Communication Protocols and Interfaces Questions

typedef void (*spi_cb_t)(int status, void *ctx);

typedef enum { SPI_NO_CS, SPI_CS_ACTIVE_LOW, SPI_CS_ACTIVE_HIGH } cs_type_t;

typedef struct {
  uint8_t device_id;
  uint8_t mode;          // 0..3
  uint32_t max_hz;
  cs_type_t cs_type;
  gpio_pin_t cs_pin;
} spi_device_cfg_t;

typedef struct {
  uint8_t device_id;
  const uint8_t *tx;
  uint8_t *rx;
  size_t len;
  uint32_t flags;        // e.g., KEEP_CS, HIGH_PRIORITY
  uint32_t timeout_ms;
  spi_cb_t cb;
  void *ctx;
} spi_transfer_t;

/* Initialization */
int spi_manager_init(void);

/* Register a device (called at startup) */
int spi_register_device(const spi_device_cfg_t *cfg);

/* Queue a transfer (non-blocking). Returns request id or -1 */
int spi_submit_transfer(const spi_transfer_t *xfer);

/* Optional blocking convenience */
int spi_transfer_blocking(const spi_transfer_t *xfer);

/* Cancel queued transfer */
int spi_cancel_transfer(int req_id);

#include <stdint.h>
#include <stddef.h>

/* Compute/update CRC-16-CCITT (poly 0x1021), initial value typically 0xFFFF.
   Reentrant: no static/global state. Can be called incrementally. */
uint16_t crc16_ccitt(const uint8_t *data, size_t len, uint16_t crc)
{
    static const uint16_t table[256] = {
        /* 256-entry table generated for poly 0x1021 (values omitted here for brevity).
           In real code include the full table or generate at build time. */
    };

    while (len--) {
        crc = (uint16_t)((crc << 8) ^ table[((crc >> 8) ^ *data++) & 0xFF]);
    }
    return crc;
}