Microchip Technology MCP2515DM-PTPLS Data Sheet
MCP2515
DS21801G-page 8
2003-2012 Microchip Technology Inc.
2.4.1
ACTIVE ERRORS
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.
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.
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.
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.
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.
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.
allows the bus nodes to restart bus communications
cleanly after an error has occurred.
2.5
Overload Frame
An overload frame, shown in
, 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
differentiated 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:
however, can only be generated during an interframe
space. In this way, an overload frame can be
differentiated 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.
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.
2.
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.
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.
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.
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.
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.
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.
Note:
Case 2 should never occur with the
MCP2515 due to very short internal
delays.
MCP2515 due to very short internal
delays.