Pull-up resistor and pull-down resistor (reprint)

First, the definition:

The pull-up is to pass the indeterminate signal through a resistor clamp at high level! Resistance at the same time to limit the flow of action! Pull the same!

Pull is to inject current into the device, the drop is the output current, the weak strong is only the resistance of the pull-up resistor is different, there is no strict distinction between the non-collector (or drain) open-circuit output

The capacity of the output circuit (such as ordinary gate circuit) to raise current and voltage is limited, and the function of the pull-up resistor is mainly for the open-circuit output current channel of the collector.

Two, the upper and lower pull resistance effect:

1, raise the voltage quasi-position:

A. When the TTL circuit drives the COMS circuit, if the high level of the TTL circuit output is lower than the lowest high level of the COMS circuit (typically 3.5V), this requires

The output of the TTL is terminated with a pull-up resistor to increase the value of the output high level.

B. The OC gate circuit must be coupled with a pull resistor to increase the high level value of the output.

2, increase the driving capacity of the output pin , and some single-chip computer pin also often use the pull-up resistor.

3, N/A pin anti-static, anti-jamming: on the COMs chip, in order to prevent damage caused by static electricity, not the feet can not be suspended, generally connected to the pull resistor to reduce the input resistance , provide a discharge path. It is easier to accept external electromagnetic interference when the pins are floating.

4, resistance matching, suppression of reflection wave interference: long-term transmission of resistance mismatch easily caused by reflection wave interference, coupled with the pull-down resistor is a resistance matching, effective suppression of reflected wave dry interference.

5. Preset space state/default potential: on some CMOS inputs or pull-down resistors are to preset the default potential. When you don't use these pins, these inputs

pull 0 or pull-up 1. On the bus, such as the I²c bus, the state of idle time is obtained by the pull-down resistor

6. Increase the noise tolerance of the chip input signal: If the input is in a high-impedance state, or the impedance input is in a dangling state, you need to add pull or pull-down to avoid receiving

The random level affects the circuit operation. Also if the output is in a passive state, you need to add a pull or pull-down, such as the output is just a transistor collector. thereby improving

The noise tolerance of the chip input signal enhances the anti-jamming ability.

{Power to the component of the call pull-up resistor, the role is usually to make the foot high-level to the element called the pull-down resistor, the role is usually to make the foot for low-level pull- up resistance and power down

The range of resistance is determined by the device (we typically use 10K)

+vcc
+------+ = pull-up resistor
| element | 
|+-----+ 
+------+ = Drop-down resistors  
/span>

-gnd

Generally, the pull-up or pull-down resistor acts as an increase in current, which strengthens the drive capability of the circuit.

For example, 51 P1 mouth.

Also, the P0 port must be connected with a pull-up resistor to be used as an IO port

The difference between the pull and the pull-down is one for the pull current, one for the sink current

In general, the sink current is large

That is, the capacity of the current-sinking drive is stronger}

Third, the choice of pull-up resistor resistance principle includes:

1, from the power consumption and the capacity of the chip sink should be large enough to consider, the resistance is large, the current is small.

2, from ensuring enough drive current considerations should be small enough, small resistance, large current.

3, for high-speed circuit, too large pull-up resistor may be flat edge. Comprehensive consideration

The above three points are usually selected between 1k and 10k. There's a similar reason for the pull-down resistor.

Four, the principle:

The pull-up resistor is actually the load resistor for the collector output. Regardless of the switching application and analog amplification, this resistor's selection is not a pat on the head. Working in a linear range is not

More said, here is the discussion is the transistor is the switch application, so only talk about the switch mode. Find a TTL device to see the last stage alone, the internal load

Resistance according to different driving capacity and speed requirements of this resistor value, low power resistance value is large, fast resistance value is small. But it's hard for chip makers to meet the needs of applications.

Probably the same kind of function chip do many kinds of, so simply do not do this load resistance, change by the user to choose the outside of their own free, so there is OC, OD output chip. By

In digital applications, transistors work in saturation and cutoff areas, the load resistance requirements are not high, the resistance value as long as it is not small to damage the last stage transistor can be, large to the output rise

Time to meet the design requirements can be, casually choose one can work normally. But the details of whether a circuit design is excellent are also to be considered. The switching power of the collector output

Whether the road is open or close to the ground is always through, the transistor on the current from the load resistance through the transistor to the ground, the current from the load resistor through the input of the load

Resistance to the ground, if the load resistor select a small point of power consumption will be large, which in the battery-powered and low power consumption of the system design is to avoid, if the resistance is selected large and will take

On the rising edge of the signal, since the input capacitance of the load is charged on the rising edge by the passive pull-up resistor, the longer the resistance rises, the lower edge is passed through the active crystal

The body tube discharges, the time depends on the device itself. Therefore, designers in the choice of pull-up resistance value, according to the actual system in terms of power consumption and speed.

3. From the angle of the IC (MOS process), explain the input/output pins separately:

1. For chip input pins, it is dangerous to float on the system board (not connected to any output pins or drives). Because it is possible to accumulate capacitance charge inside the input pin at this time

To the intermediate level (such as 1.5V), and the input buffer PMOs tube and the nmos tube simultaneously conduction, so that the power and ground between the formation of a direct path, generating

Large leakage current, a long time may damage the chip. And because being in the middle level causes the internal circuitry to be confused about its logic (0 or 1). Pull up or pull down resistor

, the internal point capacity is charged (put) to the high (low) level, the internal buffer is only nMOS (PMOS) tube conduction, will not form a power to the ground DC path. (As for preventing electrostatic

Cause damage, because the chip pin design will generally add protection circuit, but it is not necessary.

2. For output pins:

1) Normal output pin (push-pull type), generally do not need to pull up or pull down resistor.

2) OD or OC (open-drain or open collector) pins,

This type of pin requires an external pull-up resistor for line and function (at which point multiple outputs can be connected directly.) Typical applications are: Int (interrupt signal) output of multiple chips on the system board

Connect directly, then connect a pull-up resistor, then input the int pin of the MCU to realize the interrupt alarm function.

It works by:

Under normal operating conditions, the nMOS tube inside the OD-type pin is closed, and externally it is in a high-impedance state, and the external pull-up resistor causes the output to be in a high level (invalid interrupt state);

When there is an interrupt requirement, the nMOS tube inside the OD pin is turned on, because its on-resistance is much smaller than the pull-up resistor, so the output is at a low level (active interrupt State). For MOS

The resistor resistance of the circuit up to dozens of to hundreds of K is advisable.

(Note: This answer does not involve the TTL process chip, also did not consider the high-frequency PCB design needs to consider the impedance matching, electromagnetic interference and other effects.)

1, the pull-up or pull-down resistor indicated on the chip pin refers to a resistor or equivalent resistor designed to be inside the chip pin. The purpose of this resistor is to be designed when the user does not need to

To use this pin function, the pin can be placed to the default state without the addition of the element. The CMOS input is not suspended. When using, be aware that if this lack of

Save value is not what you want, you should connect this input directly to the state you need.

2, this pin can be used for "line or" logic if it is a pull-up. Other chips with external open drain or open collector outputs. Make up a negative logic or input. If it's a drop-down

Can be composed of a positive logic "line or", but the external can only be CMOS high-level open-drain chip output, this is because the CMOS output of high, low-level respectively by the

The PMOS and NMOS drain gives current, which can be made into a P-drain open circuit or N-drain open circuit. The high level of the TTL is output current by the source follower and is not suitable for "line or".

3, TTL to CMOS driver or vice versa, in principle is not recommended to use the upper and lower resistor to change the level, it is best to add a level conversion circuit. If both sides of the power supply are 5 volts, can be straight

Successive but affecting performance and stability, especially when CMOS drive TTL. On both sides of the logic level is not at the same time, be sure to use the power-level conversion. When the supply voltage is 3 volts or less, it is recommended not

It is not possible to use a resistor to pull the level with direct connection.

4, the chip plus resistor is determined by the application, but it is not feasible to use resistors to pull levels or improve drive capability in logic circuits. Need to improve the drive should be added drive circuit. Change

The level conversion circuit should be added to the variable level. Including long-term reception have a special chip.

Pull-up resistor and pull-down resistor (reprint)