esp32 SPI arduino

### 使用ESP32通过SPI与Arduino通信

#### 连接硬件
为了实现ESP32和Arduino之间的SPI通信，需要按照特定的方式连接两个设备。通常情况下，ESP32作为主机（Master），而Arduino作为从机（Slave）。以下是具体的连线方式：

| ESP32 Pin | Arduino Uno Pin |
|-----------|-----------------|
| GND | GND |
| 3V3 | VCC / VIN |
| MOSI (GPIO23) | MISO |
| MISO (GPIO19)| MOSI |
| SCK (GPIO18) | SCLK |
| SS/CS (GPIO5) | SS |

表中的`SS/CS`表示片选信号线，在某些资料中可能被标记为`CS`。

#### 初始化库文件
对于ESP32端，需引入SPI库来配置并操作SPI接口；而对于Arduino，则可以利用内置的SPI Slave功能来进行数据交换[^1]。

// ESP32 Master Code Example
#include <SPI.h>

#define SLAVE_SELECT_PIN 5 // CS pin connected to slave device

void setup() {
    pinMode(SLAVE_SELECT_PIN, OUTPUT);
    digitalWrite(SLAVE_SELECT_PIN, HIGH); // Deselect the chip at startup
    
    Serial.begin(115200);
    
    SPI.begin(); // Initialize SPI bus
}

void loop() {
    const char* message = "Hello from ESP32!";
    int length = strlen(message);

    digitalWrite(SLAVE_SELECT_PIN, LOW); // Select the slave device
    delayMicroseconds(10);

    for(int i=0;i<length;++i){
        SPI.transfer(message[i]); // Send each character over SPI one by one
    }

    delayMicroseconds(10);
    digitalWrite(SLAVE_SELECT_PIN, HIGH); // Release the slave select line after transmission completes

    delay(1000); // Wait before sending next message
}

// Arduino Slave Code Example
#include <SPI.h>
volatile boolean receiveFlag = false;
char receivedData[20];
byte index;

void setup(){
    pinMode(SS, INPUT_PULLUP); // Set SS as input pullup mode since we're acting like a slave here.
    SPI.setClockDivider(SPI_CLOCK_DIV64); // Adjust clock speed according to your needs.

    SPCR |= _BV(SPE); // Enable SPI in slave mode
    SPDR = 0;         // Start with dummy data on buffer

    attachInterrupt(digitalPinToInterrupt(SS), spiInterruptHandler, FALLING);
    Serial.begin(115200);
}

ISR(TIMER1_COMPA_vect){} // Dummy ISR just so that interrupts are enabled globally

void spiInterruptHandler(){
    byte c = SPDR; // Read incoming byte into variable 'c'
    if(c != '\n'){
        receivedData[index++] = c;
        if(index >= sizeof(receivedData)){
            index = sizeof(receivedData)-1;
        }
    }else{
        receivedData[index]='\0';
        receiveFlag=true;
        index=0;
    }
}

void loop(){
    if(receiveFlag==true){
        Serial.println(receivedData);
        memset(&receivedData,0,sizeof(receivedData));
        receiveFlag=false;
    }
}

这段代码展示了如何设置ESP32为主控方发送字符串消息给扮演从属角色的Arduino，并由后者接收处理这些信息。