crc.c

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#include "icarus/crc.h"
#include "icarus/config.h"
#include <stddef.h>
 
#ifndef HOST_TEST
 
#include "icarus/kernel.h"   /* enter_critical / exit_critical */
#include "stm32h7xx.h"
 
/* CRC->CR field encodings (from RM0433):
 *   bit 0      RESET     — write 1 to reload INIT into the data register
 *   bits 4:3   POLYSIZE  — 00=32, 01=16, 10=8, 11=7
 *   bits 6:5   REV_IN    — 00 = no input bit/byte reversal
 *   bit 7      REV_OUT   — 0  = no output bit reversal
 */
#define CRC_CR_POLYSIZE_16   (0x1u << CRC_CR_POLYSIZE_Pos)
 
static volatile uint8_t crc_initialised;
 
ITCM_FUNC static void crc_hw_init(void) {
    /* Enable the CRC peripheral clock on AHB4. The dummy read settles
     * the bus per the H7 reference manual recommendation. */
    RCC->AHB4ENR |= RCC_AHB4ENR_CRCEN;
    (void)RCC->AHB4ENR;
 
    /* Configure poly = 0x1021, init = 0xFFFF, 16-bit polysize,
     * no reversal. */
    CRC->POL  = 0x1021u;
    CRC->INIT = 0xFFFFu;
    CRC->CR   = CRC_CR_POLYSIZE_16;
 
    crc_initialised = 1u;
}
 
ITCM_FUNC uint16_t crc16_ccitt(const uint8_t *data, uint16_t len) {
    if (data == NULL || len == 0) {
        return 0xFFFFu;
    }
 
    __enter_critical();
 
    if (!crc_initialised) {
        crc_hw_init();
    }
 
    /* Reset the running value back to INIT (0xFFFF) before this run. */
    CRC->CR |= CRC_CR_RESET;
 
    /* Feed bytes one at a time. The peripheral allows 8/16/32-bit
     * accesses to DR; an 8-bit write consumes one byte regardless of
     * word alignment. */
    volatile uint8_t *const dr8 = (volatile uint8_t *)&CRC->DR;
    for (uint16_t i = 0; i < len; i++) {
        *dr8 = data[i];
    }
 
    uint16_t crc = (uint16_t)(CRC->DR & 0xFFFFu);
 
    __exit_critical();
    return crc;
}
 
#else /* HOST_TEST — portable bytewise loop */
 
uint16_t crc16_ccitt(const uint8_t *data, uint16_t len) {
    if ((data == NULL) || (len == 0u)) {
        return 0xFFFFu;
    }
    uint16_t crc = 0xFFFFu;
    for (uint16_t i = 0u; i < len; i++) {
        crc ^= (uint16_t)((uint16_t)data[i] << 8u);
        for (uint8_t bit = 0u; bit < 8u; bit++) {
            if ((crc & 0x8000u) != 0u) {
                crc = (uint16_t)((crc << 1) ^ 0x1021u);
            } else {
                crc = (uint16_t)(crc << 1);
            }
        }
    }
    return crc;
}
 
#endif /* HOST_TEST */