DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

caution required after a short or loss of electrical contact or when hot-swapping slave devices.
Electrical shorts typically occur when an iButton is placed on an iButton probe and the data
contact of the iButton bridges the probe’s DATA and GND contacts. The duration of such a
short can vary from a few milliseconds up to a second or even more. Although a short does not
damage 1 – Wire® devices on the bus, it causes a communication bus reset which, in turn, resets
the 1 – Wire® data rate to standard speed. If the bus master were using overdrive speed and was
unaware of the short, subsequent communication attempts at overdrive speed would fail. From
an iButton’s perspective, loss of contact is equivalent to a short; in either case there is no
voltage between DATA and GND. Consequently, the reset to standard speed also occurs if the
connection between the iButton and probe is lost [71].
Table 4.4 Time Savings for a Read ROM Function [71].
Step Time Slots STD Speed Time OD Speed Time
1 – Wire® Reset/Presence Detect Cycle at
Standard Speed
N/A 960µs 960µs
Command Overdrive Skip ROM 8 N/A 8 x 65µs
1 – Wire® Reset/Presence Detect Cycle at
N/A N/A 96µs
Overdrive Speed
Command Read ROM 8 8 x 65µs 8 x 8µs
Read Device Identification Number 64 64 x 65µs 64 x 8µs
Total Time 5640µs 2152µs
From the above discussion with overdrive and standard speeds it is easy to see that there
is a savings factor greater than 2.5 when using overdrive speed. The main disadvantages are:
loss of bus power forcing a reset and loss of device contact. Although not really a disadvantage,
using overdrive speed on a 1 – Wire® network does require additional programming to be done
for the 1 – Wire® Master if both standard and overdrive capable devices reside on the same
branch of a network. Ideally, if both speeds are desired on the same network, it is recommended
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