[14]ESP32之SPI使用备忘录

说明：以下为在官方库文件SPI Master driver release/v3.3基础上的总结，如果使用其他版本库，可能不适用

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一、学习文档目录

1. 乐鑫官方API ： SPI Master driver release/v3.3
2. 以上中文翻译版本可以查阅（以英文版为准）：ESP32 学习笔记（八）SPI - SPI Master
3. ESP32之软件SPI驱动及SPI、HSPI和VSPI的理解

二、关于SPI引脚的选择

1.优先推荐使用IOMUX映射的引脚

Pin Name	HSPI	VSPI
	GPIO Number	GPIO Number
CS0*	15	5
SCLK	14	18
MISO	12	19
MOSI	13	23
QUADWP	2	22
QUADHD	4	21

note * Only the first device attaching to the bus can use CS0 pin.

2.注意GPIO matrix的效率损失

如果IOMUX映射的引脚已经作为他用，并且无法替代。可以使用GPIO matrix映射到其他引脚，但是因为要经过中间期间，会造成时间的损失，进而直接降低极限速度。如果超过速度的限制在编译阶段会有提示。

三、两种运行模式的区别

比较项目	Interrupt transactions	Polling transactions
官方介绍	The interrupt transactions use an interrupt-driven logic when the transactions are in-flight. The routine will get blocked, allowing the CPU to run other tasks, while it is waiting for a transaction to be finished.	The polling transactions don’t rely on the interrupt, the routine keeps polling the status bit of the SPI peripheral until the transaction is done.
中断通知	有	无
时间损失	有	无
CPU损失	低	高

1. Polling transactions 可以节约掉队列处理和上下文切换的时间，但是CPU始终处于繁忙的状态。
2. Interrupt transactions 支持传输队列，A task can queue several transactions, and then do something else before the transactions are finished.

四、关于大小端问题

1. 首先ESP32 存储为小端模式，即低地址保存数据的低位，高地址保存数据的高位。
2. SPI工作在MSB first模式，也就意味着uint8_t的类型，8个bit的发送顺序是从7->0
3. 综上，如果一个uint16_t数据被发送，byte按照地址顺序从低到高发送，bit按照字节顺序从高到低发送，7 is first sent, then bit 6 to 0, then comes its bit 15 to bit 8.

五、SPI 模式/时序

    //Configure polarity
    if (dev->cfg.mode==0) {
        host->hw->pin.ck_idle_edge=0;
        host->hw->user.ck_out_edge=0;
    } else if (dev->cfg.mode==1) {
        host->hw->pin.ck_idle_edge=0;
        host->hw->user.ck_out_edge=1;
    } else if (dev->cfg.mode==2) {
        host->hw->pin.ck_idle_edge=1;
        host->hw->user.ck_out_edge=1;
    } else if (dev->cfg.mode==3) {
        host->hw->pin.ck_idle_edge=1;
        host->hw->user.ck_out_edge=0;
    }

六、已知问题

1. Half duplex mode is not compatible with DMA when both writing and reading phases exist.
   If such transactions are required, you have to use one of the alternative solutions:
   1. use full-duplex mode instead.
   2. disable the DMA by setting the last parameter to 0 in bus initialization function just as below: ret=spi_bus_initialize(VSPI_HOST, &buscfg, 0); this may prohibit you from transmitting and receiving data longer than 64 bytes.
   3. try to use command and address field to replace the write phase.
2. Full duplex mode is not compatible with the dummy bit workaround, hence the frequency is limited. See dummy bit speed-up workaround.
3. cs_ena_pretrans is not compatible with command, address phases in full duplex mode.

七、官方例程分析