IMX6 CAN Transceiver

[jakepollex]

I have designed a custom SoM using the i.MX6Q and have been working with the CAN interface.  Following the Sabre AI board we used a 3.3v transceiver so there was no need for level translation between the processor and transceiver.  We have run into problems in that the signals coming from the i.MX6Q are not what would be expected.  Both the CAN_RX and CAN_TX signals are being driven when sending data.  I connected a scope to the CAN_TX and CAN_RX signals without the transceiver being connected and observed the following;

These signals look exactly like what I would expect to see after the transceiver and not coming directly from the i.MX6.  I tried connecting these lines directly to another board without using the transceiver and was able to transmit and receive without issue.

I would like for someone to shed some light on exactly what is happening here as I can find no mention in the reference design or reference manual that alludes to what could possibly be going on.  Is the transceiver built into the SoC and just not mentioned anywhere or do I have the FlexCAN configured in some weird way and it just happens to work?

Thanks,

Jake

[igorpadykov]

Hi Jake

TJA1041AT datasheet used in Sabre AI board

http://www.nxp.com/documents/data_sheet/TJA1041A.pdf

on Figure 9 Timing Diagram shows signals from  transceiver (CANH and CANL)

and from processor (CAN_RXD, CAN_TXD)

http://www.nxp.com/documents/data_sheet/TJA1041A.pdf

The transceiver receiver will convert the differential analog bus signal on pins CANH

and CANL into processor CAN_RXD digital data,  The transceiver transmitter will

convert processor digital data  from CAN_TXD  into  bus CANH and CANL bus

differential analog signal.

Best regards

igor

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[jakepollex]

Thank you for the quick response.  The scope shot that I posted is WITH NO CAN TRANSCEIVER CONNECTED.  Therefore nothing should be driving the CAN_RX signal.  There was nothing connected to the CAN lines when I took the scope shot.  How and why is the CAN_RX signal being driven.  It is clearly inverted from the CAN_TX signal and not at all what should be present on the CAN_RX even if the transceiver was connected.

Thanks,

Jake

[igorpadykov]

what are amplitudes for both channels, it is not clear from picture?

Also could you attach pull-down 10Kohm to CAN_RX.

[jakepollex]

Here is a rough drawing of the signals I am seeing coming directly from the i.MX6 with NO transceiver connected.  The drawing is only to show the voltage levels.

[igorpadykov]

seems smth wrong in schematic or measuring, because

these signals should comply with Table in

sect.4.6.2 General Purpose I/O (GPIO) DC Parameters

i.MX6DQ Datasheet  [OVDD is power domain]

http://cache.freescale.com/files/32bit/doc/data_sheet/IMX6DQCEC.pdf

Had you tried to attach pull-down 10Kohm to CAN_RX and test?