MCP2515
2.4.1
ACTIVE ERRORS
2.5
Overload Frame
If an error-active node detects a bus error, the node
interrupts transmission of the current message by
generating an active error flag. The active error flag is
composed of six consecutive dominant bits. This bit
sequence actively violates the bit-stuffing rule. All other
stations recognize the resulting bit-stuffing error and, in
turn, generate error frames themselves, called error
echo flags.
The error flag field, therefore, consists of between six
and twelve consecutive dominant bits (generated by
one or more nodes). The error delimiter field (eight
recessive bits) completes the error frame. Upon
completion of the error frame, bus activity returns to
normal and the interrupted node attempts to resend the
aborted message.
Note:
Error echo flags typically occur when a
localized disturbance causes one or more
(but not all) nodes to send an error flag.
The remaining nodes generate error flags
in response (echo) to the original error
flag.
An overload frame, shown in Figure 2-5, has the same
format as an active error frame. An overload frame,
however, can only be generated during an interframe
space. In this way, an overload frame can be differen-
tiated from an error frame (an error frame is sent during
the transmission of a message). The overload frame
consists of two fields: an overload flag followed by an
overload delimiter. The overload flag consists of six
dominant bits followed by overload flags generated by
other nodes (and, as for an active error flag, giving a
maximum of twelve dominant bits). The overload
delimiter consists of eight recessive bits. An overload
frame can be generated by a node as a result of two
conditions:
1.
The node detects a dominant bit during the
interframe space, an illegal condition.
Exception:
The dominant bit is detected during
the third bit of IFS. In this case, the receivers will
interpret this as a SOF.
Due to internal conditions, the node is not yet
able to begin reception of the next message. A
node may generate a maximum of two
sequential overload frames to delay the start of
the next message.
Note:
Case 2 should never occur with the
MCP2515 due to very short internal
delays.
2.
2.4.2
PASSIVE ERRORS
If an error-passive node detects a bus error, the node
transmits an error-passive flag followed by the error
delimiter field. The error-passive flag consists of six
consecutive recessive bits. The error frame for an error-
passive node consists of 14 recessive bits. From this it
follows that, unless the bus error is detected by an error-
active node or the transmitting node, the message will
continue transmission because the error-passive flag
does not interfere with the bus.
If the transmitting node generates an error-passive flag,
it will cause other nodes to generate error frames due
to the resulting bit-stuffing violation. After transmission
of an error frame, an error-passive node must wait for
six consecutive recessive bits on the bus before
attempting to rejoin bus communications.
The error delimiter consists of eight recessive bits and
allows the bus nodes to restart bus communications
cleanly after an error has occurred.
2.6
Interframe Space
The interframe space separates a preceding frame (of
any type) from a subsequent data or remote frame.
The interframe space is composed of at least three
recessive bits called the Intermission. This allows
nodes time for internal processing before the start of
the next message frame. After the intermission, the
bus line remains in the recessive state (bus idle) until
the next transmission starts.
DS21801D-page 8
Preliminary
©
2005 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
