Rainbow Electronics CAN интерфейс - Manual (Page 33)

BOSCH
ROBERT BOSCH GmbH, Postfach 50, D-7000 Stuttgart 1
Sep. 1991
Part A - page 31
9 INCREASING CAN OSCILLATOR TOLERANCE
This section describes an upwards compatible modification of the CAN protocol, as
specified in sections 1 to 8.
9.1 Protocol Modifications
In order to increase the maximum oscillator tolerance from the 0.5% currently possible
to 1.5%, the following modifications, which are upwards compatible to the existing CAN
specification, are necessary:
[1] If a CAN node samples a dominant bit at the third bit of INTERMISSION, then it will
interpret this bit as a START OF FRAME bit.
[2] If a CAN node has a message waiting for transmission and it samples a dominant
bit at the third bit of INTERMISSION, it will interpret this as a START OF FRAME
bit, and, with the next bit, start transmitting its message with the first bit of the
IDENTIFIER without first transmitting a START OF FRAME bit and without
becoming a receiver.
[3] If a CAN node samples a dominant bit at the eighth bit (the last bit) of an ERROR
DELIMITER or OVERLOAD DELIMITER, it will, at the next bit, start transmitting an
OVERLOAD FRAME (not an ERROR FRAME). The Error Counters will not be
incremented.
[4] Only recessive to dominant edges will be used for synchronization.
In agreement with the existing specification, the following rules are still valid.
[5] All CAN controllers synchronize on the START OF FRAME bit with a hard
synchronization.
[6] No CAN controller will send a START OF FRAME bit until it has counted three
recessive bits of INTERMISSION.
This modifications allow a maximum oscillator tolerance of 1.58% and the use of a
ceramic resonator at a bus speed of up to 125 Kbits/second. For the full bus speed
range of the CAN protocol, still a quartz oscillator is required. The compatibility of the
enhanced and the existing protocol is maintained, as long as:
[7] CAN controllers with the enhanced and existing protocols, used in one and the
same network, have all to be provided with a quartz oscillator.
The chip with the highest requirement for its oscillator accuracy determines the
oscillator accuracy which is required from all the other nodes. Ceramic resonators can
only be used when all the nodes in the network use the enhanced protocol.
Oscillator Tolerance