DS2480 Dallas Semiconducotr, DS2480 Datasheet - Page 7

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

ator control flag is to be set. If the flag is set to a 1 (on) the
device translates every byte received in data mode into
a 12–bit sequence on the 1–Wire bus. For details on
how the Search Accelerator works please refer to the
section Search Accelerator Operation. Before activat-
ing the Search Accelerator, one must make sure that the
strong pull–up after every byte is disarmed (see Pulse
Command). The Search Accelerator command does
not generate a command response byte.
Although the Search Accelerator Control command
itself does not generate any 1–Wire activity, it can be
used to select the communication speed on the 1–Wire
bus. The speed selection (if different from the previous
setting, e.g., from a Reset command) will take effect
immediately.
Pulse
The Pulse command serves several functions that are
selected by the contents of bit 1 and bit 4 of the com-
mand code. The main functions are generating a strong
pull–up to 5V and generating 12V programming pulses
for EPROM devices (if the 12V are available at the V
pin). The secondary function of the pulse command is
arming and disarming a strong pull–up after every sub-
sequent byte in data mode. The arm/disarm function is
controlled by bit 1 of the command code. Bit 4 deter-
mines whether the device will generate a strong pull–up
to 5V or a 12V programming pulse. The table below
summarizes these options.
The strong pull–up to 5V is required to program
EEPROM devices or to operate special function
devices that require a higher current for a limited time
after having received a “go and convert” command.
Therefore and because it significantly reduces the
effective data throughput on the 1–Wire bus, the strong
pull–up is disarmed most of the time. Although arming or
disarming is simultaneously possible while generating a
programming pulse, this is not recommended since it is
BIT
4
0
1
0
1
BIT
1
0
0
1
1
strong pull–up to 5V and disarm
12V Programming Pulse and disarm
strong pull–up to 5V and arm
12V Programming Pulse and arm
FUNCTION
PP
likely to destroy the DS2480 if non–EPROM devices are
connected to the 1–Wire bus.
The duration of the strong pull–up or programming
pulse is determined by configuration parameters and
ranges from a few microseconds up to unlimited (see
section Configuration Commands). However, unlimited
duration is not allowed in conjunction with arming the
strong–pull–up after every byte. As long as the DS2480
is in Command Mode the host may terminate a strong
pull–up or programming pulse prematurely at any time
by sending the command code F1h.
The response byte is generated as soon as the strong
pull–up or programming pulse is over (either because
the predefined time has elapsed or due to termination).
The response byte mainly returns the command code
as sent by the host, but the two least significant bits are
undefined.
If the strong pull–up is armed and the device is in Data
Mode, the end of the strong pull–up will be signaled as
code F6h if the most significant bit of the preceding data
byte on the 1–Wire bus was a 1 and 76h otherwise. The
host will see this response byte in addition to the
response on the data byte sent (see also section Wave
Forms later in this document).
SEARCH ACCELERATOR INTRODUCTION
The Search Accelerator is a logic block inside the
DS2480 that allows using the Search ROM function
very efficiently under modern operating systems such
as Windows and Windows 95/NT. Without the DS2480
all 1–Wire port adapters have to involve the computer’s
CPU for every single time slot or pulse to be generated
on the 1–Wire bus.
Under DOS, accessing peripherals such as the UART
or parallel port is very straight forward and therefore
fast. Under Windows the situation is different and it may
take several milliseconds or more to get the first time
slot generated on the 1–Wire bus. Every subsequent
time slot will be generated in much less time, since the
computer simply sends out (“streams”) a long chain of
bytes. This works reasonably well when reading or writ-
ing large blocks of data.
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