DS2480 Dallas Semiconducotr, DS2480 Datasheet - Page 12

• Manufacturer Part Number: DS2480
• Description: Serial 1.Wire Line Driver
• Manufacturer: Dallas Semiconducotr

CONTROLLED EDGES
One of the tasks of the DS2480 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 pull–up of the rising edges reduces the rise
time on the 1–Wire bus significantly compared to a sim-
ple resistive pull–up. Figure 4 shows how the DS2480 is
involved in shaping a rising edge.
ACTIVE PULL–UP Figure 4
The circuit operates as follows: At t
(induced by the DS2480 or a device on the bus) ends.
From this point on the 1–Wire bus is pulled high by the
weak pull–up current I
The slope is determined by the load on the bus and the
value of the pull–up current. At t
threshold voltage V
over from the weak pull–up current I
higher current I
on the bus now rises faster. As the voltage on the bus
crosses the threshold V
long as this timer is on (t
continue to flow. After the timer is expired, the DS2480
will switch back to the weak pull–up current.
DS2480
042498 12/26
5V
V
V
0V
1–WIRE BUS
IS DISCHARGED
IAPTO
IAPO
ACTPU
t
IAPO
1
WEAKPU
. As a consequence, the voltage
IAPTO
APUOT
. Now the DS2480 switches
t
APUOT
at t
provided by the DS2480.
2
), the I
the voltage crosses the
3
, a timer is started. As
t
2
ACTPU
1
t
WEAKPU
3
the pull–down
current will
to the
Falling Edges (DS2480–initiated)
Whenever the DS2480 begins pulling the 1–Wire bus
low to initiate a time slot, for example, it first turns off the
weak pull–up current I
Overdrive speed it will generate a falling edge at a slew
rate of typically 15V/ s. This value is acceptable for
short 1–Wire busses and adequate for communication
at Overdrive speed. For MicroLAN networks of more
than roughly 30 meters length one should always use
flexible speed. One of the parameters that is adjustable
at flexible speed is the slew rate of DS2480–initiated fal-
ling edges. The effect of the slew rate control is shown in
Figure 5.
SLEW RATE CONTROL Figure 5
As extensive tests have shown, MicroLAN networks at a
length of up to 300 meters will perform best if the fall time
t
rate of approximately 1V/ s. This slew rate is typically
achieved by setting the configuration parameter 001
(Pull–Down Slew Rate Control) to a value of 100 (see
Table 4). If the actual measured fall time is longer than
the target value, one should use a value code of 011 or
lower. If the fall time is shorter, one should use a value
code of 101 or higher.
Once determined, the value code for the Pull–Down
Slew Rate Control parameter should be stored in the
host and always be loaded into the DS2480 after a
power–on or master reset cycle.
F
5V
0.8V
0V
1–WIRE BUS
IS PULLED UP
is in the range of 4 0.5 s. This translates into a slew
HIGH SLEW RATE
t
1
WEAKPU
WEAK PULL–UP ENDS,
PULL–DOWN BEGINS
t
F
. Then, at regular and
LOW SLEW RATE
TARGET FOR
LONG LINES:
4
0.5 s