Datasheet

DS28CM00: I²C/SMBus Silicon Serial Number
9 of 9
Figure 6. Application Schematic
V
CC
SD
A
SCL
µC
GND
V
CC
SDA
SCL
DS28CM00
GND
R
P
R
P
V
CC
To additional
devices
For I²C systems, the rise time and fall time are measured from 30% to 70% of the pullup voltage. The maximum
bus capacitance C
B
is 400pF. The maximum rise time must not exceed 300ns. Assuming maximum rise time, the
maximum resistor value at any given capacitance C
B
is calculated as: R
PMAX
= 300ns/(C
B
*ln(7/3)). For a bus
capacitance of 400pF the maximum pullup resistor would be 885Ω.
Since a 885Ω pullup resistor, as would be required to meet the rise time specification and 400pF bus capacitance,
is lower than R
PMIN
at 5.25V, a different approach is necessary. The "Max. Load…" line in Figure 7 is generated by
first calculating the minimum pullup resistor at any given operating voltage ("Minimum R
P
" line) and then calculating
the respective bus capacitance that yields a rise time of 300ns.
Only for pullup voltages of 4V and lower can the maximum permissible bus capacitance of 400pF be maintained. A
reduced bus capacitance of 300pF is acceptable for the entire operating voltage range. The corresponding pullup
resistor value at the voltage is indicated by the "Minimum R
P
" line.
Figure 7. I²C Fast Speed Pullup Resistor Selection Chart
0
200
400
600
800
1000
1200
1.5 2 2.5 3 3.5 4 4.5 5
Pull-up Voltage
Minimum Rp (Ohms)
0
100
200
300
400
500
600
Load (pF)
"Minimum Rp" Max. Load at Min. Rp fast mode
PACKAGE INFORMATION
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to
www.maxim-ic.com/DallasPackInfo.)