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
/ @) b6 Q& M8 N# B, L2. 如何用电容来定义。
3. 这个400pF 是如何推导/计算 得来的？

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,
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
5 D& J! s  s5 s, e5 s8 xThe 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.
3 L4 v) p4 j, w9 ^; w& Z
  ?) r* d& z: T2 c# F: [
. D+ `* T, F: J7 r- _1 ?

林少230

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

wjy870825

受教了

傻大个牌纯碱

学习了。

soswelcome

谢谢，受教了。

ruanyj228

% |2 g. N2 b* T" q谢谢，受教了。

沙漠风铃10

太大，信号质量就不好了

gdutchen

谢谢，受教了。