33889 Datasheet, PDF (33/60 Pages) Freescale Semiconductor, Inc – System Basis Chip with Low Speed Fault Tolerant CAN Interface

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
(No open wire, or open wire recovery)
Rec
Dom
Rec
CANL
(CAN H open wire)
Rec Dom
Rec
CANL
CANH
CANH
-3.2V
Diff
Sampling point
-3.2V
Diff
Sampling point
S-L
S-L
S-H
S-H
1us
Sampling dominant level
= > no failure or “recovery pulse”
Sampling recessive level
= > open wire “detection pulse”
Figure 13. CAN Normal Signal Communication and CAN Open Wire
S-H 1us Sampling
CANH
Diff
CANL
Dom Rec
S-L
1us Sampling
CANH counter
L-counter +/-
(count = 4) (count = 0)
L-open
recover
Figure 14. Open Wire Detection Principle
Open wire detection, MC33889B and D:
Failure detection:
The device will detect a difference in toggling counts
between the differential receiver and one of the single ended
receivers. Every time a difference in count is detected a
counter is incremented. When the counter reaches 4, the
device detects and reports an open wire condition. The open
wire detection is performed only when the device
receives a message and not when it send message.
Open wire recovery:
When the open wire failure has recovered, the difference
in count is reduced and the device detects the open wire
recovery.
MC33889B:
When detection is complete, the counter is no longer
incremented. It can only be decremented by sampling of the
dominant level on the S-H (S-L) (recovery pulse). When it
reaches zero, the failure has recovered.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33889
33