Datasheet
27Maxim Integrated
DS2483
Single-Channel 1-Wire Master
with Adjustable Timing and Sleep Mode
Applications Information
SDA and SCL Pullup Resistors
SDA is an open-drain output on the DS2483 that requires
a pullup resistor to realize high-logic levels. Because the
DS2483 uses SCL only as input (no clock stretching), the
master can drive SCL either through an open-drain/col-
lector 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: R
P(MIN)
= (V
CI2C
- 0.4V)/3mA. With an I
2
C pul-
lup voltage V
CI2C
of 5.5V, the minimum value for the pul-
lup resistor is 1.7kI. The “Minimum R
P
” line in Figure 10
shows how the minimum pullup resistor changes with the
operating (pullup) 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 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
P(MAX)
= 300ns/(C
B
x ln(7/3)). For a
bus capacitance of 400pF, the maximum pullup resistor
would be 885I.
Because an 885I pullup resistor, as would be required
to meet the rise time specification at 400pF bus capaci-
tance, is lower than R
P(MIN)
at 5.5V, a different approach
is necessary. The “Maximum Load at Minimum R
P
Fast
Mode” line in Figure 10 is generated by first calculating
the minimum pullup resistor at any given operating volt-
age (“Minimum R
P
” line) and then calculating the respec-
tive bus capacitance that yields a 300ns rise time.
Only for pullup voltages of 3V and lower can the maxi-
mum permissible 400pF bus capacitance be maintained.
A reduced 300pF bus capacitance is acceptable for 4V
and lower pullup voltages. For fast mode operation at
any pullup voltage, the bus capacitance must not exceed
200pF. The corresponding pullup resistor value at the
voltage is indicated by the “Minimum R
P
” line.
Figure 10. I
2
C Fast Mode Pullup Resistor Selection Chart
MINIMUM R
P
MAXIMUM LOAD AT MINIMUM R
P
FAST MODE
2000
MINIMUM R
P
(Ω)
LOAD (pF)
PULLUP VOLTAGE (V)
1600
1200
800
400
1234 5
0
500
400
300
200
100
0