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This article is to share with you about how the CMOS conversion circuit is to achieve level conversion, the editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article. Let's take a look at it with the editor.

CMOS integrated circuit is the abbreviation of complementary symmetrical metal oxide semiconductor (Compiementary symmetry metal oxide semicoductor) integrated circuit. Many basic logic units of the circuit are connected by enhanced PMOS transistor and enhanced NMOS transistor, so the static power consumption is very small.

Discrimination of CMOS abbreviations

In the computer field, CMOS often refers to the chip that stores the basic startup information of the computer (such as date, time, startup settings, etc.). Sometimes people will mix CMOS and BIOS. In fact, CMOS is a read-write parallel or serial FLASH chip on the motherboard, which is used to save the hardware configuration of BIOS and the user's settings for some parameters.

Today, CMOS manufacturing technology is also used to make photosensitive elements for digital imaging equipment, especially for SLR digital cameras with large frame sizes.

In addition, CMOS can also refer to complementary metal oxide components and manufacturing process.

So today, although they are all called CMOS for technological reasons, CMOS presents very different appearance features in three application areas:

First, it is used to save computer information, and CMOS is used as an erasable chip. in this field, users usually do not care about the hardware problems of CMOS, but only care about the information written on CMOS, that is, the setting of BIOS. The most frequently mentioned is to remove the battery from the motherboard and discharge CMOS when the system fails, so as to restore the BIOS settings.

Second, in the field of digital imaging, CMOS as a low-cost photosensitive element technology has been developed, the common digital products in the market, its photosensitive elements are mainly CCD or CMOS, especially low-end camera products, and usually high-end cameras are CCD photosensitive elements.

Third, in the more professional field of integrated circuit design and manufacturing.

The supply voltage VDD of CMOS circuit can work normally in the range of + 5V to 15V, the voltage fluctuation is allowed to be ±10, when the output voltage is higher than VDD-0.5V, it is logic 1, and when the output voltage is lower than VSS+0.5V (VSS is digital), it is logic 0. The output high level of CMOS circuit is about 0.9Vcc, and the output low level is about 0.1Vcc. Logic 1 when the input voltage is higher than VDD-1.5V, and logic 0 when the input voltage is lower than VSS+1.5V (VSS is digitally).

The TTL level signal is most used because the usual data representation is specified in binary, + 5V is equivalent to logic "1" and 0V is equivalent to logic "0", which is called TTL (transistor-transistor logic level) signal system, which is the standard technology for communication between internal parts of equipment controlled by computer processors.

Standard TTL input high level minimum 2V, output high level minimum 2.4V, typical value 3.4V, input low level maximum 0.8V, output low level maximum 0.4V, typical value 0.2V (input H "2V, input L" 0.8V; output H "2.4V (3.4V), output L" 0.4V (0.2V).

Is CMOS level a digital signal or an analog signal?

The CMOS level is a digital signal, the supply voltage VDD range of the COMS circuit is relatively wide, the voltage fluctuation is allowed to be ±10, when the output voltage is higher than VDD-0.5V, it is logic 1, when the output voltage is lower than VSS+0.5V (VSS is digital) is logic 0, the general digital signal is 0 and 1. If you need PCB design students to use circuit board proofing, you can find out from the blogger.

Cmos level conversion circuit

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

Critical value of TTL circuit:

VOHmin = 2.4V VOLmax = 0.4V VIHmin = 2.0V VILmax = critical value of 0.8VCMOS circuit (supply voltage is + 5V) VOHmin = 4.99V VOLmax = 0.01V VIHmin = 3.5V VILmax = 1.5V2, logic level conversion CMOS level of TTL and CMOS can drive TTL level TTL level cannot drive CMOS level, pull resistance is needed.

3. Characteristics of using logic chip

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.

Several commonly used level conversion schemes

(1) Transistor + pull-up resistance method

Is a bipolar transistor or MOSFET,C/D pole one by one pull up resistance to the positive power supply, the input level is very flexible, the output level is roughly the positive power level.

(2) OC/OD device + pull-up resistance method

Similar to (1). It is suitable for situations where the output of the device happens to be OC/OD.

(3) boost of 74xHCT series chips (3.3V → 5V)

All input 5V TTL devices compatible with 5V CMOS level can be used as 3.3V → 5V level conversion.

This is because the 3.3V CMOS level happens to be compatible with the 5V TTL level (coincidentally), while the CMOS output level is always close to the power supply level.

Cheap options such as 74xHCT (HCT/AHCT/VHCT/AHCT1G/VHCT1G/). ) series (the letter T means TTL compatibility).

(4) over-input step-down method (5V → 3.3V, 3.3V → 1.8V,. Any logic device that allows the input level to exceed the power supply can be used to reduce the level.

The "overrun" here refers to exceeding the power supply, and many older devices do not allow the input voltage to exceed the power supply, but more and more new devices remove this restriction (changing the input stage protection circuit).

For example, the datasheets of the 74AHC/VHC series chip clearly indicates that "the input voltage range is 0 to 5.5V". If the 3.3V power supply is used, the 5V → 3.3V level conversion can be realized.

(5) Special level conversion chip

The most famous is the 164245, which can not only be used as a boost / buck, but also allows both sides of the power supply to be out of sync. This is the most common level conversion scheme, but it is also very expensive (I bought RMB 45 per piece not long ago, although it is retail, but it is also frighteningly expensive), so if it is not necessary, it is best to use the first two options.

(6) Resistance partial voltage method

The easiest way to lower the level. The 5V level, divided by the 1.6k+3.3k resistor, is 3.3V.

(7) current limiting resistance method

If the above two resistors are too much, sometimes only one current-limiting resistor can be connected in series. Although some chips do not allow the input level to exceed the power supply in principle, it is still safe as long as a current limiting resistor is connected in series to ensure that the input protection current does not exceed the limit (for example, the 74HC series is 20mA).

The above is how the CMOS conversion circuit realizes the level conversion, and the editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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