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  • Author Author: Jan Cumps
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Add external SPI Flash Memory to Raspberry Pico - 4: STL containers to return byte arrays

Jan Cumps
Add external SPI Flash Memory to Raspberry Pico - 4: STL containers to return byte arrays

A projects to learn the SPI API of the RP2040 C SDK, and to use it to control an external Flash IC. In this post, I refine the m25 c++ class again. Return byte arrays as Standard Template Library (stl) containersimage

Arrays as return values instead of output parameters

Usually, when a function returns multiple values, we pass it a buffer as one of the parameters, and the function can fill it. There are other options, but this is often how we see it done.

In c++ you can return a container as a return value. You can construct it in the function - right size and value, then hand it over to the calling function. Here's a function that reads 3 bytes from the m25 IC over SPI, then hands over the results an stl vector:

std::vector<uint8_t> m25::rdid() {
    uint8_t buf[3];
    // command
    uint8_t cmdbuf[] = {
            RDID,
    };
    setDataBits(8);
    select();
    spi_write_blocking(spi, cmdbuf, 1);
    spi_read_blocking(spi, 0, buf, 1); // Manufacturer Identification
    spi_read_blocking(spi, 0, buf + sizeof buf[0], 2); // Device Identification (Memory Type || Memory Capacity)
    deselect();
    return std::vector<uint8_t> (buf, buf + (sizeof buf/sizeof buf[0]));
}

The calling program can use it like this:

   // get and print read identifier
    bytes = eeprom->rdid();
    std::cout << "read identifier: ";
    for ( auto bte: bytes ) std::cout << std::hex << static_cast<int>(bte) << " ";
    std::cout << std::endl;

Result:

read identifier: 20 20 15 

VSCode project attached: spi_flash_20221203.zip

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