Hardware Reference
In-Depth Information
After a transmission, an error-passive node will wait before initiating further transmission.
A bus-off node is not allowed to have any influence on the bus.
The CAN protocol requires each node to implement
transmit error count
and
receive error
count
to facilitate fault confinement. These two counts are updated according to 12 rules.
These 12 rules can be found in the CAN specification. An error count value greater than
roughly 96 indicates a heavily disturbed bus. It may be advantageous to provide the means to
test for this condition. If during system start-up only one node is online and if this node trans-
mits some message, it will get no acknowledgement, detect an error, and repeat the message.
It can become error passive but not bus off for this reason.
The setting of a bit time in a CAN system must allow a bit sent out by the transmitter to
reach the far end of the CAN bus and allow the receiver to send back an acknowledgement that
reaches the transmitter. In a CAN environment, the
nominal bit rate
is defined to be the num-
ber of bits transmitted per second in the absence of resynchronization by an ideal transmitter.
The inverse of the nominal bit rate is the
nominal bit time
. A nominal bit time can be di-
vided into four nonoverlapping time segments, as shown in Figure 13.12.
Nominal bit time
sync_seg
prop_seg
phase_seg1
phase_seg2
Sample point
Figure 13.12
■
Nominal bit time
The
sync_seg
segment is used to synchronize the various nodes on the bus. An edge is
expected to lie within this segment. The
prop_seg
segment is used to compensate for the phys-
ical delay times within the network. It is twice the sum of the signal's propagation time on
the bus line, the input comparator delay, and the output driver delay. The
phase_seg1
and
phase_
seg2
segments are used to compensate for edge phase errors. These segments can be lengthened
or shortened by synchronization. The
sample point
is the point in time at which the bus level
is read and interpreted as the value of that respective bit. The sample point is located at the
end of phase_seg1. A CAN controller may implement the three-samples-per-bit option in which
the majority function is used to determine the bit value. Each sample is separated from the next
sample by half a time quanta (CAN clock cycle). The
information processing time
is the time
segment starting with the sample point reserved for calculation of the sample bit(s) level. The
segments contained in a nominal bit time are represented in the unit of
time quantum
. The time
quantum (
t
Q
) is a fixed unit of time that can be derived from the oscillator period (T
OSC
).
t
Q
is
Search WWH ::
Custom Search