TCAN2451-Q1: TCAN2451-Q1RHB questions

Hi Daniel,

For the questions:

1. The nRST pin on our SBCs are “bi-directional” in the sense that they take inputs and can output signals – however the SBC mode is what actually controls state of nRST pin if it is input or output. In cases of VCC1 fault that result in VCC1 shutting off the SBC will either go into fail-safe mode or sleep mode – in both of those states nRST is off so state of nRST pin after fault detected and mode has transitioned to sleep/FSM. When the device exits sleep/fail-safe mode it will go through restart mode where nRST is an output pin (SBC is actively driving the pin low during restart mode)  and when it enters standby mode and subsequently normal mode the nRST pin will be an input not really an output (it is open collector – we integrate the pull-up resistor internally – so while it is “outputting” high it really is looking for inputs and the SBC is not actively controlling nRST output at that point – the nRST pin is really only a true output during restart mode – it will idle high in normal/standby modes which kind of can be thought of as an output (and many applications use it this way). So the device doesn’t hard restart (via a signal at nRST pin) when VCC1 fault causes SBC to go to sleep mode / FSM (depending on configuration) – nRST is disabled when VCC1 is shut off due to fault. When you recover from the fault that caused device to enter fail-safe or sleep mode and SBC is at least in standby (can also check in Normal mode – but you would most likely check in standby) you can read register 17h (FSM_CONFIG) and if you read bits 3-1 (parameter is FSM_SLP_STAT) it will tell you the reason for entering FSM or sleep if FSM is disabled see below register table to see flags that could be raised in these bits
2. CAN TRX questions:
   1. VCC2 is the SBC’s secondary LDO output  - it can output 5V at up to 200mA with +/-2% accuracy. VCAN is the CAN transceiver voltage input for the integrated CAN transceiver – for CAN to be accessible you need a proper voltage input on VCAN (5V). In most applications VCC2 is connected to VCAN as VCAN shouldn’t draw more than 110mA during a fault condition and VCC2 can source 200mA. You do need a 100nF cap on VCAN input and I’d suggest a 4.7uF output cap on VCC2 (you can go lower – but I would strongly recommend the 4.7uF or at very least a 2.2uF). Basically VCC2 is a power output and VCAN is a power input.
   2. And c : VCC2 faults basically only impact VCC2 itself – VCC2 faults lead to VCC2 shutting off. VCAN faults lead to CAN shutting off. So if you have an issue on VCC2 but VCC2 is not used for VCAN the only part of the SBC that is compromised is VCC2 – however if you have VCC2 connected to VCAN an issue on VCC2 will also result in an issue on VCAN. So essentially they are independent of each other (except thermal sensor as VCC2 and CAN share thermal sensor – so TSD on VCC2 will impact CAN and vice versa) – but in most applications they are connected so an issue on one would result in an issue on both. However – the concern wouldn’t be the VCAN pin because as long as you have the proper voltage input there it should be fine (i.e. I have never seen an issue on the VCAN pin that was a VCAN direct issue – i.e. the source supplying VCAN is usually where the issue would occur – but for standard VCC2 to VCAN applications I have never seen an issue with the direct connection as long as there is proper capacitance.
3. For the documents, I believe we have sent some material to the field team (Victor and Beni), you are working with, so check with them first and let me know if they don't have it. 

4. It is Functional Safety: Quality-Managed rated – so no specific ASIL letter rating on this device. It can be used in ASIL-B systems but that requires additional components to act as redundancies – what exactly those additional components are will vary with specific application implementation – but basically it is adding additional voltage supervision to the board.

Regards,

Matt