DS2482-100 Dallas Semiconducotr, DS2482-100 Datasheet - Page 6

• Manufacturer Part Number: DS2482-100
• Description: Single-Channel 1-Wire Master
• Manufacturer: Dallas Semiconducotr

Active Pullup (APU)
The APU bit controls whether an active pullup (controlled slew-rate transistor) or a passive pullup (R
used to drive a 1-Wire line from low to high. When APU = 0, active pullup is disabled (resistor mode). Active Pullup
should be selected if the 1-Wire line has a substantial length (several 10m) or if there is a large number (~20 or
more) of devices connected to a 1-Wire line. The active pullup does not apply to the rising edge of a presence
pulse or a recovery after a short on the 1-Wire line.
The circuit that controls rising edges (Figure 2) operates as follows: At t1 the pulldown (from DS2482 or 1-Wire
slave) ends. From this point on the 1-Wire bus is pulled high through R
capacitive load of the 1-Wire line determine the slope. In case that active pullup is disabled (APU = 0), the resistive
pullup continues, as represented by the solid line. With active pullup enabled (APU = 1), when at t2 the voltage has
reached a level between V
slew rate, as represented by the dashed line. The active pullup continues until t
on the resistive pullup will continue.
Figure 2. Rising Edge Pullup
Presence-Pulse Masking (PPM)
The PPM bit controls whether the DS2482 masks the leading edge (falling) of presence pulses. When PPM = 0,
masking is disabled. Presence pulse masking applies only to standard 1-Wire speed (1WS = 0); this bit has no
function if 1WS = 1 (Overdrive speed). Presence-Pulse Masking can improve the performance of large 1-Wire
networks since it prevents the fast falling edge of a presence pulse generated by a 1-Wire slave device from
propagating through the network and getting reflected. Reflections can cause glitches in the network that in turn
can cause slave devices to lose synchronization with the 1-Wire master.
Figure 3 shows the timing references for the Presence-Pulse Masking. If enabled (PPM = 1), the DS2482 begins
pulling the 1-Wire line low at t
1-Wire slave, if present, is pulling the 1-Wire line low. The falling edge of the presence-pulse mask is slew-rate
controlled.
Figure 3. Presence-Pulse Masking
V
0V
CC
Pull-up
V
V
0V
V
1-Wire bus is
discharged
IL1max
CC
IH1MIN
IL1MAX
PPM1
and V
after the reset low time t
RESET PULSE
APU = 1
IH1min
t
RSTL
t
1
, the DS2482 actively pulls the 1-Wire line high applying a controlled
APU con-
trolled edge
t
2
DS2482 Pull-down
DS2482 pull-down with PPM = 1; Standard speed only
t
APUOT
6 of 21
RSTL
t
PPM1
t
3
is expired. The pulldown ends at t
t
PPM2
APU = 0
PRESENCE PULSE
WPU
internal to the DS2482. V
t
APUOT
RSTH
Resistive
pull-up
1-W Slave Pull-down
is expired at t3. From that time
PPM2
WPU
, at which a
CC
resistor) is
and the