spi.c

/*
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include "hardware/resets.h"
#include "hardware/clocks.h"
#include "hardware/spi.h"

static inline void spi_reset(spi_inst_t *spi) {
    invalid_params_if(SPI, spi != spi0 && spi != spi1);
    reset_block(spi == spi0 ? RESETS_RESET_SPI0_BITS : RESETS_RESET_SPI1_BITS);
}

static inline void spi_unreset(spi_inst_t *spi) {
    invalid_params_if(SPI, spi != spi0 && spi != spi1);
    unreset_block_wait(spi == spi0 ? RESETS_RESET_SPI0_BITS : RESETS_RESET_SPI1_BITS);
}

uint spi_init(spi_inst_t *spi, uint baudrate) {
    spi_reset(spi);
    spi_unreset(spi);

    uint baud = spi_set_baudrate(spi, baudrate);
    spi_set_format(spi, 8, SPI_CPOL_0, SPI_CPHA_0, SPI_MSB_FIRST);
    // Always enable DREQ signals -- harmless if DMA is not listening
    hw_set_bits(&spi_get_hw(spi)->dmacr, SPI_SSPDMACR_TXDMAE_BITS | SPI_SSPDMACR_RXDMAE_BITS);

    // Finally enable the SPI
    hw_set_bits(&spi_get_hw(spi)->cr1, SPI_SSPCR1_SSE_BITS);

    return baud;
}

void spi_deinit(spi_inst_t *spi) {
    hw_clear_bits(&spi_get_hw(spi)->cr1, SPI_SSPCR1_SSE_BITS);
    hw_clear_bits(&spi_get_hw(spi)->dmacr, SPI_SSPDMACR_TXDMAE_BITS | SPI_SSPDMACR_RXDMAE_BITS);
    spi_reset(spi);
}

uint spi_set_baudrate(spi_inst_t *spi, uint baudrate) {
    uint freq_in = clock_get_hz(clk_peri);
    uint prescale, postdiv;
    invalid_params_if(SPI, baudrate > freq_in);

    // Disable the SPI
    uint32_t enable_mask = spi_get_hw(spi)->cr1 & SPI_SSPCR1_SSE_BITS;
    hw_clear_bits(&spi_get_hw(spi)->cr1, SPI_SSPCR1_SSE_BITS);

    // Find smallest prescale value which puts output frequency in range of
    // post-divide. Prescale is an even number from 2 to 254 inclusive.
    for (prescale = 2; prescale <= 254; prescale += 2) {
        if (freq_in < prescale * 256 * (uint64_t) baudrate)
            break;
    }
    invalid_params_if(SPI, prescale > 254); // Frequency too low

    // Find largest post-divide which makes output <= baudrate. Post-divide is
    // an integer in the range 1 to 256 inclusive.
    for (postdiv = 256; postdiv > 1; --postdiv) {
        if (freq_in / (prescale * (postdiv - 1)) > baudrate)
            break;
    }

    spi_get_hw(spi)->cpsr = prescale;
    hw_write_masked(&spi_get_hw(spi)->cr0, (postdiv - 1) << SPI_SSPCR0_SCR_LSB, SPI_SSPCR0_SCR_BITS);

    // Re-enable the SPI
    hw_set_bits(&spi_get_hw(spi)->cr1, enable_mask);