Strong Pullup (SPU)

The SPU bit is used to activate the strong pullup func-

tion 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 computa-

tion and with parasitically powered temperature sen-

sors or A/D converters. The respective device data

sheets specify the location in the communications pro-

tocol 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 con-

trol an external p-channel MOSFET to supply additional

power beyond the drive capability of the DS2482-100 to

the 1-Wire line. See 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 3, 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 automati-

cally reset to 0. Using the strong pullup feature does

not change the state of the APU bit in the Configuration

Register. Note: Strong pullup also affects the 1-Wire

Reset command. If enabled, it can cause incorrect

reading of the presence pulse and may cause a viola-

tion of the device’s absolute maximum rating.

1-Wire Speed (1WS)

The 1WS bit determines the timing of any 1-Wire com-

munication generated by the DS2482-100. All 1-Wire

slave devices support standard speed (1WS = 0),

completed within 65µs. Many 1-Wire devices 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 1-Wire 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 as 1

imme-

diately after the 1-Wire Byte command that changes the

speed of a 1-Wire device. Writing to the Configuration

Register with the 1WS bit as 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 stan-

dard speed.

DS2482-100 RESISTIVE PULLUP

DS2482-100 PULLDOWN

DS2482-100 STRONG PULLUP

0V

PCTLZ

WRITE-ZERO CASE

WRITE-ONE CASE

LAST BIT OF 1-Wire WRITE BYTE OR 1-Wire SINGLE BIT FUNCTION

NEXT

TIME SLOT

OR 1-Wire

RESET

Figure 3. Low-Impedance Pullup Timing

Single-Channel 1-Wire Master

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