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

est-case latency occurs when the bus is operated at it maximum speed of 1 Mbps and each data
frame contains no data. Again for this case it is assumed that the values for queuing jitter (Jm)
and worst-case queuing delay (wm) are zero. Therefore operating the bus at the highest speed
possible and keeping the amount of data in each data frame as small as possible would allow
minimal latency times on the CAN bus.
In many iButton and 1 – Wire® applications, cable length and loading often preclude
the use of the overdrive mode. Cable length and loading are the two biggest factors that
contribute to message latency on 1 – Wire® networks. In those applications where the
embedded electronics are in close proximity to the slave devices, the high-speed overdrive
communication mode can greatly reduce transaction times [71]. Table 4.4 shows a comparison
of typical 1 – Wire® command transactions using both standard and overdrive speeds.
There are two ways to get an overdrive-capable slave device into overdrive mode. One
way uses the Overdrive Skip command, which puts all devices on the bus into overdrive. The
other way uses the Overdrive Match command, which only puts the device with the matching
identification number into overdrive. The device remains in overdrive mode until it gets a 1 –
Wire® reset at standard speed, or it is disconnected from the bus. Any 1 – Wire® device that is
either hot swapped (plug-and-play) or is already on the bus at power always starts at standard
speed [71].
The example in Table 4.4 shows that despite the extra command and reset cycle of
getting to overdrive speed, using overdrive obtains the device identification number in 2152µs
compared to 5640µs at standard speed. The more communication that occurs at overdrive, the
more substantial is the time savings. The downside to using overdrive speed is the additional
102