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CCS/TMS320F28027: lowest SPI CLOCK/SPIBRR

SIBAPRASAD CHAKRABARTI
Intellectual 520 points
Part Number: TMS320F28027

Tool/software: Code Composer Studio

I am using TMS320F28027, and looks like the lowest clock frequency of the SPI can be achieved 33.48Khz (=60MHz/(14*128)) with 60MHz CPU CLK. lowest LSPCLK=SYSCLKOUT/14, lowest SPIBRR=LSPCLK/128.

Is it possible to reduce the SPIBRR even lower without lowering CPUCLK from 60MHz. Please comment !!

  • 0
    Guru 30330 points
    Hello,

    No. The SPI operates off of LSPCLK so the minimum frequency at 60MHz is what you have calculated. Your next option is to decrease SYSCLK.

    Why do you need to slow down the SPI so much?

    Thanks,
    Mark
  • 0 in reply to Mark Labbato
    Intellectual 520 points
    See, I am using SPI directly to drive 4 line 20 char LCD display (SPI to SERIAL IC in-between). I am using my uC board to issue PWM for POWER Electronics Converter and SPI to LCD. While MOSFET/IGBT starts switching at 400V DC bus and the load starts drawing current, the LCD that is mounted in the front of the box starts mal-functioning. Three SPI signals at 33KHz over a wire length of 1-1.5ft picks up EMI. So, I have no other way out to reduce SPI CLK, so that I can place required filter capacitor to reduce EMI to the signals !!
    In the meantime, I had reduced CPU CLK to 30MHz (even thinking to go down to 20MHz) and manged the situation by interrupting ISR once in three PWM cycles (12KHz) to get more time for the tasks to complete. So, my ISR runs at 4KHz at this moment and PWM load is updated at 4KHz rate, even though my switching frequency is 12KHz. I am not sure whether it will be good if I need to update the PWM modulator at the rate of PWM frequency for ac application !!

    If any input from your end will be appreciated !!
  • 0 in reply to SIBAPRASAD CHAKRABARTI
    Guru 30330 points
    Hello,

    Have you been able to get your application working? Can you state explicitly how the LCD is malfunctioning? Have you scoped your SPI signals to see if they truly are being affected by the system noise?

    I see that you mention the SPI is only using 3 signals, does it include the Chip select? Especially in noisy systems, it is a good idea to use the SPISTE to help your noise immunity. If the serial link is always active, it is a good chance that an errant noise pulse on your clock line may cause the link to lose synchronization. by using SPISTE, the amount of time the link is active is reduced, improving your immunity to noise.
  • 0 in reply to Mark Labbato
    Guru 30330 points
    I am closing this post. If you have additional questions or issues that you come across, please create a new post.