DS1921H/Z Maxim Integrated Products, DS1921H/Z Datasheet - Page 27

• Manufacturer Part Number: DS1921H/Z
• Description: High Resolution Thermochron Ibutton Range H
• Manufacturer: Maxim Integrated Products

DS1921H/Z
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 sets the DS1921H/Z in the Overdrive mode (OD = 1). All communication following this
command has to occur at Overdrive speed until a reset pulse of minimum 480µs duration resets all de-
vices on the bus to standard speed (OD = 0).
When issued on a multidrop bus this command will set 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 will speed 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 will occur on the bus as
multiple slaves transmit simultaneously (open-drain pull-downs will 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 DS1921H/Z on a multidrop bus and to simultaneously
set it in Overdrive mode. Only the DS1921H/Z that exactly matches the 64-bit ROM sequence will
respond to the subsequent memory function command. Slaves already in Overdrive mode from a previous
Overdrive Skip or Match command will remain in Overdrive mode. All overdrive-capable slaves will
return to standard speed at the next Reset Pulse of minimum 480µs duration. The Overdrive Match ROM
command can be used with a single or multiple devices on the bus.
1-WIRE SIGNALING
The DS1921H/Z requires strict protocols to ensure data integrity. The protocol consists of four types of
signaling on one line: Reset Sequence with Reset Pulse and Presence Pulse, Write 0, Write 1, and Read
Data. Except for the presence pulse the bus master initiates all these signals. The DS1921H/Z can
communicate at two different speeds: standard speed and Overdrive speed. If not explicitly set into the
Overdrive mode, the DS1921H/Z will communicate 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 V
below the threshold V
.
PUP
TL
To get from active to idle, the voltage needs to rise from V
past the threshold V
. The voltage
ILMAX
TH
V
is relevant for the DS1921H/Z when determining a logical level, but not for triggering any events.
ILMAX
The initialization sequence required to begin any communication with the DS1921H/Z is shown in Figure
13. A Reset Pulse followed by a Presence Pulse indicates the DS1921H/Z is ready to receive data, given
the correct ROM and memory function command. In a mixed population network, the reset low time t
RSTL
needs to be long enough for the slowest 1-Wire slave device to recognize it as a reset pulse. With the
DS1921H/Z this duration is 480µs at standard speed and 62µs at Overdrive speed. If the bus master uses
slew-rate control on the falling edge, it must pull down the line for t
+ t
to compensate for the edge.
RSTL
F
A t
duration of 480µs or longer will exit the Overdrive mode returning the device to standard speed.
RSTL
If the DS1921H/Z is in Overdrive mode and t
is no longer than 80µs, the device will remain in
RSTL
Overdrive mode.
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