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MaaxBoard-RT: FLEXIO as SPI Master - for you

tjaekel

I have converted SDK example to use FLEXIO as SPI Master implementation (in addition to regular LPSPI),
and extended/modified the example for MaaxBoard-RT:

It demonstrates how to use FLEXIO to implement a SPI Master device and receive on LPSPI2 as slave (loopback test).
ATTENTION: the Debug UART is LPUART6 (LPUART1 pins are needed for LPSPI2 Slave).

The project is here:

tjaekel/MaaxBoard-RT_flexio_spi_edma_lpspi_transfer_-master_cm7: FLEXIO as SPI master plus LSPI2 as slave (github.com)

With default clock config it works up to 47 MHz, with modified clock config (other and faster PLL for FLEXIO2) up to 258 MHz - impressive.

FLEXIO is a very powerful and flexible block in MCU in order to implement your own or additional interfaces, such as SPI, I2C, LIN, SingleWire, ...
I want to use in order to implement a QSPI interface.

  • I am sorry.
    CORRECTION - ATTENTION:

    the max. speed I get on FLEXIO as SPI master is 515 KHz (not MHz!)

    BTW:
    with a regular LPSPI4 (as master) plus LPSPI2 (as slave), I get: 45 MHz (scope shows 45.454 and 41.667 MHz) as max. speed.

    With higher speed: I get bit errors on Slave Rx and also: SCLK is not symmetrical anymore and even faster speed (e.g. 83 MHz) - the PCS signal timing is wrong (too late).

    Conclusion:

    FLEXIO is too slow for my intention (as QSPI, 40 MHz needed at least).

    Regular SPI is ok (40 MHz) but to make it even faster was not successful (I would need 160...200 MHz in order to convert into a QSPI with 40..50 MHz, e.g. via external FPGA).

    FLEXIO is ok for UART, I2C, slow SPI, single wire, LED wire protocol, LIN, ... but not really an alternative for LPSPI peripherals (FLEXIO is much slower as native devices).

  • With changing the clock sources (and stay within the spec.) - the max. SPI SCLK which works: 30 MHz

    The next higher SCLK speed - 60 MHz - has a wrong waveform:
    there are PCS pulses between bytes and not every byte transfer has the same number of clocks (looks "ugly").

    So,
    30 MHz as SPI SCLK is the max. you can get with FLEXIO2 SPI implementation.

    Remark:
    With regular SPI (e.g. LPSPI4) I can get 45 MHz working properly.
    Next speed step, 66 MHz, works still, but the waveform is not "nice" anymore (a longer SCLK pulse on bit 7).

    So, FLEXIO is slower as regular peripherals can do (in terms of SPI).
    30 MHz is the max. clock for any interface emulated with FLEXIO.