MC68HC912BD32 Datasheet, PDF (177/292 Pages) Freescale Semiconductor, Inc – Advance Information

Cyclic
Redundancy
Check c(CRC)
Freescale Semiconductor, Inc.
Byteflight™ Module
Byteflight™ Protocol
Slave:
From sync pulse: TCR2 + TCR3: slot counter becomes 1
From sync pulse: TCR2 + 2*TCR3: slot counter becomes 2
Form sync pulse: TCR2 + 3*TCR3: slot counter becomes 3
When a node wants to transmit e.g. ID=2 and the slot counter reaches
the value 2, the node starts to transmit and stops the slot counter. All
other nodes receive that message in time slot = 2 and also stop their slot
counters as soon as they detect bus activity. When a rising edge (e.g.
end of message) on the bus is detected the wait time is calculated again,
i.e. the slot counter is incremented again after TCR1+TCR3 (last bus
activity was transmitted) or TCR2+TCR3 (last bus activity was received):
Assuming that a message with ID = 2 was received or transmitted in the
time slot = 2 the slot counter is restarted in the following way:
Last bus activity was transmitted:
Waiting from rising edge: TCR1 + TCR3: slot counter is incremented by
one.
TCR3 later: slot counter is incremented again.
Last bus activity was received:
Waiting from rising edge: TCR12+ TCR3: slot counter is incremented by
one.
TCR3 later: slot counter is incremented again.
Every message transmitted onto a Byteflight™ network contains two
CRC bytes for error detection. These two bytes are produced by shifting
the ID, LEN and DATA bytes through a preset series of feedback shift
registers (For more details refer to BMW – Lastenheft SI-Bus). The
15-bit CRC result is appended to the message following the data portion.
The byte CRCH contains the upper 8 bits of the CRC result, the byte
CRCL contains the lower 7 bits, the LSB of CRCL is set to ‘0’.
Any node which receives the message recalculates the CRC value with
the same hardware and compares bytewise the result against the
received CRC value. In the case of a mismatch a CRC Error is detected.
7-sibus
Byteflight™ Module
For More Information On This Product,
Go to: www.freescale.com
MC68HC912BD32 Rev 1.0