Datasheet

DS2482-100: Single-Channel 1-Wire Master
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Strong Pullup (SPU)
The SPU bit is used to activate the strong pullup function prior to a 1-Wire Write Byte or 1-Wire Single Bit
command. Strong pullup is commonly used with 1-Wire EEPROM devices when copying scratchpad data to the
main memory or when performing an SHA-1 computation, and with parasitically powered temperature sensors or
A/D converters. The respective device data sheets specify the location in the communications protocol after which
the strong pullup should be applied. The SPU bit must be set immediately prior to issuing the command that puts
the 1-Wire device into the state where it needs the extra power. The strong pullup uses the same internal pullup
transistor as the active pullup feature. For cases where the internal strong pullup has insufficient strength, the
PCTLZ pin can be used to control an external p-channel MOSFET to supply additional power beyond the drive
capability of the DS2482-100 to the 1-Wire line. See the ΔV
STRPU
parameter in the Electrical Characteristics to
determine if the internal strong pullup is sufficient given the current load on the device.
If SPU is 1, the DS2482-100 treats the rising edge of the time slot in which the strong pullup starts as if the active
pullup was activated. However, in contrast to the active pullup, the strong pullup, i.e., the internal pullup transistor,
remains conducting, as shown in Figure 4, until one of three events occurs: the DS2482-100 receives a command
that generates 1-Wire communication (the typical case); the SPU bit in the Configuration register is written to 0; or
the DS2482-100 receives the Device Reset command. As long as the strong pullup is active, the PCTLZ output is
low. When the strong pullup ends, the SPU bit is automatically reset to 0. Using the strong pullup feature does not
change the state of the APU bit in the Configuration register.
Figure 4. Low-Impedance Pullup Timing
DS2482 resistive Pullup DS2482 Pulldown
DS2482 strong Pullup
V
cc
0V
Write 1 case
Write 0 case
Next
Time Slot
or 1-Wire
Reset
t
SLOT
Last bit of 1-Wire Write Byte or 1-Wire Single Bit Function
PCTLZ
1-Wire Speed (1WS)
The 1WS bit determines the timing of any 1-Wire communication generated by the DS2482-100. All 1-Wire slave
devices support standard speed (1WS = 0), where the transfer of a single bit (t
SLOT
in Figure 4) is completed within
65µs. Many 1-Wire device can also communicate at a higher data rate, called Overdrive speed. To change from
standard to Overdrive speed, a 1-Wire device needs to receive an Overdrive Skip ROM or Overdrive Match ROM
command, as explained in the device data sheets. The change in speed occurs immediately after the 1-Wire device
has received the speed-changing command code. The DS2482-100 must take part in this speed change to stay
synchronized. This is accomplished by writing to the Configuration register with the 1WS bit being 1 immediately
after the 1-Wire Byte command that changes the speed of a 1-Wire device. Writing to the Configuration register
with the 1WS bit being 0 followed by a 1-Wire Reset command changes the DS2482-100 and any 1-Wire devices
on the active 1-Wire line back to standard speed.