DS1925 datasheet(34/46 Pages) MAXIM

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Skip ROM [CCh]

This command can save time in a single-drop bus sys-

tem by allowing the bus master to access the memory

functions without providing the 64-bit ROM code. If more

than one slave is present on the bus and, for example, a

Read command is issued following the Skip ROM com-

mand, data collision occurs on the bus as multiple slaves

transmit simultaneously (open-drain pulldowns produces

a wired-AND result).

Resume [A5h]

The DS1925 needs to be accessed several times before

a mission starts. In a multidrop environment this means

that the 64-bit ROM code after a Match ROM command

has to be repeated for every access. To maximize the

data throughput in a multidrop environment, the Resume

function was implemented. This function checks the sta-

tus of the RC (resume control) bit and, if it is set, directly

transfers control to the memory/control functions, similar

to a Skip ROM command. The only way to set the RC bit

is through successfully executing the Match ROM, Search

ROM, or Overdrive-Match ROM command. Once the RC

bit is set, the device can repeatedly be accessed through

the Resume command function. Accessing another device

on the bus clears the RC bit, preventing two or more

devices from simultaneously responding to the Resume

Command function.

Overdrive-Skip ROM [3Ch]

On a single-drop bus this command can save time by

allowing the bus master to access the memory/control

functions without providing the 64-bit ROM code. Unlike

the normal Skip ROM command, the Overdrive-Skip ROM

sets the DS1925 into overdrive mode (OD = 1). All com-

munication following this command has to occur at over-

drive speed until a reset pulse of minimum 640µs duration

resets all devices on the bus to standard speed (OD = 0).

When issued on a multidrop bus, this command sets all

overdrive-supporting devices into overdrive mode. To

subsequently address a specific overdrive-supporting

device, a reset pulse at overdrive speed has to be issued

followed by a Match ROM or Search ROM command

sequence. This speeds up the time for the search pro-

cess. If more than one slave supporting overdrive is pres-

ent on the bus and the Overdrive-Skip ROM command is

followed by a Read command, data collision occurs on

the bus as multiple slaves transmit simultaneously (open-

drain pulldowns produce a wired-AND result).

Overdrive-Match ROM [69h]

The Overdrive-Match ROM command followed by a

64-bit ROM sequence transmitted at overdrive speed

allows the bus master to address a specific DS1925 on

a multidrop bus and to simultaneously set it in Overdrive

mode. Only the DS1925 that exactly matches the 64-bit

ROM sequence responds to the subsequent memory/

control function command. Slaves already in overdrive

mode from a previous Overdrive-Skip ROM or success-

ful Overdrive-Match ROM command remain in overdrive

mode. All overdrive-capable slaves return to standard

speed at the next reset pulse of minimum 640µs duration.

The Overdrive-Match ROM command can be used with a

single device or multiple devices on the bus.

1-Wire Signaling

The DS1925 requires strict protocols to ensure data integ-

rity. The protocol consists of four types of signaling on one

line: reset sequence with reset pulse and presence pulse,

write-zero, write-one, and read-data. Except for the pres-

ence pulse, the bus master initiates all these signals.

The DS1925 can communicate at two different speeds:

standard speed and overdrive speed. If not explicitly

set into overdrive mode, the DS1925 communicates at

standard speed. While in overdrive mode the fast timing

applies to all waveforms.

To get from idle to active, the voltage on the 1-Wire line

needs to fall from VPUP below the threshold VTL. To get

from active to idle, the voltage needs to rise from VILMAX

past the threshold VTH. The time it takes for the voltage

to make this rise is seen in Figure 12 as “ε” and its dura-

tion depends on the pullup resistor (RPUP) used and the

capacitance of the 1-Wire network attached. The voltage

VILMAX is relevant for the DS1925 when determining a

logical level, not triggering any events.

Figure 12 shows the initialization sequence required to

begin any communication with the DS1925. A reset pulse

followed by a presence pulse indicates the DS1925 is

ready to receive data, given the correct ROM and mem-

ory function command. If the bus master uses slew-rate

control on the falling edge, it must pull down the line for

tRSTL + tF to compensate for the edge. A tRSTL duration

of 640µs or longer exits overdrive mode, returning the

device to standard speed. If the DS1925 is in overdrive

mode and tRSTL is no longer than 80µs, the device

remains in overdrive mode.

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

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