Espressif ESP32-S2 Programming Manual page 390

Chapter 2. API Reference
int queue_size
Transaction queue size.
spi_device_queue_trans but not yet finished using spi_device_get_trans_result) at the same time.
transaction_cb_t pre_cb
Callback to be called before a transmission is started.
This callback is called within interrupt context should be in IRAM for best performance, see "Trans-
ferring Speed"section in the SPI Master documentation for full details. If not, the callback may crash
during flash operation when the driver is initialized with ESP_INTR_FLAG_IRAM.
transaction_cb_t post_cb
Callback to be called after a transmission has completed.
This callback is called within interrupt context should be in IRAM for best performance, see "Trans-
ferring Speed"section in the SPI Master documentation for full details. If not, the callback may crash
during flash operation when the driver is initialized with ESP_INTR_FLAG_IRAM.
struct spi_transaction_t
This structure describes one SPI transaction. The descriptor should not be modified until the transaction fin-
ishes.
Public Members
uint32_t flags
Bitwise OR of SPI_TRANS_* flags.
uint16_t cmd
Command data, of which the length is set in the command_bits of spi_device_interface_config_t.
NOTE: this field, used to be "command"in ESP-IDF 2.1 and before, is re-written to be used in
a new way in ESP-IDF 3.0.
Example: write 0x0123 and command_bits=12 to send command 0x12, 0x3_ (in previous version, you
may have to write 0x3_12).
uint64_t addr
Address data, of which the length is set in the address_bits of spi_device_interface_config_t.
NOTE: this field, used to be "address"in ESP-IDF 2.1 and before, is re-written to be used in a
new way in ESP-IDF3.0.
Example: write 0x123400 and address_bits=24 to send address of 0x12, 0x34, 0x00 (in previous version,
you may have to write 0x12340000).
size_t length
Total data length, in bits.
size_t rxlength
Total data length received, should be not greater than length in full-duplex mode (0 defaults this to the
value of length).
void *user
User-defined variable. Can be used to store eg transaction ID.
const void *tx_buffer
Pointer to transmit buffer, or NULL for no MOSI phase.
uint8_t tx_data[4]
If SPI_TRANS_USE_TXDATA is set, data set here is sent directly from this variable.
void *rx_buffer
Pointer to receive buffer, or NULL for no MISO phase. Written by 4 bytes-unit if DMA is used.
uint8_t rx_data[4]
If SPI_TRANS_USE_RXDATA is set, data is received directly to this variable.
Espressif Systems
This sets how many transactions can be 'in the air'(queued using
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