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What are CANH and CANL reset initialization values when CAN Transceiver goes to sleep ?

Started by learn 4 years ago4 replieslatest reply 4 years ago90 views
CAN Transceiver

Attached is CAN Transceiver Datasheet. This IC is wired in our Microcontroller based Electronic Control Unit.

What will be CAN High (CANH) and CAN Low (CANL) Voltage when CAN Transceiver is in sleep mode?

To verify these voltages are correct in sleep mode, measure CANH with respect to ground? And measure CANL with respect to Ground?

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Reply by matthewbarrFebruary 16, 2017

I think the answer you are looking for is in Table 34 on page 38. See Vo(dif) and Vo(rec). Refer to the voltages related to CAN Offline mode, Table 4 on page 8 shows CAN is Offline in sleep mode.

At the risk of pointing out the obvious, being able to measure these voltages depends on whether or not other active devices are connected to the CAN bus. You should be able to measure the transceiver sleep mode output voltages wrt. ground if you put 60 ohms between CANH and CANL with nothing else connected.

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Reply by JohnHPoteFebruary 17, 2017

To supplement matthewbarr's comments look at Sec. 7.2.1.3 in your data sheet.

https://en.wikipedia.org/wiki/CAN_bus migh provide useful information. Also try looking through the CAN standard to find out recomended practice for handling the bus lines for the case when all CAN transceivers are off or in sleep mode. My memory has faded on this.

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Reply by matthewbarrFebruary 17, 2017

Yes, good point. Offline in Table 4 really means Offline or Offline Bias. The transceiver won't go into Offline Bias without the right activity on CANH/L and being configured with CWE=1. Offline Bias will be transient and measuring associated voltage levels will be problematic. With just a resistor on CANH/L the transceiver should be Offline only in sleep mode.

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Reply by DNadlerFebruary 16, 2017

Shouldn't they be high-Z when the transceiver is offline?
Otherwise it would kill the CAN bus, right?
Hence "what is the voltage" is not a meaningful question, right?
Hope that helps,
Best Regards, Dave