Equation Of The Pull-Up Resistor Value - Samsung S5PV210 Hardware Design Manual

S5PV210_HARDWARE DESING GUIDE REV 1.0

12.2. Equation of the pull-up resistor value

V = 0.3 V V
IL DD
Figure 12-1) Definition of timing for High-Speed mode devices on the IIC –bus
1) tr (Rising time) which depends on Pull- up resistance and bus capacitance affects SCL frequency change
( Higher tr makes slower SCL), especially when it is High-Speed mode (400kHz)
2) tr (Rising time) maximum is 300 ns , minimum is 20 + 0.1 Cb (bus capacitance)
3) When tr (Rising time) is 300ns, SCL might be maximum 13% slower than original setting value
4) To make real SCL within 1% variation of setting value(400kHz) , tr (Rising time) should be less than
80nsec
5) User can use this formula to determine Rp , Cb and tr
Rp(Pull-up resistance) Max is a function of the rise time minimum (tr) and the estimated bus
capacitance(Cb)
V(t1)=0.3x V
V(t2)=0.7x V
T = t2 – t1 = 0.8473 x RC
R = t
p(max) r
= 0.7 V
IH DD
=V ( 1 – e^(-t1/RC)); then t1=0.3566749xRC)
dd dd
=V ( 1 – e^(-t2/RC)); then t2=1.2039729xRC)
dd dd
/ ( 0.8473 x C )
b
100