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DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
Master-to-Slave
For a write-one time slot, the voltage on the data line
must have crossed the V TH threshold before the write-one
low time t W1LMAX is expired. For a write-zero time slot,
the voltage on the data line must stay below the V TH
threshold until the write-zero low time t W0LMIN is expired.
The voltage on the data line should not exceed V ILMAX
during the entire t W0L or t W1L window. After the V TH
threshold has been crossed, the DS1922L/DS1922T need
a recovery time t REC before they are ready for the next
time slot.
Slave-to-Master
A read-data time slot begins like a write-one time slot.
The voltage on the data line must remain below V TL
until the read low time t RL is expired. During the t RL
window, when responding with a 0, the DS1922L/
DS1922T start pulling the data line low; their internal
timing generator determines when this pulldown ends
and the voltage starts rising again. When responding
with a 1, the DS1922L/DS1922T do not hold the data
line low at all, and the voltage starts rising as soon as
t RL is over.
The sum of t RL + δ (rise time) on one side and the inter-
nal timing generator of the DS1922L/DS1922T on the
other side define the master sampling window (t MSRMIN
to t MSRMAX ) in which the master must perform a read
from the data line. For most reliable communication, t RL
should be as short as permissible and the master
should read close to but no later than t MSRMAX . After
reading from the data line, the master must wait until
t SLOT is expired. This guarantees sufficient recovery
time t REC for the DS1922L/DS1922T to get ready for the
next time slot.
Improved Network Behavior
(Switchpoint Hysteresis)
In a 1-Wire environment line termination is possible
only during transients controlled by the bus master
(1-Wire driver). 1-Wire networks, therefore, are suscep-
tible to noise of various origins. Depending on the
physical size and topology of the network, reflections
from end points and branch points can add up or can-
cel each other to some extent. Such reflections are vis-
ible as glitches or ringing on the 1-Wire communication
42
line. Noise coupled onto the 1-Wire line from external
sources can also result in signal glitching. A glitch dur-
ing the rising edge of a time slot can cause a slave
device to lose synchronization with the master and, as
a consequence, result in a search ROM command
coming to a dead end or cause a device-specific func-
tion command to abort. For better performance in net-
work applications, the DS1922L/DS1922T use a new
1-Wire front-end, which makes them less sensitive to
noise and also reduces the magnitude of noise inject-
ed by the slave device itself.
The DS1922L/DS1922T’s 1-Wire front-end differs from
traditional slave devices in four characteristics:
1) The falling edge of the presence pulse has a con-
trolled slew rate. This provides a better match to the
line impedance than a digitally switched transistor,
converting the high-frequency ringing known from
traditional devices into a smoother low-bandwidth
transition. The slew-rate control is specified by the
parameter t FPD , which has different values for stan-
dard and overdrive speed.
2) There is additional lowpass filtering in the circuit that
detects the falling edge at the beginning of a time
slot. This reduces the sensitivity to high-frequency
noise. This additional filtering does not apply at over-
drive speed.
3) There is a hysteresis at the low-to-high switching
threshold V TH . If a negative glitch crosses V TH but
does not go below V TH - V HY , it is not recognized
(Figure 14, Case A). The hysteresis is effective at
any 1-Wire speed.
4) There is a time window specified by the rising edge
hold-off time t REH during which glitches are ignored,
even if they extend below V TH - V HY threshold
(Figure 14, Case B, t GL < t REH ). Deep voltage
droops or glitches that appear late after crossing the
V TH threshold and extend beyond the t REH window
cannot be filtered out and are taken as the begin-
ning of a new time slot (Figure 14, Case C, t GL ≥
t REH ).
Devices that have the parameters t FPD , V HY , and t REH
specified in their electrical characteristics use the
improved 1-Wire front-end.
Maxim Integrated
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