OC door and line and logic

Conversion) OC door and line and logic

The OC gate, also known as open collector (open drain) and non-gate circuit, open collector (open drain ). Why is OC introduced? In actual use, sometimes two or more non-door output ends need to be connected to the same wire, these and non-door data (status level) with the same wire transport out. Therefore, a new and non-gate circuit-OC gate is required to implement "line and logic ". The OC gate is mainly used in three aspects: implementation and non-logic, level conversion, and drive. Because the collector of the output tube of the OC gate circuit is suspended, an external pull-up resistor RP must be used to power the VCC. The OC door uses a pull-up resistor to output a high level. In addition, in order to increase the drive capability of the output pin, the principle of selecting the pull-up resistor resistance is as follows, it should be large enough to reduce power consumption and chip current filling capacity, and small enough to ensure sufficient driving current.

Is there a formula for calculating the selection of pull-up resistors?

Up/down resistance:
1. When the TTL circuit is used to drive the COMs circuit, if the high level output by the TTL circuit is lower than the lowest high level (generally 3.5 V) of the COMs circuit, the pulling resistance should be connected at the TTL output end, to increase the output height.
2. The open resistance must be added to the OC gate circuit to increase the output height.
3. In order to increase the drive capability of the output pin, some single-chip microcomputer pins often use the pull-up resistor.
4. On the CMOS chip, in order to prevent damage caused by static electricity, unused pins cannot be left blank. Generally, the pulling resistance is connected to reduce the input impedance and provide a drain path.
5. Increase the output level by adding a pull resistor to the Chip Pin, thereby improving the noise tolerance of the chip input signal and enhancing the anti-interference capability.
6. Improve the anti-electromagnetic interference capability of the bus. When the PIN is left blank, it is easier to accept external electromagnetic interference.
7. During long-line transmission, the interference of the reflected wave is easily caused by the mismatch of the Resistance. In addition, the drop-down resistance is used to effectively suppress the interference of the reflected wave.

The selection principles of the pull-up resistance include:
1. Considering Power Consumption Reduction and chip current filling capacity, it should be large enough; the resistance should be large and the current should be small.
2. Make sure that the driving current is sufficient, the resistance is small, and the current is large.
3. for High-Speed Circuits, excessive pull-up resistance may smooth the edge. Comprehensive Consideration
The preceding three points are usually selected from 1 K to 10 K. Similar to the drop-down Resistor

 

 

Open collector output has the following benefits:

1. the cables and functions can be achieved, that is, two or more output ends can be connected in parallel, and then pulled together to the high resistance. In this way, as long as one output is low,
The result is low, that is, the implementation and function.

2. It is similar to the above, that is, when multiple door outputs are connected together, it will not cause damage.

3. It can be used to control high levels. For typical applications, see uln2003.

4. Similar to 3, high-impedance mode is used when the output is disconnected, so that the input port can be used. For typical applications, see the I/O port structure of single-chip microcomputer 8951,
When set to 1, it is the input port.

For open collector (OC) or open drain (OD) output, If You Want To output a high level, you must connect the pulling resistance because the OC or OD gate,
When set to 1, the output is equivalent to being suspended.

Line and logic, that is, the two output ends (including more than two) can be directly connected to implement the "and" logic function. In practical applications such as bus transmission, the output ends of Multiple doors need to be connected in parallel. Generally, the output ends of TTL doors cannot be directly connected, otherwise, a large short-circuit current (fill current) is formed between the output tubes of these doors due to low impedance, and the device is burned out. On the hardware, it can be implemented by oC or tri-state gate (ST gate. When the OC door is used to achieve the line and the output port, a pulling resistance should be added at the same time.
The three-state gate (ST gate) is mainly used for multiple gate outputs to share the data bus. To prevent multiple gate outputs from occupying the data bus at the same time, the enabling signals of these gates (en) only one valid level (such as high level) is allowed, because the output of the three-state gate is a low-resistance output of the pull type, and the pulling (load) resistance is not required, therefore, the switching speed is faster than that of the OC gate, and the three-state gate is often used as the output buffer.