Rainbow Electronics DS1972 - Manual (Page 20)

DS1972: 1024-Bit EEPROM iButton
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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 DS1972 starts pulling the data
line low; its internal timing generator determines when this pulldown ends and the voltage starts rising again. When
responding with a 1, the DS1972 does 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
+ d (rise time) on one side and the internal timing generator of the DS1972 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 the
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 DS1972 to get ready for the next time slot. Note that t
REC
specified herein
applies only to a single DS1972 attached to a 1-Wire line. For multidevice configurations, t
REC
needs to be
extended to accommodate the additional 1-Wire device input capacitance. Alternatively, an interface that performs
active pullup during the 1-Wire recovery time such as the DS2482-x00 or DS2480B 1-Wire line drivers can be
used.
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 susceptible 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 cancel each other to some
extent. Such reflections are visible as glitches or ringing on the 1-Wire communication line. Noise coupled onto the
1-Wire line from external sources can also result in signal glitching. A glitch during the rising edge of a time slot can
cause a slave device to lose synchronization with the master and, consequently, result in a Search ROM command
coming to a dead end or cause a device-specific function command to abort. For better performance in network
applications, the DS1972 uses a new 1-Wire front end, which makes it less sensitive to noise.
The 1-Wire front end of the DS1972 differs from traditional slave devices in three characteristics.
1) There is additional low-pass 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 Overdrive speed.
2) 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 will not be recognized (Figure 12, Case A). The hysteresis is effective at any 1-Wire speed.
3) 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 12, 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 beginning of a new time slot (Figure 12, Case C, t
GL
³ t
REH
).
Devices that have the parameters V
HY
, and t
REH
specified in their electrical characteristics use the improved 1-Wire
front end.
Figure 12. Noise Suppression Scheme
V
PUP
V
TH
V
HY
0V
t
REH
t
GL
t
REH
t
GL
Case A Case C Case B