1024-Bit EEPROM iButton

16

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Resume [A5h]

To maximize the data throughput in a multidrop envi-

ronment, the Resume command is available. This com-

mand checks the status of the RC bit and, if it is set,

directly transfers control to the memory function com-

mands, 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 com-

mand. Accessing another device on the bus clears the

RC bit, preventing two or more devices from simultane-

ously responding to the Resume command.

Overdrive-Skip ROM [3Ch]

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

allowing the bus master to access the memory func-

tions without providing the 64-bit ROM code. Unlike the

normal Skip ROM command, the Overdrive-Skip ROM

command sets the DS1972 into the overdrive mode

(OD = 1). All communication following this command

must occur at overdrive speed until a reset pulse of

minimum 480µs 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 must be

issued followed by a Match ROM or Search ROM com-

mand sequence. This speeds up the time for the

search process. If more than one slave supporting

overdrive is present 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 DS1972 on

a multidrop bus and to simultaneously set it in overdrive

mode. Only the DS1972 that exactly matches the 64-bit

ROM sequence responds to the subsequent memory

function command. Slaves already in overdrive mode

from a previous Overdrive-Skip ROM or successful

Overdrive-Match ROM command remain in overdrive

mode. All overdrive-capable slaves return to standard

speed at the next reset pulse of minimum 480µs dura-

tion. The Overdrive-Match ROM command can be used

with a single device or multiple devices on the bus.

1-Wire Signaling

The DS1972 requires strict protocols to ensure data

integrity. The protocol consists of four types of signal-

ing on one line: reset sequence with reset pulse and

presence pulse, write-zero, write-one, and read-data.

Except for the presence pulse, the bus master initiates

all falling edges. The DS1972 can communicate at two

different speeds: standard speed and overdrive speed.

If not explicitly set into the overdrive mode, the DS1972

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 10 as

ε, and

its duration depends on the pullup resistor (RPUP) used

and the capacitance of the 1-Wire network attached.

The voltage VILMAX is relevant for the DS1972 when

determining a logical level, not triggering any events.

Figure 10 shows the initialization sequence required to

begin any communication with the DS1972. A reset

pulse followed by a presence pulse indicates that the

DS1972 is ready to receive data, given the correct

ROM and memory 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 480µs or longer exits the

overdrive mode, returning the device to standard

speed. If the DS1972 is in overdrive mode and tRSTL is

no longer than 80µs, the device remains in overdrive

mode. If the device is in overdrive mode and tRSTL is

between 80µs and 480µs, the device resets, but the

communication speed is undetermined.

After the bus master has released the line it goes into

receive mode. Now the 1-Wire bus is pulled to VPUP

through the pullup resistor or, in the case of a DS2482-

x00 or DS2480B driver, through the active circuitry.

When the threshold VTH is crossed, the DS1972 waits

for tPDH and then transmits a presence pulse by pulling

the line low for tPDL. To detect a presence pulse, the

master must test the logical state of the 1-Wire line at

tMSP.

The tRSTH window must be at least the sum of

tPDHMAX, tPDLMAX, and tRECMIN. Immediately after

tRSTH is expired, the DS1972 is ready for data commu-

nication. In a mixed population network, tRSTH should

be extended to minimum 480µs at standard speed and

48µs at overdrive speed to accommodate other 1-Wire

devices.