How to control slew rate when using SPI?

Adam Dewhirst

Other Parts Discussed in Thread: CC2650, CC2640

How to I change slew rate and drive strength settings for SPI? I’m getting poor signal integrity on my SPI output signals (SPI Clk and MOSI). I can improve the signal integrity by enabling the slow slew-rate (SLEW_RED) option in this register:

However, it looks like the slew rate & drive strength settings are hard-coded in the SPI_open() TI-RTOS function, and I don’t see a supported way to change this.

On a related note: Is there an IBIS model for the CC2650? This would help greatly with figuring out the proper way to configure the device to get good signal integrity

over 9 years ago

0 Tom Kopriva over 9 years ago

TI__Mastermind 20480 points

Hi Adam,

Adam Dewhirst said:
However, it looks like the slew rate & drive strength settings are hard-coded in the SPI_open() TI-RTOS function, and I don’t see a supported way to change this.

Sort of. BLE example projects tend to include the SPI driver as source instead of linking in with the existing TI-RTOS libraries (for code footprint reasons). I suppose in your case, you'd probably want to modify it. You can add PIN_SLEWCTRL for CLK and MOSI in SPICC26XXDMA_initIO().

    /* Configure IOs */
    /* Build local list of pins, allocate through PIN driver and map HW ports */
    if (object->mode == SPI_SLAVE) {
      /* Configure IOs for slave mode */
      spiPinTable[i++] = hwAttrs->mosiPin | PIN_INPUT_EN;
      spiPinTable[i++] = hwAttrs->misoPin | PIN_GPIO_OUTPUT_EN | PIN_GPIO_LOW | PIN_PUSHPULL | PIN_INPUT_DIS | PIN_DRVSTR_MED;
      spiPinTable[i++] = hwAttrs->clkPin  | PIN_INPUT_EN;
      spiPinTable[i++] = object->csnPin   | PIN_INPUT_EN | PIN_PULLUP;
    }
    else {
      /* Configure IOs for master mode */
      spiPinTable[i++] = hwAttrs->mosiPin | PIN_GPIO_OUTPUT_EN | PIN_GPIO_LOW | PIN_PUSHPULL | PIN_INPUT_DIS | PIN_DRVSTR_MED; // <---HERE for example
      spiPinTable[i++] = hwAttrs->misoPin | PIN_INPUT_EN | PIN_PULLDOWN;

      /* Output low signal on SCLK until SPI module drives signal if clock polarity is configured to '0' */
      /* Output high signal on SCLK until SPI module drives signal if clock polarity is configured to '1' */
      if (object->frameFormat == SPI_POL0_PHA0 || object->frameFormat == SPI_POL0_PHA1) {
          spiPinTable[i++] = hwAttrs->clkPin | PIN_GPIO_OUTPUT_EN | PIN_GPIO_LOW | PIN_PUSHPULL | PIN_INPUT_DIS | PIN_DRVSTR_MED; 
      }

I think the IBIS models might be in the design docs on the CC2650 product page.

http://www.ti.com/product/CC2640/technicaldocuments#doctype12