• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 82 subscribers
  • Views 628 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMS320F280049: what's the baud rate limit of CAN when using internal oscillator as the clock source?

Howard Zou
Genius 12365 points
Part Number: TMS320F280049

Hi,

my customer has make the board without external crystal, so they wonder if they can achieve lower baud rate CAN with internal oscillator 2 as the clock source.

The required frequency tolerance for the CAN bit clock depends on the bit timing setup and network
configuration, and can be as tight as 0.1%. Since the main system clock (in the form of PERx.SYSCLK)
may not be precise enough, the bit clock can also be connected to XTAL via the CLKSRCCTL2 register.

1. So my understanding is that the clock may not need to be as tight as 0.1%(that's just a tightest number). So how can I calculate the baud rate that could be supported by internal oscillator?

2. Why F2803x and F2806x doesn't have such limitation for CAN clock source in TRM?

  • 0
    TI__Mastermind 34535 points

    Hi Howard,

    1.) Please refer to application note  SPRAC35 (CAN Bit Timing Calculator) as guidance for determining the maximum clock tolerance using different values of bit rate, bus length and CAN clock frequency.

    2.) The internal oscillators in F2803x and F2806x are of different design and do not have the variation issue seen with F280049 device.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Genius 12365 points

    Joseph,

    but according to the datasheet spec, the internal oscillator for F2806x is also very loose, it may be ±3% tolerance among -40 to 125C range.

  • 0 in reply to Howard Zou
    TI__Mastermind 34535 points

    Hi Howard,

    The difference between F2803x/6x and F280049 internal oscillators is that the F2803x/6x has linear and predictable frequency output with respect to temperature, as shown in the datasheet plots.  This predictability of the output allows the user to compensate for the frequency (i.e. - set it to 10MHz) whenever there is a temperature change.

    There is a note on the datasheet about using the CAN with the internal oscillator as follows:

    "For information on using the CAN module with the on-chip zero-pin oscillators, see MCU CAN Module Operation Using the On-Chip Zero-Pin Oscillator."

    Please refer to the app note on how the frequency compensation is carried out on F2803x/6x devices.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Genius 12365 points

    Joseph,

    thank you.

    After compensation, the tolerance will be ±1%(according to datasheet 7.13.1.3 Internal Zero-Pin Oscillator (INTOSC1/INTOSC2) Characteristics), right? So ±1% is the limit when we choose external crystal or oscillator, right?

    Besides, with the calculation tool you show, I can see that we need oscillator tolerance at 1.58% for 100k baud rate. Does it mean that if we configure the DCAN register in that value(0x034D3), we can use internal oscillator since its tolerance is ±1.5% from -30C to 90C?

  • 0 in reply to Howard Zou
    TI__Mastermind 34535 points

    Hi Howard,

    Yes, with the compensation, the INTOSC tolerance for F2803x/6x should be within +/-1%.

    Please note that existing and past F280049 devices still have the old INTOSC tolerance of +/-3%.  The first device that had the improvement on INTOSC tolerance of +/-1.5% (-30C to 90C) is F28002x.  In order for F280049 to have the tightened INTOSC specs, production flow and datasheet changes have to happen.  F280049 is currently in the process of the production flow change for the tighter INTOSC tolerance to be similar to F28002x.  Hope this provides some clarification.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Genius 12365 points

    Joseph,

    Thank you.

    1. So external crystal or oscillator within +/-1% is good enough to be chosen, right?

    2. Sorry that I'm not clear, 

    by configuring DCAN register in that value(0x034D3), we can use internal oscillator of F28002x since its tolerance is ±1.5% from -30C to 90C to achieve 100k baud rate at that specific condition(bus length, CAN clock listed in the table)?

  • +1 in reply to Howard Zou
    TI__Mastermind 34535 points

    Hi Howard,

    1.) Yes, for this application +/-1% tolerance for the clock source would be sufficient.

    2.) No worries.  Good to know that you are referring to F28002x, so that should not be an issue for the improved tolerance at -30C to 90C.  I referred to the title of the post which was referring to TMS320F280049.

    Best regards,

    Joseph