TJA1043 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2017. All rights reserved.
Product data sheet Rev. 6 — 10 November 2017 11 of 32
NXP Semiconductors
TJA1043
High-speed CAN transceiver
7.2.7 Bus failure flag
The Bus failure flag is set if the transceiver detects a bus line short-circuit condition to
V
BAT
, V
CC
or GND during four consecutive dominant-recessive cycles on pin TXD, while
trying to drive the bus lines dominant. The Bus failure flag can be polled via the ERR_N
pin in Normal mode (see Table 5
). This flag is cleared at power-on or when the transceiver
re-enters Normal mode.
7.2.8 Local failure flag
In Normal and Listen-only modes, the transceiver can distinguish four different local
failure events, any of which will cause the Local failure flag to be set. The four local failure
events are: TXD dominant clamping, TXD-to-RXD short circuit, bus dominant clamping
and an overtemperature event. The nature and detection of these local failures is
described in Section 7.3
. The Local failure flag can be polled via the ERR_N pin in
Listen-only mode (see Table 5
). This flag is cleared at power-on, when entering Normal
mode or when RXD is dominant while TXD is recessive, provided that all local failures
have been resolved.
7.3 Local failures
The TJA1043 can detect four different local failure conditions. Any of these failures will set
the Local failure flag, and in most cases the transmitter of the transceiver will be disabled.
7.3.1 TXD dominant time-out function
A permanent LOW level on pin TXD (due to a hardware or software application failure)
would drive the CAN bus into a permanent dominant state, blocking all network
communications. The TXD dominant time-out function prevents such a network lock-up by
disabling the transmitter if pin TXD remains LOW for longer than the TXD dominant
time-out time t
to(dom)TXD
. The t
to(dom)TXD
timer defines the minimum possible bit rate of
40 kbit/s. The transmitter remains disabled until the Local failure flag has been cleared.
7.3.2 TXD-to-RXD short-circuit detection
A short-circuit between pins RXD and TXD would lock the bus in a permanent dominant
state once it had been driven dominant, because the low-side driver of RXD is typically
stronger than the high-side driver of the controller connected to TXD. TXD-to-RXD
short-circuit detection prevents such a network lock-up by disabling the transmitter. The
transmitter remains disabled until the Local failure flag has been cleared.
7.3.3 Bus dominant time-out function
A CAN bus short circuit (to V
BAT
, V
CC
or GND) or a failure in one of the other network
nodes could result in a differential voltage on the bus high enough to represent a bus
dominant state. Because a node will not start transmission if the bus is dominant, the
normal bus failure detection will not detect this failure, but the bus dominant clamping
detection will. The Local failure flag is set if the dominant state on the bus persists for
longer than t
to(dom)bus
. By checking this flag, the controller can determine if a clamped bus
is blocking network communications. There is no need to disable the transmitter. Note that
the Local failure flag does not retain a bus dominant clamping failure, and is released as
soon as the bus returns to recessive state.
