Datasheet

DS2480B
Maxim Integrated ........................................................................................................................................................................................... 14
CONTROLLED EDGES
One of the tasks of the DS2480B is to actively shape the edges of the 1-Wire communication waveforms.
This speeds up the recharging of the 1-Wire bus (rising edges) and reduces ringing of long lines (falling
edges). The circuitry for shaping rising edges is always on. The slew rate of falling edges is actively
controlled only at flexible speed and requires the parameter for slew rate control being different from its
power-on default value.
All Rising Edges
The active pullup of the rising edges reduces the rise time on the 1-Wire bus significantly compared to a
simple resistive pullup. Figure 4 shows how the DS2480B is involved in shaping a rising edge.
Figure 4. ACTIVE PULLUP
The circuit operates as follows: At t
1
the pulldown (induced by the DS2480B or a device on the bus) ends.
From this point on the 1-Wire bus is pulled high by the weak pullup current I
WEAKPU
provided by the
DS2480B. The slope is determined by the load on the bus and the value of the pullup current. At t
2
the
voltage crosses the threshold voltage V
IAPO
. Now the DS2480B switches over from the weak pullup
current I
WEAKPU
to the higher current I
ACTPU
. As a consequence, the voltage on the bus now rises faster. As
the voltage on the bus crosses the threshold V
IAPTO
at t
3
, a timer is started. As long as this timer is on
(t
APUOT
), the I
ACTPU
current will continue to flow. After the timer is expired, the DS2480B will switch
back to the weak pullup current. Excessive noise on the 1-Wire line at the V
IAPTO
level can cause an
undesirable trip of the active pullup. External R-C filtering as discussed in the HARDWARE
APPLICATION EXAMPLES section and Application Note 148 should be used to prevent false triggering.