcc: 3.3V; VOH>=2.4V; VOL<=0.4V; VIH>=2V; VIL<=0.8V.
2.5V LVTTL:
Vcc: 2.5V; VOH>=2.0V; VOL<=0.2V; VIH>=1.7V; VIL<=0.7V.
The lower LVTTL is not commonly used, so let's not talk about it. It is mostly used in high-speed chips such as processors, and it is OK to check the chip manual when using it.
TTL use attention: TTL level generally overshoot will be more serious, it may be connected with a 22 ohm or 33 ohm resistor at the start end; ????? ????????? TTL level input pin is floating when it is considered internally High level. If you want to pull it down, use a resistance of 1k or less to pull it down. TTL output cannot drive CMOS input.
CMOS: Complementary Metal Oxide Semiconductor?? PMOS+NMOS.
Vcc: 5V; VOH>=4.45V; VOL<=0.5V; VIH>=3.5V; VIL<=1.5V.
Compared with TTL, it has a larger noise margin, and the input impedance is much larger than TTL input impedance. Corresponding to 3.3V LVTTL, LVCMOS appears, which can directly drive each other with 3.3V LVTTL.
3.3V LVCMOS:
Vcc: 3.3V; VOH>=3.2V; VOL<=0.1V; VIH>=2.0V; VIL<=0.7V.
2.5V LVCMOS:
Vcc: 2.5V; VOH>=2V; VOL<=0.1V; VIH>=1.7V; VIL<=0.7V.
CMOS use note: There is a thyristor structure in the CMOS structure. When the input or input pin is higher than a certain value of VCC (for example, some chips are 0.7V), if the current is large enough, it may cause a latch-up effect and cause the chip to burn out .
ECL: Emitter Coupled Logic Emitter Coupled Logic (differential structure)
Vcc=0V; Vee: -5.2V; VOH=-0.88V; VOL=-1.72V; VIH=-1.24V; VIL=-1.36V.
High speed, strong driving ability, low noise, it is easy to reach hundreds of M applications. However, the power consumption is large and a negative power supply is required. In order to simplify the power supply, PECL (ECL structure, using positive voltage supply instead) and LVPECL appeared.
PECL: Pseudo/Positive ECL
Vcc=5V; VOH=4.12V; VOL=3.28V; VIH=3.78V; VIL=3.64V
LVPELC: Low Voltage PECL
Vcc=3.3V; VOH=2.42V; VOL=1.58V; VIH=2.06V; VIL=1.94V
ECL, PECL, LVPECL use note: different levels cannot be directly driven. AC coupling, resistance network or special chip can be used for conversion in the middle. The above three are all emitter-fed output structures, which must be pulled to a DC bias voltage by a resistor. (For example, LVPECL used for clock: 130 ohm pull-up and 82 ohm pull-down for DC matching; 82 ohm pull-up for AC matching and 130 ohm pull-down at the same time. However, the DC level is 1.95V after the two methods work about.)
The previous level standards have a relatively large swing. In order to reduce electromagnetic radiation and increase the switching speed, LVDS level standards have been introduced.
LVDS: Low Voltage Differential Signaling
Differential pair input and output, there is a constant current source 3.5-4mA inside, change the direction on the differential line to indicate 0 and 1. It is converted to a differential level of ±350mV through an external 100-ohm matching resistor (and close to the receiving end on the differential line).
Note for the use of LVDS: it can reach above 600M, PCB requirements are high, and the differential line requirements are strictly equal in length, and the difference should not exceed 10mil (0.25mm). The distance between the 100 ohm resistor and the receiving end should not exceed 500 mils, preferably within 300 mils.
The following levels may not be used very much, because of space, just a brief introduction. If you are interested, you can contact me.
CML: It is a kind of circuit that is matched internally, no need to match. The triode structure is also a differential line, and the speed can reach above 3G. Only point-to-point transmission.
GTL: A structure similar to CMOS, the input is a comparator structure, one end of the comparator is connected to the reference level, and the other end is connected to the input signal. 1.2V power supply.
Vcc=1.2V; VOH>=1.1V; VOL<=0.4V; VIH>=0.85V; VIL<=0.75V
PGTL/GTL+:
Vcc=1.5V; VOH>=1.4V; VOL<=0.46V; VIH>=1.2V; VIL<=0.8V
HSTL is a level standard mainly used for QDR memory: generally there are V¬CCIO=1.8V and V¬¬CCIO=1.5V. Similar to the GTL above, the input is a comparator structure, one end of the comparator is connected to the reference level (VCCIO/2), and the other end is connected to the input signal. The reference level is relatively high (1% accuracy).
SSTL is mainly used for DDR memory. Basically the same as HSTL. V¬¬CCIO=2.5V, the input is a comparator structure, one end of the comparator is connected to the reference level of 1.25V, and the other end is connected to the input signal. The reference level is relatively high (1% accuracy).
HSTL and SSTL are mostly used below 300M.
RS232 and RS485 are basically familiar with everyone, just briefly mention:
RS232 uses ±12-15V power supply, and the serial port behind our computer is RS232 standard. +12V means 0, -12V means 1. It can be converted with a dedicated chip such as MAX3232, or it can be reversed and voltage matched with two transistors and some peripheral circuits.
RS485 is a differential structure, which has higher anti-interference ability than RS232. Transmission distance can reach thousands of meters
The difference between LVTTL and LVCMOS
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