1. high impedance有high impedance嘅用途, 唔一定每支都需要drive/pull-down
2. pull-down/up主要係確保power on/reset時有預設值, 所以至少確保enable pin要pull up/down
3. 有啲device IO係open drain, 咁就一定要加pull up, e.g I2C
4. 如果device input係schmitt trigger, 唔drive到頂或底會食多左電

threshold係幾多唔係重點, 重點係要確保, 任何時候都唔好有undefined condition.

TOP

thxthx ching...  但由最basic講起   一個mcu output gpio pin connect左去OR gate既input到...
當m ...
clcwong 發表於 2019-12-28 15:16



1. 有咩情況係唔需要pull-down/up?
- 如果pin係做analog, 唔洗pull
- 需要慳電, 可以就唔好pull up, pull up食電

2. floating voltage會有幾多?
- 一般MCU/IC pin都有clamping diode, 會限制住voltage range起GND-0.3V to VDD+0.3V中間
- IC pin有寄生電容, 寄生電阻, etc, 好微量, 好多時會leak到支pin落0V, 但唔一定, 睇咩IC, 睇parasitic element係咩

3. 放隻手指埋去個pin到都可以另佢由0變1咁
- MCU IC pin impedance有成> 100Meg ohm to GND/VDD. 你揾電錶度下你手指幾多ohm. 你隻手指drive短落GND/VDD再drive起支pin好正常

4. 咁係無pull-up/down resistor既情況底下佢仲會唔會係咁高低高低跳黎跳去?
- IC pin微量嘅寄生電容, 寄生電阻會"穩"住條pin, 除非RF interference好勁, 或你駁條長電線落pin收RF interference, 如果唔係唔會跳黎跳去

TOP

本帖最後由 icefire 於 2019-12-28 23:42 編輯
1. 換句話講   即係理論上接每粒logic IC都應該要有一個pull-up/down resistor for each input pin?

2. ...
clcwong 發表於 2019-12-28 21:38



1. 唔好成日講到一定要點, 真係好睇你做咩, 我好多時enable pin先pull low/down, 因為pull up會食電
3. 係
4. 成條2.54mm jump wire駁左出來, 仲駁埋入bread board都唔叫長?  成條天線, 果度夠induce市電50Hz落去.

concept係
樣樣野放落電子線路都有Parasitic element, 有impedance(L/C/R). 大氣有電阻值, PCB板有電阻值, 只係大定細, 有resistance, 電流就會leak. 兩條電線放得近己經係一個capacitor同transformer, 只係數值微細, 但有capacitance/inductance, 就可以induce voltage
加電阻pull up/high就係控制impedance, 電阻impedance值 <<< Parasitic element值

TOP