的作用硬件(Pull-up,pull-downresistors,hardware)

上拉、下拉电阻的作用硬件（Pull-up, pull-down resistors,

hardware）

Pull-up, pull-down resistors play the role of , do

not give up on yourself! (mom once said to me, the moment I

turned round to go out, I burst into tears and didn't want anyone

to see it!) To see this one small heart feeling, think of when

I left home to thousands of miles away, my mother also said

something similar, but as a man, you must create a day, in order

to repay their parents!! Pull-up and pull-down resistors,

hardware, 2009-12-25, 10:28:59, read 23, comment 0: big, small,

medium and small

Turn a hardware article, read the harvest, may be for the

hardware is not what, but for our software origin, into the

embedded very helpful.

Pull up and pull down resistors

Pull-up resistor:

1, when the TTL driving circuit of COMS circuit, if the high

level is lower than the minimum high level COMS circuit output

TTL circuit (usually 3.5V), then you need to pull in the

resistance of the output end of the TTL, in order to improve

the output high level value.

2, OC gate circuit must be combined with pull resistors to use.

3, in order to increase the output pin drive capacity, and some

SCM pins also often use pull-up resistor.

4, in the COMS chip, in order to prevent damage caused by static

electricity, no feet can not be left hanging, generally

connected to the pull resistor to reduce input impedance,

providing leakage path.

5, the chip pins plus pull resistors to improve the output level,

thereby increasing the chip input signal noise margin, enhance

anti-interference ability.

6, improve the anti electromagnetic interference capability of

the bus. It is easier to accept electromagnetic interference

from the outside of the foot.

7, long line transmission in the resistance mismatch, easy to

cause reflection wave interference, plus pull-down resistance

is resistance matching, effectively suppress the reflection

wave interference.

The selection principle of the resistance value of the pull-up

resistor includes:

1, from the power consumption and chip current capacity should

be considered large enough, high resistance, small current.

2, from ensuring sufficient driving current should be

considered small enough, small resistance, large current.

3. For high speed circuits, too large pull-up resistors may

flatten at the edges. Comprehensive consideration

The above three points are usually selected from 1K to 10K.

There is a similar reason for pull-down resistors

The selection of the pull-up resistor and the pull-down

resistor should be based on the characteristics of the

switching tube and the input characteristics of the lower

circuit. The main factors to consider are the following

factors:

1. drive capacity and power balance. For example, the lower the

pull-up resistance, the greater the driving power, but the

greater the power consumption, the design should pay attention

to the balance between the two.

2. driver requirements for subordinate circuits. Similarly,

the above pull resistor, for example, when the output is high,

the switch is broken, and the pull-up resistor should be

appropriately chosen to provide sufficient current to the lower

circuit.

3. high and low level settings. The threshold levels of

different circuits are different, and the resistors should be

properly set to ensure that the correct level is output. For

example, when the output is low, the switch is switched on, the

pull-up resistor and the switch on resistance value should be

guaranteed below the zero level threshold.

4. frequency characteristic. Take the above resistance as an

example,

The delay between the pull-up resistor and the capacitor

between the drain source of the switching diode and the input

capacitor of the lower circuit will cause a RC delay, and the

greater the resistance, the greater the delay. The setup of the

pull-up resistor should take into consideration the

requirements of the circuit in this regard.

The setting principle of the pull-down resistor is the same as

the pull-up resistor.

OC gate output high level is a high resistance state, the

pull-up current to be provided by the pullup resistor with input

terminal of each port is less than 100uA, with output drive

current is about 500uA, the standard working voltage is 5V, high

and low threshold input port is 0.8V (lower than this value is

low) 2V; (high exit limit).

When the drawing resistor is selected:

500uA x 8.4K= 4.2, that is more than 8.4K election, the output

can be pulled down to less than 0.8V, this is the minimum

resistance, and then small will not pull down. If the output

current is large, then the resistance value can be reduced, and

it can be lower than 0.8V when it is pulled down.

When the output is high, the leakage current of the pipe is

ignored, and the two input port needs 200uA

200uA x15K=3V, that is, pull-up resistor, voltage drop is 3V,

the output can reach 2V, this resistance is the maximum

resistance, and then big can not pull 2V. Select 10K available.

The COMS gate can refer to the 74HC series

The design of pipe leakage current can not be ignored, IO actual

current in different level is different, the above is just the

principle, in a word: output high level to be fed back to the

input port output low level not to output feed hold (otherwise

excess current fed cascade the input port is higher than the

low limit value of exit is not reliable)

In digital circuits, the input pins must be connected to the

fixed level and connected to the high level or ground via the

1K resistor.

1. resistance action:

The L connection group is designed to prevent the input from

being suspended

L weakens the interference of external current to the chip

L protect diode in CMOS, the general current is not more than

10mA

L pull-up and pull-down, current limiting

The L 1. changes the level potential, commonly used in TTL-CMOS

matching

2. there is a definite state when the pin is left vacant

3. increase the driving power at high level output.

4. Provide current for the OC gate

L that depends on what device the output drives. If the device

needs high voltage, and the output voltage of the output is not

enough, you need to add a pull-up resistor.

If there is a l pull-up resistor the port in the default value

for the high level you want to control it must use low level

to control such as the three state gate circuit transistor or

diode cathode collector, to control the current pull-up

resistor pull down into a low level. Conversely，

L is especially used in the interface circuit, in order to get

the general level, using this method, in order to ensure the

correct state of the circuit, to avoid accidents, for example,

in motor control, the inverter upper and lower bridge arm is

not straight, if they are used with a microcontroller to drive,

you must set the initial state through preventing!

2 definitions:

L pullup is to place an uncertain signal at a high level through

a resistor! Resistance simultaneously acts as a limiting

current! Drop in the same way!

The L pull-up is the injection current to the device, and the

pull-down is the output current

The L is weak but the resistance value of the pull-up resistor

is different, and there is no strict distinction between them

For non L (collector or drain) open circuit output circuit (such

as the common gate circuit of current and voltage increase)

ability is limited, the pull-up resistor is the main function

of the open circuit output current output channel for

collector.

3. Why use pull resistors?:

L is normally used as a single button trigger. If the IC itself

does not have an internal resistance, the resistor must be

connected to the outside of the IC in order to keep the single

key in a state that is not triggered or triggered back to its

original state.

L digital circuit has three conditions: high level and low level,

and high resistance condition, some applications do not want

a high impedance state through a pull-up resistor or pull-down

resistor that is in a steady state, depending on the design

requirements and!

L in general is I/O port, some can be set, and some can not set,

there is some built-in external I/O port output, similar to a

triode C, when C connected through a resistor and power when

connected together, the resistance becomes C pull resistance,

also that is to say, if the high level of the port is normal,

C through a resistor and connected together, the resistor

called pull-down resistor, the port is usually low, effect:

For example, when a port with a pull-up resistor is set to lose

status, his normal level is high, used to detect low level

inputs.

The L pullup resistor is used to provide current when the bus

drive capability is insufficient. The general idea is to pull

the current, and the pull down resistor is used to absorb the

current, which is what your classmates say