These CAN transceivers meet the ISO11898-2 (2016) High Speed CAN (Controller Area Network) physical layer standard. They are designed for data rates in excess of 1 Mbps for CAN FD and enhanced timing margin / higher data rates in long and highly-loaded networks. These devices provide many protection features to enhance device and CAN robustness.
During normal mode (the only mode where the CAN driver is active), the TXD DTO circuit prevents the transceiver from blocking network communication in the event of a hardware or software failure where TXD is held dominant longer than the timeout period tTXD_DTO. The DTO circuit timer starts on a falling edge on TXD. The DTO circuit disables the CAN bus driver if no rising edge is seen before the timeout period expires. This frees the bus for communication between other nodes on the network. The CAN driver is re-activated when a recessive signal is seen on the TXD terminal, thus clearing the TXD DTO condition. The receiver and RXD terminal still reflect activity on the CAN bus, and the bus terminals are biased to the recessive level during a TXD dominant timeout.
The minimum dominant TXD time 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 by: Minimum Data Rate = 11 / tTXD_DTO.
If the junction temperature of the device exceeds the thermal shutdown threshold (TTSD), the device turns off the CAN driver circuits thus 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_HYS) below the thermal shutdown temperature (TTSD) of the device.
|VCC||DEVICE STATE||BUS OUTPUT||RXD|
|> UVVCC||Normal||Per TXD||Mirrors Bus(2)|
|< UVVCC||Protected||High Impedance||High Impedance|
|VCC||VIO||DEVICE STATE||BUS OUTPUT||RXD|
|> UVVCC||> UVVIO||Normal||Per TXD||Mirrors Bus(1)|
|< UVVCC||> UVVIO||STB = High: Standby Mode||Recessive||Bus Wake RXD Request(2)|
|STB =Low: Protected Mode||High Impedance||High (Recessive)|
|> UVVCC||< UVVIO||Protected||High Impedance||High Impedance|
|< UVVCC||< UVVIO||Protected||High Impedance||High Impedance|
After an undervoltage condition is cleared and the supplies have returned to valid levels, the device typically resumes normal operation within 50 µs.
The device is designed to be 'ideal passive' or 'no load' to the CAN bus if it is unpowered. The bus terminals (CANH, CANL) have extremely low leakage currents when the device is unpowered to avoid loading down the bus. This is critical if some nodes of the network are unpowered while the rest of the of network remains in operation. The logic terminals also have extremely low leakage currents when the device is unpowered to avoid loading down other circuits that may remain powered.
These devices have internal pull ups on critical terminals to place the device into known states if the terminals float. The TXD terminal is pulled up to VCC or VIO to force a recessive input level if the terminal floats. The terminal is also pulled up to force the device into low power Standby mode if the terminal floats.
The device has two protection features that limit the short circuit current when a CAN bus line is short-circuit fault condition: driver current limiting (both dominant and recessive states) and TXD dominant state time out to prevent permanent higher short circuit current of the dominant state during a system fault. During CAN communication the bus switches between dominant and recessive states, thus the short circuit current may be viewed either as the instantaneous current during each bus state or as an average current of the two states. For system current (power supply) and power considerations in the termination resistors and common-mode choke ratings, use the average short circuit current. Determine the ratio of dominant and recessive bits by the data in the CAN frame plus the following factors of the protocol and PHY that force either recessive or dominant at certain times:
Consider the short circuit current and possible fault cases of the network when sizing the power ratings of the termination resistance and other network components.
The 5-V VCC only devices are supplied by a single 5-V rail. The digital inputs have TTL input thresholds and are therefore 5 V and 3.3 V compatible. The RXD outputs on these devices are driven to the VCC rail for logic high output. Additionally, the TXD and STB pins are internally pulled up to VCC. The internal bias of the mode pins may only place the device into a known state if the terminals float, they may not be adequate for system-level biasing during transients or noisy enviroments.
TXD pull up strength and CAN bit timing require special consideration when these devices are used with CAN controllers with an open-drain TXD output. An adequate external pull up resistor must be used to ensure that the CAN controller output of the micrcontroller maintains adequate bit timing to the TXD input.
These devices use a 5 V VCC power supply for the CAN driver and high speed receiver blocks. These transceivers have a second power supply for I/O level-shifting (VIO). This supply is used to set the CMOS input thresholds of the TXD and pins and the RXD high level output voltage. Additionally, the internal pull ups on TXD and STB are pulled up to VIO.
The device has two main operating modes: Normal mode and Standby mode. Operating mode selection is made via the STB input terminal.
|STB Terminal||MODE||DRIVER||RECEIVER||RXD Terminal|
|LOW||Normal Mode||Enabled (ON)||Enabled (ON)||Mirrors Bus State(1)|
|HIGH||Standby Mode||Disabled (OFF)||Disabled (OFF) (Low Power Bus Monitor is Active)||High (Unless valid WUP has been received)|
The CAN bus has two states during powered operation of the device: dominant and recessive. A dominant bus state is when the bus is driven differentially, corresponding to a logic low on the TXD and RXD terminal. A recessive bus state is when the bus is biased to VCC / 2 via the high-resistance internal input resistors RIN of the receiver, corresponding to a logic high on the TXD and RXD terminals.
Select the Normal mode of device operation by setting STB terminal low. The CAN driver and receiver are fully operational and CAN communication is bi-directional. The driver translates a digital input on TXD to a differential output on CANH and CANL. The receiver translates the differential signal from CANH and CANL to a digital output on RXD.
Activate low power Standby mode by setting STB terminal high. In this mode the bus transmitter will not send data nor will the normal mode receiver accept data as the bus lines are biased to ground minimizing the system supply current. Only the low power receiver will be actively monitoring the bus for activity. RXD indicates a valid wake up event after a wake-up pattern (WUP) has been detected on the Bus. The low power receiver is powered using only the VIO pin. This allows VCC to be removed reducing power consumption further.
The TCAN1042 family offers a remote wake request feature that is used to indicate to the host micrcontroller that the bus is active and the node should return to normal operation.
These devices use the multiple filtered dominant wake up pattern (WUP) from the ISO11898-2 (2016) to qualify bus activity. Once a valid WUP has been received the wake request will be indicated to the micrcontroller by a falling edge and low corresponding to a "filtered" dominant on the RXD output terminal.
The WUP consists of a filtered dominant pulse, followed by a filtered recessive pulse, and finally by a second filtered dominant pulse. These filtered dominant, recessive, dominant pulses do not need to occur in immediate succession. There is no timeout that will occur between filtered bits of the WUP. Once a full WUP has been detected the device will continue to drive the RXD output low every time an additional filtered dominant signal is received from the bus.
For a dominant or recessive signal to be considered "filtered", the bus must continually remain in that state for more than tWK_FILTER. Due to variability in the tWK_FILTER, the following three scenarios can exist:
Once the first filtered dominant signal is received, the device is now waiting on a filtered recessive signal, other bus traffic will not reset the bus monitor. Once the filtered recessive signal is received, the monitor is now waiting on a second filtered dominant signal, and again other bus traffic will not reset the monitor. After reception of the full WUP, the device will transition to driving the RXD output pin low for the remainder of any dominant signal that remains on the bus for longer than tWK_FILTER.
|DEVICE||INPUTS||OUTPUTS||DRIVEN BUS STATE|
|H or Open||Z||Z||Recessive|
|H or Open||X||Z||Z||Recessive|
|DEVICE MODE||CAN DIFFERENTIAL INPUTS
VID = VCANH – VCANL
|BUS STATE||RXD TERMINAL(1)|
|Normal||VID ≥ VIT+(MAX)||Dominant||L(2)|
|VIT-(MIN) < VID < VIT+(MAX)||?||?(2)|
|VID ≤ VIT-(MIN)||Recessive||H(2)|
|Open (VID ≈ 0 V)||Open||H|