SPI 从机驱动程序

SPI 从机驱动程序控制在 ESP32-S2 中作为从机的 SPI 外设。

ESP32-S2 中 SPI 外设概述

ESP32-S2 集成了 2 个通用的 SPI 控制器,可用作片外 SPI 主机驱动的从机节点。

SPI2 和 SPI3 各自具有一个与之同名的独立总线信号。

术语

下表为 SPI 主机驱动的相关术语。

术语

定义

主机 (Host)

ESP32-S2 外部的 SPI 控制器外设。用作 SPI 主机,在总线上发起 SPI 传输。

从机设备 (Device)

SPI 从机设备(通用 SPI 控制器)。每个从机设备共享 MOSI、MISO 和 SCLK 信号,但只有当主机向从机设备的专属 CS 线发出信号时,从机设备才会在总线上处于激活状态。

总线 (Bus)

信号总线,由连接到同一主机的所有从机设备共用。一般来说,一条总线包括以下线路:MISO、MOSI、SCLK、一条或多条 CS 线,以及可选的 QUADWP 和 QUADHD。每个从机设备都有单独的 CS 线,除此之外,所有从机设备都连接在相同的线路下。如果以菊花链的方式连接,几个从机设备也可以共享一条 CS 线。

MISO

主机输入,从机输出,也写作 Q。数据从从机设备发送至主机。

MOSI

主机输出,从机输入,也写作 D。数据从主机发送至从机设备。

SCLK

串行时钟。由主机产生的振荡信号,使数据位的传输保持同步。

CS

片选。允许主机选择连接到总线上的单个从机设备,以便发送或接收数据。

QUADWP

写保护信号。只用于 4 位 (qio/qout) 传输。

QUADHD

保持信号。只用于 4 位 (qio/qout) 传输。

断言 (Assertion)

指激活一条线的操作。反之,将线路恢复到非活动状态(回到空闲状态)的操作则称为 去断言

传输事务 (Transaction)

即主机断言从机设备的 CS 线,向从机设备传输数据,接着去断言 CS 线的过程。传输事务为原子操作,不可打断。

发射沿 (Launch Edge)

源寄存器将信号 发射 到线路上的时钟边沿。

锁存沿 (Latch Edge)

目的寄存器 锁存 信号的时钟边沿。

驱动程序的功能

SPI 从机驱动程序允许将 SPI 外设作为全双工设备使用。驱动程序可以发送/接收长度不超过 72 字节的传输事务,或者利用 DMA 来发送/接收更长的传输事务。然而,存在一些与 DMA 有关的 已知问题

SPI 传输事务

主机断言 CS 线并在 SCLK 线上发出时钟脉冲时,一次全双工 SPI 传输事务就此开始。每个时钟脉冲都意味着通过 MOSI 线从主机转移一个数据位到从机设备上,并同时通过 MISO 线返回一个数据位。传输事务结束后,主机去断言 CS 线。

传输事务的属性由作为从机设备的 SPI 外设的配置结构体 spi_slave_interface_config_t 和传输事务配置结构体 spi_slave_transaction_t 决定。

由于并非每次传输事务都需要写入和读取数据,您可以选择配置 spi_transaction_t 为仅 TX、仅 RX 或同时 TX 和 RX 传输事务。如果将 spi_slave_transaction_t::rx_buffer 设置为 NULL,读取阶段将被跳过。如果将 spi_slave_transaction_t::tx_buffer 设置为 NULL,则写入阶段将被跳过。

备注

主机应在从机设备准备好接收数据之后再进行传输事务。建议使用另外一个 GPIO 管脚作为握手信号来同步设备。更多细节,请参阅 传输事务间隔

使用驱动程序

  • 调用函数 cpp:func:spi_slave_initialize,将 SPI 外设初始化为从机设备。请确保在 bus_config 中设置正确的 I/O 管脚,并将未使用的信号设置为 -1

如果传输事务的数据大于 32 字节,需要在主机上设置参数 dma_chan 以使能 DMA 通道。若数据小于 32 字节,则应将 dma_chan 设为 0

-(可选)如需卸载 SPI 从机驱动程序,请调用 spi_slave_free()

传输事务数据和主/从机长度不匹配

通常,通过从机设备进行传输的数据会被读取或写入到由 spi_slave_transaction_t::rx_bufferspi_slave_transaction_t::tx_buffer 指示的大块内存中。可以配置 SPI 驱动程序,使用 DMA 进行传输。在这种情况下,则必须使用 pvPortMallocCaps(size, MALLOC_CAP_DMA) 将缓存区分配到具备 DMA 功能的内存中。

驱动程序可以读取或写入缓存区的数据量取决于 spi_slave_transaction_t::length,但其并不会定义一次 SPI 传输的实际长度。传输事务的长度由主机的时钟线和 CS 线决定,且只有在传输事务完成后,才能从 spi_slave_transaction_t::trans_len 中读取实际长度。

如果传输长度超过缓存区长度,则只有在 spi_slave_transaction_t::length 中指定的初始比特数会被发送和接收。此时, spi_slave_transaction_t::trans_len 被设置为 spi_slave_transaction_t::length 而非实际传输事务长度。若需满足实际传输事务长度的要求,请将 spi_slave_transaction_t::length 设置为大于 spi_slave_transaction_t::trans_len 预期最大值的值。如果传输长度短于缓存区长度,则只传输与缓存区长度相等的数据。

速度与时钟

传输事务间隔

ESP32-S2 的 SPI 从机外设是由 CPU 控制的通用从机设备。与专用的从机相比,在内嵌 CPU 的 SPI 从机设备中,预定义寄存器的数量有限,所有的传输事务都必须由 CPU 处理。也就是说,传输和响应并不是实时的,且可能存在明显的延迟。

解决方案为,首先使用函数 spi_slave_queue_trans(),然后使用 spi_slave_get_trans_result(),来代替 spi_slave_transmit()。由此一来,可使从机设备的响应速度提高一倍。

您也可以配置一个 GPIO 管脚,当从机设备开始新一次传输事务前,它将通过该管脚向主机发出信号。示例代码存放在 peripherals/spi_slave 目录下。

时钟频率要求

SPI 从机的工作频率最高可达 40 MHz。如果时钟频率过快或占空比不足 50%,数据就无法被正确识别或接收。

限制条件和已知问题

  1. 若启用了 DMA,则 RX 缓冲区应该以字对齐(从 32 位边界开始,字节长度为 4 的倍数)。否则,DMA 可能无法正确写入或无法实现边界对齐。若此项条件不满足,驱动程序将会报错。

    此外,主机写入字节长度应为 4 的倍数。长度不符合的数据将被丢弃。

应用示例

从机设备/主机通信的示例代码存放在 ESP-IDF 示例项目的 peripherals/spi_slave 目录下。

API 参考

Header File

Functions

esp_err_t spi_slave_initialize(spi_host_device_t host, const spi_bus_config_t *bus_config, const spi_slave_interface_config_t *slave_config, spi_dma_chan_t dma_chan)

Initialize a SPI bus as a slave interface.

警告

SPI0/1 is not supported

警告

If a DMA channel is selected, any transmit and receive buffer used should be allocated in DMA-capable memory.

警告

The ISR of SPI is always executed on the core which calls this function. Never starve the ISR on this core or the SPI transactions will not be handled.

参数
  • host – SPI peripheral to use as a SPI slave interface

  • bus_config – Pointer to a spi_bus_config_t struct specifying how the host should be initialized

  • slave_config – Pointer to a spi_slave_interface_config_t struct specifying the details for the slave interface

  • dma_chan – - Selecting a DMA channel for an SPI bus allows transactions on the bus with size only limited by the amount of internal memory.

    • Selecting SPI_DMA_DISABLED limits the size of transactions.

    • Set to SPI_DMA_DISABLED if only the SPI flash uses this bus.

    • Set to SPI_DMA_CH_AUTO to let the driver to allocate the DMA channel.

返回

  • ESP_ERR_INVALID_ARG if configuration is invalid

  • ESP_ERR_INVALID_STATE if host already is in use

  • ESP_ERR_NOT_FOUND if there is no available DMA channel

  • ESP_ERR_NO_MEM if out of memory

  • ESP_OK on success

esp_err_t spi_slave_free(spi_host_device_t host)

Free a SPI bus claimed as a SPI slave interface.

参数

host – SPI peripheral to free

返回

  • ESP_ERR_INVALID_ARG if parameter is invalid

  • ESP_ERR_INVALID_STATE if not all devices on the bus are freed

  • ESP_OK on success

esp_err_t spi_slave_queue_trans(spi_host_device_t host, const spi_slave_transaction_t *trans_desc, TickType_t ticks_to_wait)

Queue a SPI transaction for execution.

Queues a SPI transaction to be executed by this slave device. (The transaction queue size was specified when the slave device was initialised via spi_slave_initialize.) This function may block if the queue is full (depending on the ticks_to_wait parameter). No SPI operation is directly initiated by this function, the next queued transaction will happen when the master initiates a SPI transaction by pulling down CS and sending out clock signals.

This function hands over ownership of the buffers in trans_desc to the SPI slave driver; the application is not to access this memory until spi_slave_queue_trans is called to hand ownership back to the application.

参数
  • host – SPI peripheral that is acting as a slave

  • trans_desc – Description of transaction to execute. Not const because we may want to write status back into the transaction description.

  • ticks_to_wait – Ticks to wait until there’s room in the queue; use portMAX_DELAY to never time out.

返回

  • ESP_ERR_INVALID_ARG if parameter is invalid

  • ESP_OK on success

esp_err_t spi_slave_get_trans_result(spi_host_device_t host, spi_slave_transaction_t **trans_desc, TickType_t ticks_to_wait)

Get the result of a SPI transaction queued earlier.

This routine will wait until a transaction to the given device (queued earlier with spi_slave_queue_trans) has succesfully completed. It will then return the description of the completed transaction so software can inspect the result and e.g. free the memory or re-use the buffers.

It is mandatory to eventually use this function for any transaction queued by spi_slave_queue_trans.

参数
  • host – SPI peripheral to that is acting as a slave

  • trans_desc[out] Pointer to variable able to contain a pointer to the description of the transaction that is executed

  • ticks_to_wait – Ticks to wait until there’s a returned item; use portMAX_DELAY to never time out.

返回

  • ESP_ERR_INVALID_ARG if parameter is invalid

  • ESP_ERR_NOT_SUPPORTED if flag SPI_SLAVE_NO_RETURN_RESULT is set

  • ESP_OK on success

esp_err_t spi_slave_transmit(spi_host_device_t host, spi_slave_transaction_t *trans_desc, TickType_t ticks_to_wait)

Do a SPI transaction.

Essentially does the same as spi_slave_queue_trans followed by spi_slave_get_trans_result. Do not use this when there is still a transaction queued that hasn’t been finalized using spi_slave_get_trans_result.

参数
  • host – SPI peripheral to that is acting as a slave

  • trans_desc – Pointer to variable able to contain a pointer to the description of the transaction that is executed. Not const because we may want to write status back into the transaction description.

  • ticks_to_wait – Ticks to wait until there’s a returned item; use portMAX_DELAY to never time out.

返回

  • ESP_ERR_INVALID_ARG if parameter is invalid

  • ESP_OK on success

Structures

struct spi_slave_interface_config_t

This is a configuration for a SPI host acting as a slave device.

Public Members

int spics_io_num

CS GPIO pin for this device.

uint32_t flags

Bitwise OR of SPI_SLAVE_* flags.

int queue_size

Transaction queue size. This sets how many transactions can be ‘in the air’ (queued using spi_slave_queue_trans but not yet finished using spi_slave_get_trans_result) at the same time.

uint8_t mode

SPI mode, representing a pair of (CPOL, CPHA) configuration:

  • 0: (0, 0)

  • 1: (0, 1)

  • 2: (1, 0)

  • 3: (1, 1)

slave_transaction_cb_t post_setup_cb

Callback called after the SPI registers are loaded with new data.

This callback is called within interrupt context should be in IRAM for best performance, see “Transferring 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.

slave_transaction_cb_t post_trans_cb

Callback called after a transaction is done.

This callback is called within interrupt context should be in IRAM for best performance, see “Transferring 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_slave_transaction_t

This structure describes one SPI transaction

Public Members

size_t length

Total data length, in bits.

size_t trans_len

Transaction data length, in bits.

const void *tx_buffer

Pointer to transmit buffer, or NULL for no MOSI phase.

void *rx_buffer

Pointer to receive buffer, or NULL for no MISO phase. When the DMA is anabled, must start at WORD boundary (rx_buffer%4==0), and has length of a multiple of 4 bytes.

void *user

User-defined variable. Can be used to store eg transaction ID.

Macros

SPI_SLAVE_TXBIT_LSBFIRST

Transmit command/address/data LSB first instead of the default MSB first.

SPI_SLAVE_RXBIT_LSBFIRST

Receive data LSB first instead of the default MSB first.

SPI_SLAVE_BIT_LSBFIRST

Transmit and receive LSB first.

SPI_SLAVE_NO_RETURN_RESULT

Don’t return the descriptor to the host on completion (use post_trans_cb to notify instead)

Type Definitions

typedef struct spi_slave_transaction_t spi_slave_transaction_t
typedef void (*slave_transaction_cb_t)(spi_slave_transaction_t *trans)