RXD

(receiver)

TXD

(driver)

CAN

Bus

Signal

Normal CAN

communication

TXD fault stuck dominant

Example: PCB failure or bad software

Driver disabled freeing bus for other nodes

Bus would be stuck dominant, blocking communication for the

whole network but TXD DTO prevents this and frees the bus for

communication after the tTXD_DTO time.

tTXD_DTO

tTXD_DTO

Fault is repaired and transmission

capability is restored

Communication from

other bus nodes

Communication from

repaired nodes

Communication from

other bus nodes

Communication from

repaired nodes

Communication from

local node

Figure 8-3. Example Timing Diagram for TXD DTO

Note

The minimum dominant TXD time (tTXD_DTO) allowed by the TXD DTO circuit limits the minimum

possible transmitted data rate of the device. The CAN protocol allows a maximum of eleven

successive dominant bits (on TXD) for the worst case, where five successive dominant bits are

followed immediately by an error frame. This, along with the tTXD_DTO minimum, limits the minimum

data rate. Calculate the minimum transmitted data rate with Equation 1.

Minimum Data Rate = 11 / tTXD_DTO

(1)

8.3.3.2 Thermal Shutdown (TSD)

If the junction temperature of the device exceeds the thermal shutdown threshold (TTSD), the device turns off the

CAN driver circuits, blocking the TXD-to-bus transmission path. The CAN bus terminals are biased to the

recessive level during a thermal shutdown, and the receiver-to-RXD path remains operational. The shutdown

condition is cleared when the junction temperature drops at least the thermal shutdown hysteresis temperature

(TTSD_HYST) below the thermal shutdown temperature (TTSD) of the device.

8.3.3.3 Undervoltage Lockout and Default State

The supply pins have undervoltage detection that places the device in protected or default mode which protects

the bus during an undervoltage event on the VCC1 or VCC2 supply pins. If the bus-side power supply, VCC2, is

less than about 4 V, the power shutdown circuits in the ISO1044B device disable the transceiver to prevent false

transmissions because of an unstable supply. If the VCC1 supply is still active when this occurs, the receiver

output (RXD) goes to a default HIGH (recessive) value. Table 8-1 summarizes the undervoltage lockout and fail-

safe behavior.

Table 8-1. Undervoltage Lockout and Default State

VCC1

VCC2

DEVICE STATE

BUS OUTPUT

RXD

> UVVCC1

> UVVCC2

Functional

Per Device State and TXD

Mirrors Bus

<UVVCC1

> UVVCC2

Protected

Recessive

Undetermined

>UVVCC1

< UVVCC2

Protected

High Impedance

Recessive (Default High)

ISO1044

SLLSFB0A – MARCH 2020 – REVISED JULY 2020

www.ti.com

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