为何I2C 的连接数， 用400pF 限制？

Quantum_

1. 如题 ："The number of ICs that can be connected to the same bus is limited only by a maximum bus capacitance of 400 pF."---THE I2C-BUS SPECIFICATION
, c0 E8 |6 y" S6 U& V2. 如何用电容来定义。
' m; u3 y; T3 c3. 这个400pF 是如何推导/计算 得来的？

$ t* h( h/ v: }) p/ m谢谢！

Aubrey

IIC线上电平从低到变化时, 电源通过上拉电阻RP对线上负载电容CL充电, 这需要一定的时间, 即上升时间, 端口信号的上升时间t可近似用充电时间常数RPCL乘积表示,
9 P; ~/ e4 ~! l. F- ^如果RC 充电时间常数过大, 将使得信号上升沿变化缓慢, 达不到传输要求.  

超級狗

嗯插屁（NXP）I2C-Bus Specification and User Manual Revision 6.0
/ V1 E8 k" }& e5 L( @7 kThe switched pull-up circuit in Figure 44 is for a supply voltage of VDD = 5 V ± 10 % and a maximum capacitive load of 400 pF. Since it is controlled by the bus levels, it needs no additional switching control signals. During the rising/falling edges, the bilateral switch in the HCT4066 switches pull-up resistor Rp2 on/off at bus levels between 0.8 V and 2.0 V. Combined resistors Rp1 and Rp2 can pull up the bus line within the maximum specified rise time (tr) of 300 ns.
% d9 i1 Z5 @; c  |2 ?1 O+ m

gdutchen

谢谢，受教了。

沙漠风铃10

太大，信号质量就不好了

ruanyj228

0 E7 O5 A/ @0 L谢谢，受教了。

soswelcome

谢谢，受教了。

傻大个牌纯碱

学习了。

wjy870825

受教了

tomatogua

根据时序要求，上升时间来定了，负载电容大了上升时间会变慢，时序会变差

林少230

受教了