SPI transfers fail once clock rate is set higher than 6Mhz

[Stephen Murphy]

Hi there 👋 First off, thank you for all of your work in updating and maintaining this lib. It's been easy to integrate and I appreciate the open-source aspect of it.

However, I am having some issues. I am integrating this lib into my C application and attempting to send SPI communications with a FT2232HL bridge and verifying the output using a Salae Logic analyzer. My issues are as follows

• SPI Writes give what just looks like erratic bouncing lines (Actually very similar results to my next issue) - I have had to replace my Writes with a Transfer call in order for it to work properly
• SPI transactions (Read/Write/Transfer) can not exceed 6Mhz. Otherwise the IO lines appear erratic and unpredictable.

Below is a snippet of the code used to initiate a transfer. The _write function opens up the channel with our SPI configuration retrieved from my defined xfer_t which then calls the spi_write function where the transfer is actually initiated and then cleaned up.

void SPI::_write(shost_xfer_t *xfer) {
    int ret = 0;
    // Open an MPSSE instance for SPI0 and store the context
    this->mpsse = MPSSE(modes::SPI0, xfer->clk, MSB);

    // Ensure the context is not NULL and and the mpsse channel is open
    if (this->mpsse != NULL && !this->mpsse->open) {
        // TODO what should be something like: "Failed to initialize MPSSE: %s\n", ErrorString(this->mpsse)
        throw std::system_error(EIO, std::generic_category(), ErrorString(this->mpsse));
    }

    ret = spi_write(xfer->buff, xfer->len);

    Close(this->mpsse);

    switch (ret) {
        case 1:
            // TODO printing address whould be nice
            throw std::system_error(ENXIO, std::generic_category(), "No response from peripheral.");
            break;
        case 2:
            throw std::system_error(EIO, std::generic_category(), "Hardware failure.");
            break;
        default:
            // all is good, no need to do anything.
            break;
    }
    xfer->bytesTranferred = xfer->len;
}

int8_t SPI::spi_write(uint8_t *src_buffer, size_t buffer_len) {
    int8_t retVal = 0;
    char *writeBuffer = (char *) malloc(buffer_len);
    char *readBuffer = (char *) malloc(buffer_len+1);
    memcpy((void*)writeBuffer, (const void*)src_buffer, buffer_len);

    // Send the start condition for SPI
    if( MPSSE_OK != Start(mpsse) ) {
        retVal = 1;
        goto CLEANUP;
    }
    // Write our buffer
    readBuffer = Transfer(mpsse, writeBuffer, buffer_len);
    // Send the stop condition for SPI
    Stop(mpsse);

CLEANUP:
    // Free the write buffer we created
    free(writeBuffer);
    free(readBuffer);

    return retVal;
}

Here is a preview of a Read transaction at what is supposed to be 12Mhz.

1.) As a sanity check, do you see anything off in the transfer code.
2.) Have you seen related issues to pushing the clock rate too far on certain hardware?