DS2482S-800+TR MAXIM [Maxim Integrated Products], DS2482S-800+TR Datasheet - Page 21

• Manufacturer Part Number: DS2482S-800+TR
• Description: Eight-Channel 1-Wire Master
• Manufacturer: MAXIM [Maxim Integrated Products]

DS2482-800: Eight-Channel 1-Wire Master
Case C: 1-Wire busy (1WB = 1)
S
AD,0
A
1WT
A\
P
The master should stop and restart as soon as the DS2482 does not acknowledge the command code.
Figure 10. Application Schematic
Application Information
SDA and SCL Pullup Resistors
SDA is an open-drain output on the DS2482 that requires a pullup resistor to realize high logic levels. Because the
DS2482 uses SCL only as input (no clock stretching) the master can drive SCL either through an open-
drain/collector output with a pullup resistor or a push-pull output.
Pullup Resistor R
Sizing
P
According to the I²C specification, a slave device must be able to sink at least 3mA at a V
of 0.4V. This DC
OL
condition determines the minimum value of the pullup resistor: Rpmin = (V
- 0.4V)/3mA. With an operating
CC
voltage of 5.5V, the minimum value for the pullup resistor is 1.7k. The "Minimum RP" line in Figure 11 shows how
the minimum pullup resistor changes with the operating voltage.
For I²C systems, the rise time and fall time are measured from 30% to 70% of the pullup voltage. The maximum
bus capacitance C
is 400pF. The maximum rise time at standard speed must not exceed 1000ns and 300ns at
B
fast speed. Assuming maximum rise time, the maximum resistor value at any given capacitance C
is calculated
b
as: Rpmaxs = 1000ns/(C
*ln(7/3)) (standard speed) and Rpmaxf = 300ns/(C
*ln(7/3)) (fast speed). For a bus
B
B
capacitance of 400pF the maximum pullup resistor values are 2.95k at standard speed and 885 at fast speed. A
value between of 1.7k and 2.95k meets all requirements at standard speed.
Since a 885 pullup resistor, as would be required to meet the rise time specification at fast speed and 400pF bus
capacitance, is lower than Rpmin at 5.5V, a different approach is necessary. The "Max. Load…" line in Figure 11 is
generated by first calculating the minimum pullup resistor at any given operating voltage ("Minimum Rp" line) and
then calculating the respective bus capacitance that yields a rise time of 300ns.
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