Datasheet

DS2482-100: Single-Channel 1-Wire Master
20 of 22
Figure 11. Application Schematic
1-Wire Device #1
(with special power
requirements
)
1-Wire line
DS2482-100
IO
PCTLZ
Current Limiting
Resistor
*R
t
*R
P
(I²C port)
µC
SDA
SCL
A
D0
A
D1
V
CC
DS2482-100
IO
PCTLZ
1-Wire
Device #2
1-Wire line*R
t
V
CC
* R
t
Line termination resistor, typically 100
Ω
R
P
I²C pull-up resistor, see Application
Information for R
P
sizing.
V
CC
A
D0
A
D1
SDA
SCL
APPLICATION INFORMATION
SDA and SCL Pullup Resistors
SDA is an open-drain output on the DS2482-100 that requires a pullup resistor to realize high logic levels. Because
the DS2482-100 uses SCL only as input (no clock stretching) the master may drive SCL either through an open-
drain/collector output with a pullup resistor or a push-pull output.
Pullup Resistor R
P
Sizing
According to the I
2
C specification, a slave device must be able to sink at least 3mA at a V
OL
of 0.4V. This DC
condition determines the minimum value of the pullup resistor: Rpmin = (V
CC
- 0.4V)/3mA. With an operating
voltage of 5.5V, the minimum value for the pullup resistor is 1.7kΩ. The "Minimum RP" line in Figure 12 shows how
the minimum pullup resistor changes with the operating voltage.
For I
2
C systems, the rise time and fall time are measured from 30% to 70% of the pullup voltage. The maximum
bus capacitance C
B
is 400 pF. The maximum rise time at standard speed must not exceed 1000ns and 300ns at
fast speed. Assuming maximum rise time, the maximum resistor value at any given capacitance C
B
is calculated
as: Rpmaxs = 1000ns/(C
B
*ln(7/3)) (standard speed) and Rpmaxf = 300ns/(C
B
*ln(7/3)) (fast speed). For a bus
capacitance of 400pF the maximum pullup resistor values are 2.95kΩ at standard speed and 885Ω at fast speed. A
value between of 1.7kΩ and 2.95kΩ meets all requirements at standard speed.