TM4C129XNCZAD: CANTST Tx:01 Sample Mode

John Duquette

Part Number: TM4C129XNCZAD

I have this device showing an ACK error when on a long cable (30m) and about 600 ns of total round trip delay from CANnTX to CANnRX. With 15 quanta per 1 Mbps CAN I have the sample point set well past this, but I'm seeing intermittent operation (occasionally 600 ns will fail while 640 ns will pass).

Tiva 120 MHz

Clocks/bit = 15

Predivide = 8

TSEG1 13

TSEG2 1

SJW 1

I want to verify the bit timing with the "Sample Point" mode (CANTST Tx:01), but after enabling this mode:

Init CAN

Enable CAN

Set TST bit in CANCTL

Set CANTST:Tx to "01" for Sample Point

I am still only seeing typical CAN data on the Tx pin (ACK after receiving a CAN message, Normal bit stream when sending a message). I'm expecting to see an edge per bit time at the Sample Point, but I'm not seeing that. Is there more documentation on the "Sample Point" testing somewhere? Or example code? Do I need to disable some other CAN operation for the "Sample Point" test mode to take over?

over 5 years ago

0 Bob Crosby over 5 years ago

TI__Guru 72500 points

How did you put the CAN module in sample point mode? Here are the register writes that I used in a different project.

    // Put the CAN module in sample point mode
    HWREG(CAN0_BASE + CAN_O_CTL) |= 0x80u; //Enable test
    HWREG(CAN0_BASE + CAN_O_TST) = 0x20u;  //Enable sample point

Here is the logic analyzer shot. In this project the baudrate is 125K and the sample point is centered. Also, the TX pin from node B is disconnected from the transceiver to avoid causing errors on the CAN bus.

0 John Duquette over 5 years ago in reply to Bob Crosby

Intellectual 710 points

Oops, wrong button, this isn't resolved yet.

That's about what I was expecting, except I'm surprised the sample pt is showing as a pulse and not an edge. Can you zoom in on that to see how wide the sample pt pulse is? Maybe I'm sampling too slow to capture it.

0 John Duquette over 5 years ago in reply to John Duquette

Intellectual 710 points

That's not it. I'm now sampling at 500 Msps and not seeing anyting, so this should even catch 120 MHz system clock pulses.

0 Bob Crosby over 5 years ago in reply to John Duquette

TI__Guru 72500 points

The sample point signal is a positive pulse equal to the period of the system clock. In your case (120MHz) it is 8.3nS..

0 John Duquette over 5 years ago in reply to Bob Crosby

Intellectual 710 points

Thanks. I'm still only seeing typical CAN traffic on the Tx pin. Here's my full initialization sequence:

void InitializeCAN( uint8_t CANHardwareNumber,

uint32_t CANBase,

uint32_t CANBitRate,

uint32_t ReceiveFilter1,

uint32_t ReceiveFilter2,

uint32_t ReceiveFilter3,

uint32_t IDMask,

uint32_t SourceClock )

{

uint16_t ui16CANCTL;

CANInit( CANBase );

//for 120MHz clock

tCANBitClkParms CANBitClkSettings = {13, 1, 1, 8};

CANBitTimingSet(CANBase, &CANBitClkSettings);

#endif

{

CANEnable( CANBase );

//Set Test bit in control register, which write-enables test register.

ui16CANCTL = HWREG(CANBase + CAN_O_CTL);

HWREG(CANBase + CAN_O_CTL) = ui16CANCTL | CAN_CTL_TEST;

//Enable sample point test

HWREG(CANBase + CAN_O_TST) = CAN_TST_TX_SAMPLE;

CANIntEnable( CANBase, CAN_INT_MASTER | CAN_INT_ERROR );

/* Configure the receive message FIFO. */

CANReceiveFIFO( CANHardwareNumber, ReceiveFilter1, ReceiveFilter2, ReceiveFilter3, IDMask );

if( CANHardwareNumber == CAN0_BUS )

{

IntEnable( INT_CAN0 );

}

else if( CANHardwareNumber == CAN1_BUS )

{

IntEnable( INT_CAN1 );

}

CANControl[CAN0_BUS].StatusControl = 0;

CANControl[CAN1_BUS].StatusControl = 0;

}

CANSendingMonitorValues = 0;

MonitorValuesRequestedBy = 0;

CANLargePayloadLastSerialNumber = 0;

MessageBackupToCANHardwareCounter[CAN0_BUS] = 0;

MessageBackupToCANHardwareCounter[CAN1_BUS] = 0;

}

Is it possible that any of that additional setup is disabling the test mode?

0 Bob Crosby over 5 years ago in reply to John Duquette

TI__Guru 72500 points

What is the purpose of these two lines? Are these mapped back to the hardware CAN control register? Can you provide a screen shot of the relevant CAN registers after running the initialization code?

      CANControl[CAN0_BUS].StatusControl = 0;

      CANControl[CAN1_BUS].StatusControl = 0;

(Next time you want to insert code snippets in the thread, use the </> symbol from the menu bar. It will keep code formatting.)

0 John Duquette over 5 years ago in reply to Bob Crosby

Intellectual 710 points

Thanks Bob, for the tip on code snippets. This will take some deeper digging next week.

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TM4C129XNCZAD: CANTST Tx:01 Sample Mode