Issues Writing Registers on ADXL375 with SPI

RE: Issues Writing Registers on ADXL375 with SPI

asharkl — Mon, 29 Jul 2019 16:06:29 GMT

You are completely right, thank you for pointing that out. In this case it did not cause any problems because it address is being OR-ed with 0x00 (ADXL375_SPI_WRITE) so functionally does not change, but for a more generalized solution the OR-ing should happen before copying into packet[].

RE: Issues Writing Registers on ADXL375 with SPI

awneil — Thu, 25 Jul 2019 23:47:25 GMT

This doesn't look right:

    uint8_t packet[2] = {address, data};
    address = address | ADXL375_SPI_WRITE;    
    err_code = nrf_drv_spi_transfer(p_spi_master, packet, 2, NULL, 0);

You copy the address into packet[0], and you use the packet[] array for your transfer.

Therefore OR-ing the address with ADXL375_SPI_WRITE after you've copied it into the array won't have any effect on the transmission!

RE: Issues Writing Registers on ADXL375 with SPI

asharkl — Thu, 25 Jul 2019 18:26:06 GMT

Thank you all for the help. I rewrote the SPITransfer function using an array as suggested by hmolesworth , and made sure that I had somewhere to store my data. The biggest difference was made by sending the write data as an array instead of 2 separate transmissions. The code that worked correctly was this, case this is of use to anyone:

uint32_t oneByteWrite(uint8_t address, uint8_t data) 
{
    uint32_t err_code;
    uint8_t packet[2] = {address, data};
    address = address | ADXL375_SPI_WRITE;    
    err_code = nrf_drv_spi_transfer(p_spi_master, packet, 2, NULL, 0);
    if(err_code != NRF_SUCCESS)
    {
        return err_code;
    }
    return err_code;
}

RE: Issues Writing Registers on ADXL375 with SPI

awneil — Thu, 25 Jul 2019 15:33:34 GMT

[quote userid="65515" url="~/f/nordic-q-a/50164/issues-writing-registers-on-adxl375-with-spi/200583"]the use of SPI in not quite as you hope[/quote]

Indeed.

    uint8_t p_tx_data;
    uint8_t p_rx_data;

asharkl - The use of "p_" as a prefix like that usually indicates that the variable is a pointer.

Is that what you intended?

If so, you also need to define somewhere to store the actual data!

RE: Issues Writing Registers on ADXL375 with SPI

hmolesworth — Thu, 25 Jul 2019 15:18:18 GMT

I can't spend more than a few minutes, but it's clear the use of SPI in not quite as you hope. The code above shows this:

uint8_t SPItransfer(uint8_t txData)
{
    uint8_t p_tx_data;
    uint8_t p_rx_data;
    p_tx_data = txData;
    uint32_t err_code = nrf_drv_spi_transfer(p_spi_master, &p_tx_data, 1, &p_rx_data, 2);
    APP_ERROR_CHECK(err_code);
    return p_rx_data;
}

That code will not work correctly, since the rx buffer will corrupt the stack when 2 bytes are stuffed into an area for 1 byte. This is a corrected version:

uint8_t SPItransfer(uint8_t txData)
{
    uint8_t p_tx_data;
    uint8_t p_rx_data[2];  // Make this an array to receive more bytes, 2 in this case
    p_tx_data = txData;
    uint32_t err_code = nrf_drv_spi_transfer(p_spi_master, &p_tx_data, 1, &p_rx_data, 2);
    APP_ERROR_CHECK(err_code);
    return p_rx_data[1];  // Return the second byte, not the first
}

This corrected version only works for a single command byte expecting a single response byte which will be the second of two bytes received over SPI. To read more than a single byte after a single command byte do not use a loop, ask for all the (sequential) bytes in a single transfer.

uint8_t tx_data;
uint8_t rx_data[20];  // Make this an array to receive lots ofbytes, say 20

uint32_t SPItransfer(uint8_t txData)
{
    tx_data = txData;
    uint32_t err_code = nrf_drv_spi_transfer(p_spi_master, &tx_data, 1, &rx_data, sizeof(rx_data));
    APP_ERROR_CHECK(err_code);
    return err_code;  // Return the error code, look in p_rx_data for response string
}

void multiByteRead(int startAddress, char* buffer, int size) {
    uint8_t tx = (ADXL375_SPI_READ | ADXL375_MULTI_BYTE | (startAddress & 0x3F));
    //Send address to start reading from.
    SPItransfer(tx);
    for (int i = 0; i < size; i++) {
        // this is just a simple example, but try not to be silly
        if (i >= sizeof(rx_data)) break;
        buffer[i] = rx_data[i];
    }
}

There are other potential issues; you might have to increase port pin drive strength on the SCK pin; 4MHz might be too high a clock speed so maybe try something lower until it works then speed up until it doesn't; the accelerometer may require a delay between the command byte and the subsequent response bytes (some do, some don't).

RE: Issues Writing Registers on ADXL375 with SPI

awneil — Thu, 25 Jul 2019 09:34:22 GMT

You're welcome.

Yes, it is particularly badly presented!

You may wish to provide feedback here:

https://devzone.nordicsemi.com/nordic/backstage/f/backstage-forum/41454/options-for-posting-source-code-are-confusing

RE: Issues Writing Registers on ADXL375 with SPI

awneil — Thu, 25 Jul 2019 09:22:39 GMT

[quote userid="13562" url="~/f/nordic-q-a/50164/issues-writing-registers-on-adxl375-with-spi/200429"]the signal looks good I guess[/quote]

It looks like the scope is in "digital" mode - so it won't be showing any analogue "anomolies" ... ?

[quote userid="13562" url="~/f/nordic-q-a/50164/issues-writing-registers-on-adxl375-with-spi/200429"]I suggest you also get an analog scope of the signal[/quote]

Indeed - for the above reason!

asharkl - Presumably, this scope has the facility to decode serial bus comms?

If you don't already have it, now would be a good time to at least try the free evaluation!

[quote userid="13562" url="~/f/nordic-q-a/50164/issues-writing-registers-on-adxl375-with-spi/200429"] I suggest you ask AD for help as well[/quote]

RE: Issues Writing Registers on ADXL375 with SPI

haakonsh — Thu, 25 Jul 2019 08:27:03 GMT

I can't really read what the bits states on the data lines are, but the signal looks good I guess.

I suggest you also get an analog scope of the signal.

If the SPI communication checks out then I suggest you ask AD for help as well, they should be able to help you. Have you tried their driver?

RE: Issues Writing Registers on ADXL375 with SPI

asharkl — Wed, 24 Jul 2019 20:45:12 GMT

I removed the manual control over the SS pin and my read function still works but the write function is still broken. I have verified proper procedure against the ADXL data sheet. Here is the SPI Write procedure as per the data sheet.

In order to set the ADXL into measurement mode, 0x08 must be sent to the Power Control Register (0x2D). As per the data sheet I OR'ed the SPI Write command (0x00) with the address and sent that via SPI, immediately followed by sending 0x08. Below is a scope image of just a single write.

And below is a picture of a single write, immediately followed by attempting to read the register. Even though I sent 0x08 to the Power Control register, it reads as 0x0C regardless of what is sent. The same applies to all the register I write to, they will get set to an arbitrary value and nothing short of power cycling the board will get it to revert to the default value.

Thank you for your help, and please let me know if you require closeups of any part of the logic signal.

RE: Issues Writing Registers on ADXL375 with SPI

asharkl — Wed, 24 Jul 2019 20:24:52 GMT

Thank you for that! I could not for the life of me figure out how to do that before.

RE: Issues Writing Registers on ADXL375 with SPI

awneil — Wed, 24 Jul 2019 08:35:34 GMT

How to properly post source code:

RE: Issues Writing Registers on ADXL375 with SPI

haakonsh — Wed, 24 Jul 2019 08:19:02 GMT

First, you should not need to control the SS pin manually, the SPI driver will handle it.

Second, writing certain registers might require a special procedure of some kind. I suggest you triple-check with the adxl datasheet.

Do you have a scope of the SPI writes?