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November 18, 2019, 09:19:26 21:19


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Author Topic: Suggest High-Speed CAN transceiver with Disabled and Stand-by modes.  (Read 798 times)
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Signal
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« on: August 21, 2019, 01:14:47 13:14 »

Hi, I need help.

TJA1051T/E transceiver has three modes of operation: Normal, Silent and Disabled. These modes are selected by two pins: EN and S.
In Silent mode transmitter is disabled but receiver works in fully operational mode. That mode I do not need.
In Disabled mode not only transmitter is disabled but also internal load/terminal resistors are disconnected from bus. That feature is what I found unique and what I need!
Unfortunately, in Disabled mode receiver is also completely disabled and there are no Stand-by mode as normally present in most of high speed CAN bus transceivers. In Stand-by mode transceiver detects activity on bus and interrupts/wakes up CAN controller.

So I am searching for high-speed CAN transceiver with Stand-by and Disabled (in TJA1051T/E sense) modes. Does anybody knows it?

As alternative could you suggest IC or simple circuit that could detect differential signal on bus to wake up a node that do not compromise the bus and with similar level of protection from bus?
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bigtoy
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« Reply #1 on: August 21, 2019, 02:31:43 14:31 »

At my work we build a lot of CAN devices, and build fairly large CAN networks for industrial type applications. What I'm not understanding is WHY you want to have a disable pin on the CAN transceiver. In my experience it's not necessary. The reason I say that is that CAN is a wired-OR multidrop network. Which means the transceivers sit across the bus in a quite high-impedance state, and they don't noticeably affect the bus unless they transmit. Hence, if you want to guarantee the transceiver won't transmit, it's a lot easier to just gate the TX pin on the logic bus side of the transceiver, than it is to attempt to find a special transceiver.

You mention load resistors. They only exist on the ends of the bus. At each end of the bus there needs to be a 120 ohm resistor between CAN-H and CAN-L. You do not have load resistors at each transceiver. No matter how many transceivers you have on the bus, when a given transceiver is driving the bus it sees a load of 60 ohms (the two 120-ohms in parallel). One purpose of these two termination resistors is to bring CAN-H and CAN-L together when nobody is driving the bus. When transmitting, the transceivers drive the CAN-L / CAN-H lines apart, typically around 5V. That's all they do, and the resistors bring them back together. To create the logic zeros and ones on the bus.

Regarding transceiver choices, there are many. We use the Texas Instruments ISO1050 on most of our products. This supports isolation between the logic side and the CAN bus side, which is useful when you have different devices talking to each other which can have different grounds. If you're talking CAN within a single board, or if you have the system grounds well controlled and equal, then isolation is likely not a concern for you. In terms of protection from "bad things" on the bus (surges, ESD, etc), putting a common-mode choke on the CAN-L / CAN-H signals, close to the transceiver, can do a great deal to help slow or block such nastiness.

Waking up a node is either simple or complicated depending upon your needs. To detect bus activity it's simple - just look at the RX pin from any transceiver - it'll tell you if the bus gets driven apart (to 5V). More complicated is to know if there's a packet on the bus being addressed specifically to you - do you want to wake up for everything, or just for data for you? If you want to wake up on data just for you, you need to use the logic built into your CAN peripheral, in whatever microcontroller / CPU / etc you're using. I've worked with a number of them, and all have address filtering, interrupts and status flags, etc, for this purpose.

I hope this helps.
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Signal
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« Reply #2 on: August 22, 2019, 06:44:18 06:44 »

What I'm not understanding is WHY you want to have a disable pin on the CAN transceiver.
<...>
I hope this helps.
Nope. It does not help at all. But thank you for willing to help!
I'm asking the specific question. Sorry that you do not understand why but I still need it for my specific purpose.

About "load" resistance, maybe it is my fault not to be clear or explicit enough. I wrote "internal resistors" that in data sheets are named as "input resistance". Did you noticed them before? Purpose of them (surprise) also is to be a "load" or "terminal" (for stub) but somehow weaker than main ones. Did you know about it? Even to add external additional resistors if needed is a known measure.

I am quite experienced using CAN bus as a device designer too so please do not explain me the basics. Please consider more seriously that I ask a specific question for a reason.
If this is still difficult for you then you can try to realize that you also do not understand why NXP developed TJA1051T/E with this particular feature. Do you want to convince them that it was not necessary? ;)

Quote
When transmitting, the transceivers drive the CAN-L / CAN-H lines apart, typically around 5V.
Strange part. Hope you mean 2.5 instead of 5.
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